# 5 Dedekind extensions

Size: px
Start display at page:

Transcription

1 Number theory I Fall 2016 Lecture #5 09/22/ Dedekind extensions In this lecture we prove that the integral closure of a Dedekind domain in a finite extension of its fraction field is also a Dedekind domain; this implies, in particular, that the ring of integers of a number field is a Dedekind domain. Having accomplished this task we consider the factorization of prime ideals in Dedekind extensions. 5.1 Dual modules, pairings, and lattices In this section we work in a more general setting, where A is any commutative (unital) ring. Definition 5.1. Let A be a commutative ring and M an A-module. The dual module M is the A-module Hom A (M, A) with scalar multiplication (af)(m) = af(m), where a A, f Hom A (M, A), and m M. If ϕ: M N is an A-module homomorphism, the dual homomorphism ϕ : N M is defined by ϕ (g)(m) = g(ϕ(m)), for g N and m M. It is easy to check that taking duals preserves identity maps and is compatible with composition: if ϕ 1 : M N and ϕ 2 : N P are A-module homomorphisms, then (ϕ 2 ϕ 1 ) = ϕ 1 ϕ 2. We thus have a contravariant functor from the category of A-modules to itself. We have already seen some examples of dual modules: invertible fractional ideals are dual to their inverses. Proposition 5.2. Let A be an integral domain with fraction field K and let M be a nonzero A-submodule of K. Then M (A : M) := {x K : xm A}; in particular, if M is an invertible fractional ideal then M M 1 and M M. Proof. For any x (A : M) the map m xm is an A-linear map from M to A, hence an element of M, and this defines an A-module homomorphism ϕ: (A : M) M, since the map x (m xm) is itself A-linear. Since M K is a nonzero A-module, it contains some nonzero a A (if a/b M, so is ba/b = a). If f M and m = b/c M then f(m) = f ( ) b c = ac ac f ( b c ) = b ( ac ) ac f = b f(a) = f(a)a 1 m, c ac where we have used the fact that a 1 f(a 2 /a 3 ) = a 2 f(a 1 /a 3 ) for any a 1, a 2, a 3 A with a 1 /a 3, a 2 /a 3 M, by the A-linearity of f. It follows that f corresponds to multiplication by x = f(a)a 1, which lies in (A : M) since xm = f(m) A for all m M. The map f f(a)a 1 defines an A-module homomorphism M (A : M) inverse to ϕ, so ϕ is an isomorphism. When M is an invertible fractional ideal we have M (A : M) = M 1, by Lemma 3.8, and M (M 1 ) 1 = M follows. If A is a field, then M is a vector space and M is its dual space and we have M M, as in the proposition above. But in general not every A-module is isomorphic to its double dual; those that are are said to be reflexive. Example 5.3. As a Z-module, we have Q = {0} because there are no non-trivial Z-linear homomorphisms from Q to Z; indeed, Q is a divisible group and Z contains no non-trivial divisible subgroups. It follows that Q = {0} (but as Q-modules we have Q Q Q ). Similarly, the dual of any finite Z-module (any finite abelian group) is the zero module, as is the double dual. More generally, if A is an integral domain every dual (and double dual) A-module must be torsion free, but not all A-modules are torsion free. Lecture by Andrew Sutherland

2 One situation where we can recover many of the standard results that hold for vector spaces of finite dimension (with essentially the same proofs), is when M is a free module of finite rank. In particular, not only is M reflexive, we have M M (non-canonically) and may explicitly construct a dual basis. Theorem 5.4. Let A be a commutative ring and let M be a free A-module of rank n. Then M is also a free A-module of rank n, and each basis (e 1,..., e n ) of M uniquely determines a dual basis (e 1,... e n) of M with the property { i e 1 = j, i (e j ) = δ ij := 0 i j. Proof. If n = 0 then M = M = {0} and the theorem holds. Now assume n 1 and fix an A-basis e := (e 1,..., e n ) for M. For each a := (a 1,..., a n ) A n, define f a M by setting f a (e i ) = a i and extending A-linearly. The map a f a gives an A-module homomorphism A n M with inverse f (f(e 1 ),..., f(e n )) and is therefore an isomorphism. It follows that M A n is a free A-module of rank n. Now let e i := f ai, where a i = (0,..., 0, 1, 0,..., 0) A n has a 1 in the ith position. Then (e 1,..., e n) is a basis for M, since (a 1,..., a n ) is a basis for A n, and we have e i (e j) = δ ij. This choice of e i is unique because it must be the image of (a 1,..., a n ) under the isomorphism a f a. Definition 5.5. Let A be a commutative ring and M an A-module. A (bilinear) pairing on M is an A-linear map, : M M A. Explicitly, this means that for all u, v, w M and λ A we have u + v, w = u, w + v, w, u, v + w = u, v + u, w, λu, v = u, λv = λ u, v. If v, w = w, v then, is symmetric, if v, w = w, v then, is skew-symmetric, and if v, v = 0 then, is alternating (the last two are equivalent provided char(a) 2). The pairing, induces an A-module homomorphism ϕ: M M m (n m, n ) If ker ϕ = {0} then, is nondegenerate, and if ϕ is an isomorphism then, is perfect. Every perfect pairing is necessarily nondegenerate. If M is a vector space of finite dimension the converse holds, but this is not true in general, not even for free modules of finite rank: consider the pairing x, y := 2xy on Z, which is non-degenerate but not perfect. If M is a free A-module with basis (e 1,..., e n ) and, is a perfect pairing, we can apply the inverse of the isomorphism ϕ: M M induced by the pairing to the dual basis (e 1,..., e n) given by Theorem 5.4 to obtain a basis (e 1,..., e n) for M that satisfies e i, e j = δ ij. When, is symmetric we can similarly recover (e 1,..., e n ) from (e 1,..., e n) in the same way. We record this fact in the following proposition Fall 2016, Lecture #5, Page 2

3 Proposition 5.6. Let A be a commutative ring and let M be a free A-module of rank n with a perfect pairing,. For each A-basis (e 1,..., e n ) of M there is a unique basis (e 1,..., e n) for M such that e i, e j = δ ij. Proof. Existence follows from the discussion above: apply the inverse of the isomorphism ϕ: V V induced by, to the dual basis (e 1,..., e n) given by Theorem 5.4 to obtain a basis (e 1,..., e n) for M with e i = ϕ 1 (e i ). We then have e i = ϕ(e i ) = m e i, m and e i, e j = ϕ(e i)(e j ) = e i (e j ) = δ ij for 1 i, j n. If (f 1,..., f n ) is another basis for M with the same property then for each i we have e i f i, e j = δ ij δ ij = 0 for every e j, and therefore e i f i, m = 0 for all m M, but then e i f i ker ϕ = {0}, since the perfect pairing, is nondegenerate, and therefore f i = e i for each i; uniqueness follows. Remark 5.7. In what follows the commutative ring A in Proposition 5.6 will typically be a field K and the free A-module M will be a K-vector space that we will denote V. We may then use A to denote a subring of K and M to denote an A-submodule of V. A perfect paring, on the K-vector space V will typically not restrict to a perfect pairing on the A-module M. For example, the perfect pairing x, y = xy on Q does not restrict to a perfect pairing on the Z-module 2Z because the induced map ϕ: 2Z 2Z defined by ϕ(m) = (n mn) is not surjective: the map x x/2 lies in 2Z = Hom Z (2Z, Z) but it is not in the image of ϕ. We now introduce the notion of a lattice in a vector space. Definition 5.8. Let A be an integral domain with fraction field K and let V be a K-vector space of finite dimension. A (full) A-lattice in V is a finitely generated A-submodule M of V that spans V as a K-vector space. Remark 5.9. Some authors require A-lattices to be free A-modules. When A = Z (or any PID) this is not a restriction because M is necessarily torsion-free (it lies in a vector space) and any finitely generated torsion-free module over a PID is free (by the structure theorem for finitely generated modules over a PID). But when A is not a PID, finitely generated torsion-free A-modules will typically not be free. We do not want to exclude this case! In particular if L/K is an extension of number fields the ring of integers O L will typically not be a free O K -module (even though it is a free Z-module, as we shall shortly prove), but we still want to treat O L as an O K -lattice in L (this will be important in later lectures when we define the different ideal D L/K ). Definition Let A be a noetherian domain with fraction field K, and let V be a K-vector space of finite dimension with a perfect pairing,. If M is an A-lattice in V, its dual lattice (with respect to the pairing, ) is the A-module M := {x V : x, m A for all m M}. It is clear that M is an A-submodule of V, but it is not clear that it is an A-lattice in V (it must be finitely generated and to span V ), nor is it obvious that it is isomorphic to the dual module M. In order to justify the term dual lattice, let us now prove both facts. We will need to use the hypothesis that A is noetherian, since in general the dual of a finitely generated A-module need not be finitely generated. Notice that, is a perfect pairing Fall 2016, Lecture #5, Page 3

4 on the K-module V ; it typically does not restrict to a perfect pairing on the A-module M (so Proposition 5.6 applies with A = K and M = V, but typically not with A = A and M = M). Theorem Let A be a noetherian domain with fraction field K, let V be a K-vector space with a perfect pairing,, and let M be an A-lattice in V. The dual lattice M is an A-lattice in V isomorphic to M. Proof. Let e := (e 1,..., e n ) be a K-basis for V that lies in M, and let (e 1,..., e n) be the unique K-basis for V given by Proposition 5.6 that satisfies e i, e j = δ ij. To see that M spans V, if we write a finite set of generators for M in terms of the basis e with coefficients in K and let d be the product of all denominators that appear, then de lies in M, and it is a K-basis for V ; here we are using the bilinearity of, and (crucially) the fact that e i, e j = δ ij A (since the ring A contains 0 and 1). We now show M is finitely generated. Let N := {a 1 e a n e n : a 1,..., a n A} A n be the free A-submodule of M spanned by e. The A-module N spans V and is finitely generated, so it is an A-lattice in V. The K-basis e for V lies in N, since e i, e j = δ ij A, and we claim it is an A-basis for N. Given x N n, write x = i=1 x i e i with x i K; then x, e i = x i e i, e i = x i A, so x lies in the A-span of e. It follows that N is a free A-module of rank n, and in particular, a finitely generated over a noetherian ring and therefore a noetherian module (all submodules finitely generated); see [1, Thm ]. From the definition of the dual lattice we have N M M N, so M is a submodule of a noetherian module, hence finitely generated. We now show M M. We have an obvious A-module homomorphism ϕ: M M given by x (m x, m ). We now proceed as in the proof of Theorem 5.2. Given f M, we can write any m M as 1 d i a ie i for some d, a 1,..., a n A. We then have ( ) f(m) = d f 1 n n a i n a i e i = d d d f(e a i n i) = f(e j ) e d j, e i = x, m A, i=1 i=1 i=1 j=1 where x = n n i=1 f(e i)e i M does not depend on m. The map f i=1 f(e i)e i an A-module homomorphism M M inverse to ϕ, so ϕ is an isomorphism. is thus Corollary Let A be a noetherian domain with fraction field K, let V be a K-vector space with a perfect pairing,, and let M be a free A-lattice in V with A-basis (e 1,..., e n ). The dual lattice M is a free A-lattice in V that has a unique A-basis (e 1,..., e n) that satisfies e i, e j = δ ij. Proof. This follows from the proof of Theorem 5.11 with N = M and e i = e i. You might wonder whether M = M for an A-lattice M in a vector space V. This is false in general, but it is true when A is a Dedekind domain and we have a symmetric perfect pairing on V. To prove this we first show that the dual lattice respects localization. Lemma Let A be a noetherian domain with fraction field K, let V be a K-vector space of finite dimension with a perfect pairing,, let M be an A-lattice in V, and let S be a multiplicative subset of A. Then S 1 M and S 1 M are S 1 A-lattices with (S 1 M) = S 1 M as S 1 A Fall 2016, Lecture #5, Page 4

5 Proof. It is clear that S 1 M is an S 1 M are both S 1 A-lattices: each contains a basis for V (since M and M do), and both are finitely generated as S 1 A-modules (since M and M are finitely generated as A-modules). Let m 1,... m 1 n be A-module generators for M (and therefore S A-module generators for S 1 M). If x is an element of (S 1 M) then for each m i we have x, m i = a i /s i for some a i A and s i S, and if we put s = s 1 s n then sx, m i A for every m i, hence for all m M; thus sx M and x S 1 M. Conversely, if x = y/s is an element of S 1 M with y M and s S, then y, m i A for every m i and x, m i = y, m i /s S 1 A for every m 1 1 i, hence for all m S M, and it follows that x (S M). Proposition Let A be a Dedekind domain with fraction field K, let V be a K-vector space of finite dimension with a symmetric perfect pairing,, and let M be an A-lattice in V. Then M = M. Proof. By Proposition 2.7, it suffices to show (M ) p = M p for each maximal ideal p of A. By Lemma 5.13 we have (M ) p = M p, so it is enough to show that the proposition holds when A is replaced by one of its localizations A p (a DVR, since A is a Dedekind domain). So let us assume A that is a DVR. Then A is a PID and M and M are both torsion-free modules over a PID, hence free A-modules. So let us choose an A-basis (e 1,..., e n ) for M, and let (e 1,..., e n) be the unique dual A-basis for M that satisfies e i, e j = δ ij (given by Corollary 5.12). If we now let (e 1,..., e n ) be the unique A-basis for M that satisfies e i, e j = δ ij and note that e i, e j = δ ij (since, is symmetric), by uniqueness, we must have e i = e i for all i, and therefore M = M. 5.2 Extensions of Dedekind domains Let A be a Dedekind domain with fraction field K, let L/K be a finite extension, and let B be the integral closure of A in L. We wish to prove that B is a Dedekind domain, which we will do by showing that it is an A-lattice in L; this will imply, in particular, that B is finitely generated, which is really the only difficult thing to show. Let us first show that B spans L as a vector space (and in fact L is its fraction field). Proposition Let A be a Dedekind domain with fraction field K, let L/K be a finite extension, and let B be the integral closure of A in L. Every element of L can be written as b/a with a A and b B. In particular, B spans L as a K-vector space and L is the fraction field of B. Proof. Let α L. By multiplying the minimal polynomial of α in K[x] by the product of the denominators of its coefficients, we obtain an irreducible polynomial in A[x]: g(x) = an x n + a n n 1 x a 1 x + a 0, with a n 0, that has α as a root. We can make this polynomial monic by replacing x with x/a n and multiplying through by a n 1 n to obtain a n 1 n g(x/a n ) = x n 1 + an 1x n + a n a n x n a n 2 n a 1 1 x + a n n a 0. This is a monic polynomial with coefficients in A that has a n α L as a root. Therefore a n α B, since B is the integral closure of A in L, and α = b/a n for some b B and a n A 1 as claimed. It follows that B generates L as a K-vector space (we have α = b a n with 1 a n K), and B L Frac B implies L = Frac B (no smaller field can contain B) Fall 2016, Lecture #5, Page 5

6 Proposition Let A be a Dedekind domain with fraction field K, let L/K be a finite extension, and let B be the integral closure of A in L. Then N L/K (b) A and T L/K (b) A for all b B. Proof. The minimal polynomial f = d i i=0 a ix K[x] of b has coefficients in A, by Proposition 1.25, and it then follows from Proposition 4.44 that N L/K (b) = ( 1) de a e 0 A and T L/K (b) = ea d 1 A (where e = [L : K(b)] Z). Definition Let L/K be a finite extension of fields. The trace pairing on L is the map L L K defined by x, y L/K := T L/K (xy). Proposition Let L/K be a finite extension of fields. The trace pairing, L/K is a symmetric bilinear pairing; it is perfect if and only if L/K is separable. Proof. Bilinearity follows from the K-linearity of the trace map T L/K, and symmetry is immediate. As proved on Problem Set 2, T L/K is the zero map if and only if L/K is inseparable. If T L/K is the zero map then the trace pairing is clearly not perfect, and otherwise we may pick z L for which T L/K (z) 0. Then for every x L we have x, z/x L/K = T L/K (z) 0, which means that map x x, y L/K is not the zero map, so the kernel of the map L L induced by, L/K is trivial. Thus the pairing, L/K is non-degenerate and therefore perfect, since K is a field. We now assume that L/K is separable. For the next several lectures we will be working in the following setting: A is a Dedekind domain with fraction field K, the extension L/K is finite separable, and B is the integral closure of A in L (which we will shortly prove is a Dedekind domain). As a convenient shorthand, we will write assume AKLB to indicate that we are using this setup. Proposition Assume AKLB. Then B is an A-lattice in L under the trace pairing, L/K. In particular, B is finitely generated as an A-module. Proof. By Proposition 5.15, B spans L as a K-vector space, so it contains a basis (e 1,..., e n ) for L as a K-vector space. Let M B be the A-span of (e 1,..., e n ). Then M is an A-lattice in L contained in B, and it has a dual lattice M that contains the A-module B := {x L : x, b A for all b B}. Proposition 5.16 implies that B B, and we thus have inclusions M B B M. By Theorem 5.11, M is an A-lattice in L, hence finitely generated, hence noetherian. It follows that its A-submodule B is finitely generated and thus an A-lattice in L. Remark When L/K is inseparable, B need not be finitely generated as an A-module, not even when A is a PID; see [2, Ex. 11, p. 205]. We used the separability hypothesis in order to get a perfect pairing, which plays a crucial role in the proof of Theorem Lemma Let B/A be an extension of domains with B integral over A, and let q 0 q 1 be primes of B. Then q 0 A q 1 A and dim A dim B Fall 2016, Lecture #5, Page 6

7 Proof. We first replace B with B/q 0 and replace A, q 0, and q 1 with their images in B/q 0 (the new B is integral over the new A, since the image of a monic polynomial in A[x] is a monic polynomial in (A/(q 0 A))[x]). Then q 0 = (0) and q 1 is a nonzero prime ideal. Let α q 1 be nonzero. Its minimal polynomial x n + a n 1 n 1 x + + a 0 = 0 has coefficients in A and nonzero constant coefficient a 0 (otherwise divide by x). We have a 0 = a 1 α α n q 1, thus 0 a 0 q 1 A. So q 1 A is not the zero ideal and therefore properly contains q 0 A = {0}. We can apply this result repeatedly to any chain of distinct prime ideals in B to get a corresponding chain of distinct prime ideals in A, thus dim A dim B. Theorem Let A be a Dedekind domain with fraction field K, let L/K be a finite separable extension, and let B be the integral closure of A in L. Then B is a Dedekind domain. Proof. Recall that we defined a Dedekind domain as an integrally closed noetherian domain of dimension at most one. Let us verify that each of these conditions holds: B is an integrally closed domain (by definition); B is finitely generated over the noetherian ring A (by Prop. 5.19), hence noetherian; B has dimension at most 1, since dim B dim A 1, by Lemma Thus B is a Dedekind domain. Remark Theorem 5.22 holds without the assumption that L/K is separable. This follows from the Krull-Akizuki Theorem, see [4, Thm. 11.7] or [3, VII.2.5], which is used to prove that B is noetherian even when it is not finitely generated as an A-module. Corollary The ring of integers of a number field is a Dedekind domain. 5.3 Splitting primes in Dedekind extensions We continue in the AKLB setup, in which A is a Dedekind domain, K is its fraction field, L/K is a finite separable 1 extension, and B is the integral closure of A, which we now know is a Dedekind domain with fraction field L. As we proved in earlier lectures, every nonzero ideal in a Dedekind domain can be uniquely factored into prime ideals. Understanding the ideal structure of a Dedekind domain thus boils down to understanding its prime ideals. In order to simplify the language, whenever we have a Dedekind domain A, by a prime of A (or of its fraction field K), we always mean a nonzero prime ideal; the prime elements of A are precisely those that generate nonzero principal prime ideals, so this generalizes the usual terminology. Note that 0 is (by definition) not prime, even though (0) is a prime ideal; thus when we refer to a prime of A we are specifically excluding the zero ideal. If A has dimension zero then so does B there are no primes to consider, so we may as well assume dim A = 1, in which case dim B = 1 as well (if B is a field than so is B K = A). Henceforth our AKLB setup will include the assumption that A K. Given a prime p of A, we can consider the ideal pb it generates in B (its extension to B under the inclusion map). The ideal pb need not be prime, but it can be uniquely factored 1 Most of our proofs will not actually use the separability hypothesis (and even when they do, there may be another way to prove the same result, as with Theorem 5.22). In order to simplify the presentation we will use the separability assumption whenever it would be awkward not to. The cases we care most about (extensions of local and global fields) are going to be separable in any case Fall 2016, Lecture #5, Page 7

8 into nonzero prime ideals in the Dedekind domain B. We thus have pb = q eq, where q ranges over primes of B and the exponents e q 0 are zero for all but finitely many primes q. The primes q for which e q > 0 are said to lie over or above the prime ideal p. As an abuse of notation, we will often write q p to indicate this relationship (there is little risk of confusion, the prime ideal p is maximal hence not divisible by any prime ideals of A other than itself). Lemma Let A be a ring of dimension one contained in a Dedekind domain B. Let p be a prime of A and let q be a prime of B. Then q p if and only if q A = p. Proof. If q divides pb then it contains pb (to divide is to contain), and therefore q A contains pb A which contains p; the ideal p is maximal and q A A (since 1 q), so q A = p. Conversely, if q A = p then q = qb certainly contains (q A)B = pb, and B is a Dedekind domain, so q divides pb (in a Dedekind domain to contain is to divide). Lemma 5.25 implies that contraction gives us a surjective map Spec B Spec A defined by q q A; to see why it is surjective, note that (0) A = (0), and if p is a nonzero element of Spec A then pb is nonzero and not the unit ideal, and therefore divisible by at least one q Spec B. The fibers of this map are finite; we use {q p} to denote the fiber above a prime p of A. The primes p of A are all maximal ideals (since dim A = 1), so each has an associated residue field A/p, and similarly for primes q of B. If q lies above p then we may regard the residue field B/q as a field extension of q: the kernel of the map A B B/q is p = A q, and the induced map A/p = A/(q A) B/q is a ring homomorphism of fields, hence injective. Definition Assume AKLB, and let p be a prime of A. The exponent e q in the factorization pb = q p q eq is the ramification index of q, and the degree f q = [B/q : A/p] of the corresponding residue field extension is the residue degree (or inertia degree) of q. In situations where more than one extension of Dedekind domains is under consideration, we may write e q/p for e q and f q/p for f q. Lemma Let A be a Dedekind domain with fraction field K, let M/L/K be a tower of finite separable extension, and let B and C be the integral closures of A in L and M respectively. Then C is the integral closure of B in M, and if r is a prime of M lying above a prime q of L lying above a prime p of K then e r/p = e r/q e q/p and f r/p = f r/q f q/p. Proof. It follows from Proposition 1.18 that the integral closure of B in M lies in C, and it contains C, since A B. We thus have a tower of Dedekind extensions C/B/A. If r q p then the factorization of pc in C refines the factorization of pb in B, so e r/p = e r/q e q/p, and the residue field embedding A/p C/r factors as A/p B/q C/r, so f r/p = f r/q f q/p. Example Let A = Z, with K = Frac A = Q, and let L = Q(i) with [L : K] = 2. The prime p = (5) factors in B = Z[i] into two distinct prime ideals: 5Z[i] = (2 + i)(2 i). q Fall 2016, Lecture #5, Page 8

9 The prime (2 + i) has ramification index e (2+i) = 1, and e (2 i) = 1 as well. The residue field Z/(5) is isomorphic to the finite field F 5, and we also have Z[i]/(2 + i) F 5 (this can be determined by counting the Z[i]-lattice points in a fundamental parallelogram of the sublattice (2 + i) in Z[i]), so f (2+i) = 1; we similarly have f (2 i) = 1. The p = (7) remains prime in B = Z[i]; its prime factorization is simply 7Z[i] = (7), where the (7) on the RHS denotes a principal ideal in B (this is clear from context). The ramification index of (7) is thus e (7) = 1, but its residue field degree is f (7) = 2, because Z/(7) F 7, but Z[i]/(7) F 49 has dimension 2 has an F 7 -vector space. The prime p = (2) factors as (2) = (1 + i) 2, since (1 + i) 2 = (1 + 2i 1) = (2i) = (2) (note that i is a unit). You might be thinking that (2) = (1 + i)(1 i) factors into distinct primes, but note that (1 + i) = i(1 + i) = (1 i). Thus e (1+i) = 2, and f (1+i) = 1 because Z/(2) F 2 Z[i]/(1 + i). Let us now compute the sum q p e q f q for each of the primes p we factored above: eq f q = e (1+i) f (1+i) = 2 1 = 2, q (2) e q f q = e (2+i) f (2+i) + e (2 i) f (2 i) = = 2, q (5) q (7) e q f q = e (7) f (7) = 2 1 = 2. In all three cases we obtain 2 = [Q(i) : Q]; as we shall shortly prove, this is not an accident. Example Let A = C[x], with K = Frac A = C(x), and let L = C( x) = Frac B, where B = C[x, y]/(y 2 x). Then [L : K] = 2. The prime p = (x 4) factors in B into two distinct prime ideals: (x 4) = (y 2 4) = (y + 2)(y 2). We thus have e (y+2) = 1, and f (y+2) = [B/(y + 2) : A/(x 4)] = [C : C] = 1. Similarly, e (y 2) = 1 and f (y 2) = 1. The prime p = x factors in B as (x) = (y 2 ) = (y) 2, and e (y) = 2 and f (y) = 1. As in the previous example, q p e qf q = [L : K] in both cases: eq f q = e (y+2) f (y+2) + e (y 2) f (y 2) = = 2, q (x 4) q (x) e q f q = e (y) f (y) = 2 1 = 2. Before proving that q p e qf q = [L : K] always holds, let us consider the quotient ring B/pB. The ring B/pB is in general not a field extension of A/p (because it is not necessarily a field), but it is an A/p-algebra, and in particular, an A/p-vector space. Let us determine its dimension Fall 2016, Lecture #5, Page 9

10 Lemma Assume AKLB and let p be a prime of A. The dimension of B/pB as an A/p-vector space is equal to the dimension of L as a K-vector space. Proof. Let S = A p, let A = S 1 A = A p and let B = S 1 B (note that S is closed under finite products, both as a subset of A and as a subset of B, so this makes sense). Then and A /pa = (S 1 A)/(pS 1 A) = A p /(pa p ) A/p, B /pb = S 1 B/pS 1 B B/pB, Thus if the lemma holds when A = A p is a DVR then it also holds for A; we now assume without loss of generality that A is a DVR. By Proposition 5.19, B is finitely generated as an A module, and as an integral domain containing A, it must be torsion free. It follows from the structure theorem for finitely generated modules over a PID that B is free of finite rank over A. By Proposition 5.15, B spans L as a K-vector space, so any A-basis for B is a K-basis for L. Conversely, after clearing denominators any K-basis for L yields an A-basis for B. It follows that B has rank [L : K] as a free A-module, and it follows that B/pB has rank [L : K] as a a free A/p-module (in fact an A/p vector space). To see this explicitly, let e = (e 1,..., e n ) be an A-basis for B. Its image ē in B/pB spans B/pB as an A/p-module, since the reduction map is surjective. If i a ie i pb for some a 1,..., a n A, then i a ie i = j b jp j for some b j = i a ije i B with a ij A and p j p. Each a i = a ij p j therefore lies in p, which shows that ē is A/p-linearly independent. It follows that ē is an A/p-basis for B/pB, which therefore has rank n = [L : K]. Theorem Assume AKLB. For each prime p of A we have eq f q = [L : K]. Proof. We have q p Applying the previous proposition gives B/pB B/q eq q p [L : K] = [B/pB : A/p] = [B/q eq : A/p] q p = e q [B/q : A/p] q p = e q f q. q p The second equality comes from the Chinese Remainder Theorem, and the third uses the fact that B/q eq has dimension e q as a B/q-vector space. Indeed, we have q eq = {x B : v q (x) e q }, and if π q is a uniformizer for B q (a generator qb q that we can force to lie in q by clearing denominators), the images of (π 0, π 1,..., π eq 1 ) in B/q eq are a B/q-basis for B/q eq Fall 2016, Lecture #5, Page 10

11 For each prime p of A, let g p := #{q p} denote the cardinality of the fiber above p. Corollary Assume AKLB and let p be a prime of A. Then g p is an integer in the interval [1, n], where n = [L : K], as are e q and f q for each q p. We now define some standard terminology that we may use in the AKLB setting to describe how a prime p of K splits in L (that is, for a nonzero prime ideal p of A, how the ideal pb factors into nonzero prime ideals q of B). Definition Assume AKLB, let p be a prime of A. L/K is totally ramified at q if e q = [L : K] (equivalently, f q = 1 = g p = 1). L/K is unramified at q if e q = 1 and B/q is a separable extension of A/p. L/K is unramified above p if it is unramified at all q p, equivalently, if B/pB is a finite étale algebra over A/p. When L/K is unramified above p we say that p remains inert in L if pb is prime (equivalently, e q = g p = 1, and f q = [L : k]). p splits completely in L if g p = [L : K] (equivalently, e q = f q = 1 for all q p). References [1] Allen Altman and Steven Kleiman, A term of commutative algebra, Worldwide Center of Mathematics, [2] Z.I. Borevich and I.R. Shafarevich, Number theory, Academic Press, [3] N. Bourbaki, Commutative Algebra: Chapters 1 7, Springer, [4] H. Matsumura, Commutative ring theory, Cambridge University Press, Fall 2016, Lecture #5, Page 11

### 5 Dedekind extensions

18.785 Number theory I Fall 2017 Lecture #5 09/20/2017 5 Dedekind extensions In this lecture we prove that the integral closure of a Dedekind domain in a finite extension of its fraction field is also

### 6 Ideal norms and the Dedekind-Kummer theorem

18.785 Number theory I Fall 2016 Lecture #6 09/27/2016 6 Ideal norms and the Dedekind-Kummer theorem Recall that for a ring extension B/A in which B is a free A-module of finite rank, we defined the (relative)

### 7 Orders in Dedekind domains, primes in Galois extensions

18.785 Number theory I Lecture #7 Fall 2015 10/01/2015 7 Orders in Dedekind domains, primes in Galois extensions 7.1 Orders in Dedekind domains Let S/R be an extension of rings. The conductor c of R (in

### (1) A frac = b : a, b A, b 0. We can define addition and multiplication of fractions as we normally would. a b + c d

The Algebraic Method 0.1. Integral Domains. Emmy Noether and others quickly realized that the classical algebraic number theory of Dedekind could be abstracted completely. In particular, rings of integers

### 12 Ramification, Haar measure, the product formula

18.785 Number theory I Lecture #12 Fall 2015 10/22/2015 12 Ramification, Haar measure, the product formula 12.1 Ramification in terms of the different and discriminant We conclude our discussion of the

### Algebraic Number Theory

TIFR VSRP Programme Project Report Algebraic Number Theory Milind Hegde Under the guidance of Prof. Sandeep Varma July 4, 2015 A C K N O W L E D G M E N T S I would like to express my thanks to TIFR for

### Thus, the integral closure A i of A in F i is a finitely generated (and torsion-free) A-module. It is not a priori clear if the A i s are locally

Math 248A. Discriminants and étale algebras Let A be a noetherian domain with fraction field F. Let B be an A-algebra that is finitely generated and torsion-free as an A-module with B also locally free

### Dedekind Domains. Mathematics 601

Dedekind Domains Mathematics 601 In this note we prove several facts about Dedekind domains that we will use in the course of proving the Riemann-Roch theorem. The main theorem shows that if K/F is a finite

### 1 Absolute values and discrete valuations

18.785 Number theory I Lecture #1 Fall 2015 09/10/2015 1 Absolute values and discrete valuations 1.1 Introduction At its core, number theory is the study of the ring Z and its fraction field Q. Many questions

### 8 Complete fields and valuation rings

18.785 Number theory I Fall 2017 Lecture #8 10/02/2017 8 Complete fields and valuation rings In order to make further progress in our investigation of finite extensions L/K of the fraction field K of a

### Introduction to Arithmetic Geometry Fall 2013 Lecture #18 11/07/2013

18.782 Introduction to Arithmetic Geometry Fall 2013 Lecture #18 11/07/2013 As usual, all the rings we consider are commutative rings with an identity element. 18.1 Regular local rings Consider a local

### Introduction to Arithmetic Geometry Fall 2013 Lecture #7 09/26/2013

18.782 Introduction to Arithmetic Geometry Fall 2013 Lecture #7 09/26/2013 In Lecture 6 we proved (most of) Ostrowski s theorem for number fields, and we saw the product formula for absolute values on

### n P say, then (X A Y ) P

COMMUTATIVE ALGEBRA 35 7.2. The Picard group of a ring. Definition. A line bundle over a ring A is a finitely generated projective A-module such that the rank function Spec A N is constant with value 1.

### These warmup exercises do not need to be written up or turned in.

18.785 Number Theory Fall 2017 Problem Set #3 Description These problems are related to the material in Lectures 5 7. Your solutions should be written up in latex and submitted as a pdf-file via e-mail

### ADVANCED COMMUTATIVE ALGEBRA: PROBLEM SETS

ADVANCED COMMUTATIVE ALGEBRA: PROBLEM SETS UZI VISHNE The 11 problem sets below were composed by Michael Schein, according to his course. Take into account that we are covering slightly different material.

### 4 Etale algebras, norm and trace

18.785 Number theory I Fall 2017 Lecture #4 09/18/2017 4 Etale algebras, norm and trace 4.1 Separability In this section we briefly review some standard facts about separable and inseparable field extensions

### Notes on p-divisible Groups

Notes on p-divisible Groups March 24, 2006 This is a note for the talk in STAGE in MIT. The content is basically following the paper [T]. 1 Preliminaries and Notations Notation 1.1. Let R be a complete

### IDEAL CLASSES AND RELATIVE INTEGERS

IDEAL CLASSES AND RELATIVE INTEGERS KEITH CONRAD The ring of integers of a number field is free as a Z-module. It is a module not just over Z, but also over any intermediate ring of integers. That is,

### Elliptic Curves Spring 2015 Lecture #7 02/26/2015

18.783 Elliptic Curves Spring 2015 Lecture #7 02/26/2015 7 Endomorphism rings 7.1 The n-torsion subgroup E[n] Now that we know the degree of the multiplication-by-n map, we can determine the structure

### ON THE ISOMORPHISM BETWEEN THE DUALIZING SHEAF AND THE CANONICAL SHEAF

ON THE ISOMORPHISM BETWEEN THE DUALIZING SHEAF AND THE CANONICAL SHEAF MATTHEW H. BAKER AND JÁNOS A. CSIRIK Abstract. We give a new proof of the isomorphism between the dualizing sheaf and the canonical

### Assigned homework problems S. L. Kleiman, fall 2008

18.705 Assigned homework problems S. L. Kleiman, fall 2008 Problem Set 1. Due 9/11 Problem R 1.5 Let ϕ: A B be a ring homomorphism. Prove that ϕ 1 takes prime ideals P of B to prime ideals of A. Prove

### 24 Artin reciprocity in the unramified case

18.785 Number theory I Fall 2017 ecture #24 11/29/2017 24 Artin reciprocity in the unramified case et be an abelian extension of number fields. In ecture 22 we defined the norm group T m := N (I m )R m

### Number Theory Fall 2016 Problem Set #6

18.785 Number Theory Fall 2016 Problem Set #6 Description These problems are related to the material covered in Lectures 10-12. Your solutions are to be written up in latex (you can use the latex source

### Honors Algebra 4, MATH 371 Winter 2010 Assignment 4 Due Wednesday, February 17 at 08:35

Honors Algebra 4, MATH 371 Winter 2010 Assignment 4 Due Wednesday, February 17 at 08:35 1. Let R be a commutative ring with 1 0. (a) Prove that the nilradical of R is equal to the intersection of the prime

### ALGEBRA HW 4. M 0 is an exact sequence of R-modules, then M is Noetherian if and only if M and M are.

ALGEBRA HW 4 CLAY SHONKWILER (a): Show that if 0 M f M g M 0 is an exact sequence of R-modules, then M is Noetherian if and only if M and M are. Proof. ( ) Suppose M is Noetherian. Then M injects into

### Number Theory Fall 2016 Problem Set #3

18.785 Number Theory Fall 2016 Problem Set #3 Description These problems are related to the material covered in Lectures 5-7. Your solutions are to be written up in latex and submitted as a pdf-file via

### ABSTRACT NONSINGULAR CURVES

ABSTRACT NONSINGULAR CURVES Affine Varieties Notation. Let k be a field, such as the rational numbers Q or the complex numbers C. We call affine n-space the collection A n k of points P = a 1, a,..., a

### Problem 1. Characterizing Dedekind domains (64 points)

18.785 Number Theory I Fall 2017 Problem Set #2 Description These problems are related to the material covered in Lectures 3 5. Your solutions are to be written up in latex (you can use the latex source

### Injective Modules and Matlis Duality

Appendix A Injective Modules and Matlis Duality Notes on 24 Hours of Local Cohomology William D. Taylor We take R to be a commutative ring, and will discuss the theory of injective R-modules. The following

### Math 762 Spring h Y (Z 1 ) (1) h X (Z 2 ) h X (Z 1 ) Φ Z 1. h Y (Z 2 )

Math 762 Spring 2016 Homework 3 Drew Armstrong Problem 1. Yoneda s Lemma. We have seen that the bifunctor Hom C (, ) : C C Set is analogous to a bilinear form on a K-vector space, : V V K. Recall that

### ALGEBRA QUALIFYING EXAM PROBLEMS LINEAR ALGEBRA

ALGEBRA QUALIFYING EXAM PROBLEMS LINEAR ALGEBRA Kent State University Department of Mathematical Sciences Compiled and Maintained by Donald L. White Version: August 29, 2017 CONTENTS LINEAR ALGEBRA AND

### 11. Finitely-generated modules

11. Finitely-generated modules 11.1 Free modules 11.2 Finitely-generated modules over domains 11.3 PIDs are UFDs 11.4 Structure theorem, again 11.5 Recovering the earlier structure theorem 11.6 Submodules

### A BRIEF INTRODUCTION TO LOCAL FIELDS

A BRIEF INTRODUCTION TO LOCAL FIELDS TOM WESTON The purpose of these notes is to give a survey of the basic Galois theory of local fields and number fields. We cover much of the same material as [2, Chapters

### Math 121 Homework 5: Notes on Selected Problems

Math 121 Homework 5: Notes on Selected Problems 12.1.2. Let M be a module over the integral domain R. (a) Assume that M has rank n and that x 1,..., x n is any maximal set of linearly independent elements

### NOTES ON FINITE FIELDS

NOTES ON FINITE FIELDS AARON LANDESMAN CONTENTS 1. Introduction to finite fields 2 2. Definition and constructions of fields 3 2.1. The definition of a field 3 2.2. Constructing field extensions by adjoining

### 4.4 Noetherian Rings

4.4 Noetherian Rings Recall that a ring A is Noetherian if it satisfies the following three equivalent conditions: (1) Every nonempty set of ideals of A has a maximal element (the maximal condition); (2)

### MATH 8254 ALGEBRAIC GEOMETRY HOMEWORK 1

MATH 8254 ALGEBRAIC GEOMETRY HOMEWORK 1 CİHAN BAHRAN I discussed several of the problems here with Cheuk Yu Mak and Chen Wan. 4.1.12. Let X be a normal and proper algebraic variety over a field k. Show

### MINKOWSKI THEORY AND THE CLASS NUMBER

MINKOWSKI THEORY AND THE CLASS NUMBER BROOKE ULLERY Abstract. This paper gives a basic introduction to Minkowski Theory and the class group, leading up to a proof that the class number (the order of the

### Norm Groups with Tame Ramification

ECTURE 3 orm Groups with Tame Ramification et K be a field with char(k) 6=. Then K /(K ) ' {continuous homomorphisms Gal(K) Z/Z} ' {degree étale algebras over K} which is dual to our original statement

### Homework 2 - Math 603 Fall 05 Solutions

Homework 2 - Math 603 Fall 05 Solutions 1. (a): In the notation of Atiyah-Macdonald, Prop. 5.17, we have B n j=1 Av j. Since A is Noetherian, this implies that B is f.g. as an A-module. (b): By Noether

### Raynaud on F -vector schemes and prolongation

Raynaud on F -vector schemes and prolongation Melanie Matchett Wood November 7, 2010 1 Introduction and Motivation Given a finite, flat commutative group scheme G killed by p over R of mixed characteristic

### Algebra Homework, Edition 2 9 September 2010

Algebra Homework, Edition 2 9 September 2010 Problem 6. (1) Let I and J be ideals of a commutative ring R with I + J = R. Prove that IJ = I J. (2) Let I, J, and K be ideals of a principal ideal domain.

### Algebraic Geometry Spring 2009

MIT OpenCourseWare http://ocw.mit.edu 18.726 Algebraic Geometry Spring 2009 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms. 18.726: Algebraic Geometry

### NOTES ON LINEAR ALGEBRA OVER INTEGRAL DOMAINS. Contents. 1. Introduction 1 2. Rank and basis 1 3. The set of linear maps 4. 1.

NOTES ON LINEAR ALGEBRA OVER INTEGRAL DOMAINS Contents 1. Introduction 1 2. Rank and basis 1 3. The set of linear maps 4 1. Introduction These notes establish some basic results about linear algebra over

### Integral Extensions. Chapter Integral Elements Definitions and Comments Lemma

Chapter 2 Integral Extensions 2.1 Integral Elements 2.1.1 Definitions and Comments Let R be a subring of the ring S, and let α S. We say that α is integral over R if α isarootofamonic polynomial with coefficients

### ALGEBRA EXERCISES, PhD EXAMINATION LEVEL

ALGEBRA EXERCISES, PhD EXAMINATION LEVEL 1. Suppose that G is a finite group. (a) Prove that if G is nilpotent, and H is any proper subgroup, then H is a proper subgroup of its normalizer. (b) Use (a)

### DIVISORS ON NONSINGULAR CURVES

DIVISORS ON NONSINGULAR CURVES BRIAN OSSERMAN We now begin a closer study of the behavior of projective nonsingular curves, and morphisms between them, as well as to projective space. To this end, we introduce

### Math 711: Lecture of September 7, Symbolic powers

Math 711: Lecture of September 7, 2007 Symbolic powers We want to make a number of comments about the behavior of symbolic powers of prime ideals in Noetherian rings, and to give at least one example of

Algebra Qualifying Exam August 2001 Do all 5 problems. 1. Let G be afinite group of order 504 = 23 32 7. a. Show that G cannot be isomorphic to a subgroup of the alternating group Alt 7. (5 points) b.

### FILTERED RINGS AND MODULES. GRADINGS AND COMPLETIONS.

FILTERED RINGS AND MODULES. GRADINGS AND COMPLETIONS. Let A be a ring, for simplicity assumed commutative. A filtering, or filtration, of an A module M means a descending sequence of submodules M = M 0

### ALGEBRAIC GEOMETRY COURSE NOTES, LECTURE 2: HILBERT S NULLSTELLENSATZ.

ALGEBRAIC GEOMETRY COURSE NOTES, LECTURE 2: HILBERT S NULLSTELLENSATZ. ANDREW SALCH 1. Hilbert s Nullstellensatz. The last lecture left off with the claim that, if J k[x 1,..., x n ] is an ideal, then

### 18.785: Algebraic Number Theory

18.785: Algebraic Number Theory (lecture notes) Taught by Bjorn Poonen Fall 2014, MIT Last updated: January 17, 2015 1 Disclaimer These are my notes from Prof. Poonen s course on algebraic number theory,

### ORAL QUALIFYING EXAM QUESTIONS. 1. Algebra

ORAL QUALIFYING EXAM QUESTIONS JOHN VOIGHT Below are some questions that I have asked on oral qualifying exams (starting in fall 2015). 1.1. Core questions. 1. Algebra (1) Let R be a noetherian (commutative)

### Reid 5.2. Describe the irreducible components of V (J) for J = (y 2 x 4, x 2 2x 3 x 2 y + 2xy + y 2 y) in k[x, y, z]. Here k is algebraically closed.

Reid 5.2. Describe the irreducible components of V (J) for J = (y 2 x 4, x 2 2x 3 x 2 y + 2xy + y 2 y) in k[x, y, z]. Here k is algebraically closed. Answer: Note that the first generator factors as (y

### Algebraic structures I

MTH5100 Assignment 1-10 Algebraic structures I For handing in on various dates January March 2011 1 FUNCTIONS. Say which of the following rules successfully define functions, giving reasons. For each one

### INJECTIVE MODULES: PREPARATORY MATERIAL FOR THE SNOWBIRD SUMMER SCHOOL ON COMMUTATIVE ALGEBRA

INJECTIVE MODULES: PREPARATORY MATERIAL FOR THE SNOWBIRD SUMMER SCHOOL ON COMMUTATIVE ALGEBRA These notes are intended to give the reader an idea what injective modules are, where they show up, and, to

### Chapter 3. Rings. The basic commutative rings in mathematics are the integers Z, the. Examples

Chapter 3 Rings Rings are additive abelian groups with a second operation called multiplication. The connection between the two operations is provided by the distributive law. Assuming the results of Chapter

### 1 Flat, Smooth, Unramified, and Étale Morphisms

1 Flat, Smooth, Unramified, and Étale Morphisms 1.1 Flat morphisms Definition 1.1. An A-module M is flat if the (right-exact) functor A M is exact. It is faithfully flat if a complex of A-modules P N Q

Graduate Preliminary Examination Algebra II 18.2.2005: 3 hours Problem 1. Prove or give a counter-example to the following statement: If M/L and L/K are algebraic extensions of fields, then M/K is algebraic.

### Homework 4 Solutions

Homework 4 Solutions November 11, 2016 You were asked to do problems 3,4,7,9,10 in Chapter 7 of Lang. Problem 3. Let A be an integral domain, integrally closed in its field of fractions K. Let L be a finite

### ALGEBRA HW 3 CLAY SHONKWILER

ALGEBRA HW 3 CLAY SHONKWILER (a): Show that R[x] is a flat R-module. 1 Proof. Consider the set A = {1, x, x 2,...}. Then certainly A generates R[x] as an R-module. Suppose there is some finite linear combination

### HARTSHORNE EXERCISES

HARTSHORNE EXERCISES J. WARNER Hartshorne, Exercise I.5.6. Blowing Up Curve Singularities (a) Let Y be the cusp x 3 = y 2 + x 4 + y 4 or the node xy = x 6 + y 6. Show that the curve Ỹ obtained by blowing

### 9. Integral Ring Extensions

80 Andreas Gathmann 9. Integral ing Extensions In this chapter we want to discuss a concept in commutative algebra that has its original motivation in algebra, but turns out to have surprisingly many applications

### Algebraic number theory, Michaelmas 2015

Algebraic number theory, Michaelmas 2015 January 22, 2016 Contents 1 Discrete valuation rings 2 2 Extensions of Dedekind domains 7 3 Complete discrete valuation rings 8 4 The p-adic numbers 10 5 Extensions

### SUMMARY ALGEBRA I LOUIS-PHILIPPE THIBAULT

SUMMARY ALGEBRA I LOUIS-PHILIPPE THIBAULT Contents 1. Group Theory 1 1.1. Basic Notions 1 1.2. Isomorphism Theorems 2 1.3. Jordan- Holder Theorem 2 1.4. Symmetric Group 3 1.5. Group action on Sets 3 1.6.

### CRing Project, Chapter 5

Contents 5 Noetherian rings and modules 3 1 Basics........................................... 3 1.1 The noetherian condition............................ 3 1.2 Stability properties................................

### MATH 8253 ALGEBRAIC GEOMETRY WEEK 12

MATH 8253 ALGEBRAIC GEOMETRY WEEK 2 CİHAN BAHRAN 3.2.. Let Y be a Noetherian scheme. Show that any Y -scheme X of finite type is Noetherian. Moreover, if Y is of finite dimension, then so is X. Write f

### The most important result in this section is undoubtedly the following theorem.

28 COMMUTATIVE ALGEBRA 6.4. Examples of Noetherian rings. So far the only rings we can easily prove are Noetherian are principal ideal domains, like Z and k[x], or finite. Our goal now is to develop theorems

### Math 210B. Artin Rees and completions

Math 210B. Artin Rees and completions 1. Definitions and an example Let A be a ring, I an ideal, and M an A-module. In class we defined the I-adic completion of M to be M = lim M/I n M. We will soon show

### ALGEBRA QUALIFYING EXAM SPRING 2012

ALGEBRA QUALIFYING EXAM SPRING 2012 Work all of the problems. Justify the statements in your solutions by reference to specific results, as appropriate. Partial credit is awarded for partial solutions.

### CHAPTER 0 PRELIMINARY MATERIAL. Paul Vojta. University of California, Berkeley. 18 February 1998

CHAPTER 0 PRELIMINARY MATERIAL Paul Vojta University of California, Berkeley 18 February 1998 This chapter gives some preliminary material on number theory and algebraic geometry. Section 1 gives basic

### where Σ is a finite discrete Gal(K sep /K)-set unramified along U and F s is a finite Gal(k(s) sep /k(s))-subset

Classification of quasi-finite étale separated schemes As we saw in lecture, Zariski s Main Theorem provides a very visual picture of quasi-finite étale separated schemes X over a henselian local ring

### Topics in linear algebra

Chapter 6 Topics in linear algebra 6.1 Change of basis I want to remind you of one of the basic ideas in linear algebra: change of basis. Let F be a field, V and W be finite dimensional vector spaces over

### REPRESENTATION THEORY, LECTURE 0. BASICS

REPRESENTATION THEORY, LECTURE 0. BASICS IVAN LOSEV Introduction The aim of this lecture is to recall some standard basic things about the representation theory of finite dimensional algebras and finite

### Lecture 2. (1) Every P L A (M) has a maximal element, (2) Every ascending chain of submodules stabilizes (ACC).

Lecture 2 1. Noetherian and Artinian rings and modules Let A be a commutative ring with identity, A M a module, and φ : M N an A-linear map. Then ker φ = {m M : φ(m) = 0} is a submodule of M and im φ is

### Introduction to modules

Chapter 3 Introduction to modules 3.1 Modules, submodules and homomorphisms The problem of classifying all rings is much too general to ever hope for an answer. But one of the most important tools available

### Structure of rings. Chapter Algebras

Chapter 5 Structure of rings 5.1 Algebras It is time to introduce the notion of an algebra over a commutative ring. So let R be a commutative ring. An R-algebra is a ring A (unital as always) together

### Introduction to Arithmetic Geometry Fall 2013 Lecture #17 11/05/2013

18.782 Introduction to Arithmetic Geometry Fall 2013 Lecture #17 11/05/2013 Throughout this lecture k denotes an algebraically closed field. 17.1 Tangent spaces and hypersurfaces For any polynomial f k[x

### Projective and Injective Modules

Projective and Injective Modules Push-outs and Pull-backs. Proposition. Let P be an R-module. The following conditions are equivalent: (1) P is projective. (2) Hom R (P, ) is an exact functor. (3) Every

### Math 120 HW 9 Solutions

Math 120 HW 9 Solutions June 8, 2018 Question 1 Write down a ring homomorphism (no proof required) f from R = Z[ 11] = {a + b 11 a, b Z} to S = Z/35Z. The main difficulty is to find an element x Z/35Z

### Elliptic Curves Spring 2013 Lecture #14 04/02/2013

18.783 Elliptic Curves Spring 2013 Lecture #14 04/02/2013 A key ingredient to improving the efficiency of elliptic curve primality proving (and many other algorithms) is the ability to directly construct

### THROUGH THE FIELDS AND FAR AWAY

THROUGH THE FIELDS AND FAR AWAY JONATHAN TAYLOR I d like to thank Prof. Stephen Donkin for helping me come up with the topic of my project and also guiding me through its various complications. Contents

### 11. Dimension. 96 Andreas Gathmann

96 Andreas Gathmann 11. Dimension We have already met several situations in this course in which it seemed to be desirable to have a notion of dimension (of a variety, or more generally of a ring): for

### Rings and groups. Ya. Sysak

Rings and groups. Ya. Sysak 1 Noetherian rings Let R be a ring. A (right) R -module M is called noetherian if it satisfies the maximum condition for its submodules. In other words, if M 1... M i M i+1...

### Math 145. Codimension

Math 145. Codimension 1. Main result and some interesting examples In class we have seen that the dimension theory of an affine variety (irreducible!) is linked to the structure of the function field in

### Theorem 5.3. Let E/F, E = F (u), be a simple field extension. Then u is algebraic if and only if E/F is finite. In this case, [E : F ] = deg f u.

5. Fields 5.1. Field extensions. Let F E be a subfield of the field E. We also describe this situation by saying that E is an extension field of F, and we write E/F to express this fact. If E/F is a field

### Course 311: Michaelmas Term 2005 Part III: Topics in Commutative Algebra

Course 311: Michaelmas Term 2005 Part III: Topics in Commutative Algebra D. R. Wilkins Contents 3 Topics in Commutative Algebra 2 3.1 Rings and Fields......................... 2 3.2 Ideals...............................

### Inverse Galois Problem for Totally Real Number Fields

Cornell University Mathematics Department Senior Thesis Inverse Galois Problem for Totally Real Number Fields Sudesh Kalyanswamy Class of 2012 April, 2012 Advisor: Prof. Ravi Ramakrishna, Department of

### AN ALTERNATIVE APPROACH TO SERRE DUALITY FOR PROJECTIVE VARIETIES

AN ALTERNATIVE APPROACH TO SERRE DUALITY FOR PROJECTIVE VARIETIES MATTHEW H. BAKER AND JÁNOS A. CSIRIK This paper was written in conjunction with R. Hartshorne s Spring 1996 Algebraic Geometry course at

### LECTURE 2. Hilbert Symbols

LECTURE 2 Hilbert Symbols Let be a local field over Q p (though any local field suffices) with char() 2. Note that this includes fields over Q 2, since it is the characteristic of the field, and not the

### RINGS: SUMMARY OF MATERIAL

RINGS: SUMMARY OF MATERIAL BRIAN OSSERMAN This is a summary of terms used and main results proved in the subject of rings, from Chapters 11-13 of Artin. Definitions not included here may be considered

### 3.1. Derivations. Let A be a commutative k-algebra. Let M be a left A-module. A derivation of A in M is a linear map D : A M such that

ALGEBRAIC GROUPS 33 3. Lie algebras Now we introduce the Lie algebra of an algebraic group. First, we need to do some more algebraic geometry to understand the tangent space to an algebraic variety at

### Math 210B:Algebra, Homework 2

Math 210B:Algebra, Homework 2 Ian Coley January 21, 2014 Problem 1. Is f = 2X 5 6X + 6 irreducible in Z[X], (S 1 Z)[X], for S = {2 n, n 0}, Q[X], R[X], C[X]? To begin, note that 2 divides all coefficients

### Introduction to Arithmetic Geometry Fall 2013 Lecture #15 10/29/2013

18.782 Introduction to Arithmetic Geometry Fall 2013 Lecture #15 10/29/2013 As usual, k is a perfect field and k is a fixed algebraic closure of k. Recall that an affine (resp. projective) variety is an

### 3 Properties of Dedekind domains

18.785 Number theory I Fall 2016 Lecture #3 09/15/2016 3 Proerties of Dedekind domains In the revious lecture we defined a Dedekind domain as a noetherian domain A that satisfies either of the following

### COURSE SUMMARY FOR MATH 504, FALL QUARTER : MODERN ALGEBRA

COURSE SUMMARY FOR MATH 504, FALL QUARTER 2017-8: MODERN ALGEBRA JAROD ALPER Week 1, Sept 27, 29: Introduction to Groups Lecture 1: Introduction to groups. Defined a group and discussed basic properties

### 11 Annihilators. Suppose that R, S, and T are rings, that R P S, S Q T, and R U T are bimodules, and finally, that

11 Annihilators. In this Section we take a brief look at the important notion of annihilators. Although we shall use these in only very limited contexts, we will give a fairly general initial treatment,

### Extension theorems for homomorphisms

Algebraic Geometry Fall 2009 Extension theorems for homomorphisms In this note, we prove some extension theorems for homomorphisms from rings to algebraically closed fields. The prototype is the following