# IDEAL CLASSES AND RELATIVE INTEGERS

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1 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, if E Q we can consider O E as an O -module. Since O E is finitely generated over Z, it is also finitely generated over O (just a larger ring of scalars), but O E may or may not have a basis over O. When we treat O E as a module over O, rather than over Z, we speak about a relative extension of integers. If O is a PID then O E will be a free O -module, so O E will have a basis over O. Such a basis is called a relative integral basis for E over. The next three examples illustrate some possibilities when O is not a PID. Example 1. Let = Q( 5) and E = Q(i, 5) = (i). Although O = Z[ 5] is not a PID, O E is a free O -module with relative integral basis {1, i+ 5 }. Example. Let = Q( 15) and E = Q( 15, 6) = ( 6). Then h( ) =, so O is not a PID, but it turns out that O E = O O 6, so OE is a free O -module. Example 3. Let = Q( 6) and E = Q( 6, 3) = ( 3). Then h( ) =, so O is not a PID, and it turns out that (1) O E = O e 1 pe, where e 1 = 1+ 3, e = 1 3, and p = (3, 6). (Although e is not in O E, there isn t a problem with the direct sum decomposition (1) for O E over O since the coefficients of e run not over O but over the ideal p, which doesn t include 1, so e pe.) Equation (1) says that as an O -module, O E p. The ideal p is not principal and this suggests O E is not a free O -module, although that does require an argument. To reinforce this point, p p does not look like a free O -module, since p is not principal, but p p has a second direct sum decomposition that admits an O -basis, so a direct sum of two non-free modules can be free. We will see how in Example 9 below. What we are after is a classification of finitely generated torsion-free modules over a Dedekind domain, which will then be applied in the number field setting to describe O E as an O -module. The extent to which O E could fail to have an O -basis will be related to ideal classes in. A technical concept we need to describe modules over a Dedekind domain is projective modules. Definition 4. Let A be any commutative ring. An A-module P is called projective if every surjective linear map f : M P from any A-module M onto P looks like a projection out of a direct sum: there is an isomorphism h: M = P N for some A-module N such that h(m) = (f(m), ) for all m M. The isomorphism h is not unique. or example, taking A = Z, P = Z, and M = Z Z with f(a, b) = a b, we can use h: M P Z by h(a, b) = (a b, b) or h(a, b) = 1

2 KEITH CONRAD (a b, a b). Each of these works since the first coordinate of h(a, b) is f(a, b) and h is obviously invertible. The complementary summand N in the definition of a projective module is isomorphic to the kernel of f. Indeed, the condition h(m) = (f(m), ) means f(m) = 0 if and only if h(m) is in {0} N, which means h restricts to an isomorphism between ker f and {0} N = N. It is easy to give examples of non-projective modules. or instance, if P is a projective A-module with n generators there is a surjective A-linear map A n P, so A n = P Q for some A-module Q. When A is a domain, any submodule of A n is torsion-free, so a finitely generated projective module over a domain is torsion-free. Therefore a finitely generated module over a domain that has torsion is not projective: Z Z/() is not a projective Z-module. More importantly for us, though, is that fractional ideals in a Dedekind domain are projective modules. Lemma 5. or a domain A, any invertible fractional A-ideal is a projective A-module. In particular, when A is a Dedekind domain all fractional A-ideals are projective A-modules. Proof. Let a be an invertible fractional A-ideal. Then k x iy i = 1 for some x i a and y i a 1. or each x a, x = 1 x = x 1 (x 1x) + + x k (x k x) and x i x a 1 a = A, so a k Ax i a, so a = Ax Ax k. In a similar way, a 1 = Ay Ay k. Suppose f : M a is a surjective A-linear map. Choose m i M such that f(m i ) = x i. Define g : a M by g(x) = k (xy i)m i. Note xy i aa 1 = A for all i, so g(x) makes sense and g is A-linear. Then f(g(x)) = k (xy i )f(m i ) = n (xy i )x i = x n x i y i = x. Check the A-linear map h: M a ker f given by the formula h(m) = (f(m), m g(f(m))) has inverse (x, y) g(x) + y. Here is the main structure theorem. Theorem 6. Every finitely generated torsion-free module over a Dedekind domain A is isomorphic to a direct sum of ideals in A. Proof. Let M be a finitely generated torsion-free A-module. We can assume M 0 and will show there is an embedding M A d for some d 1 such that the image of M intersects each standard coordinate axis of A d. Let be the fraction field of A and x 1,..., x n be a generating set for M as an A-module. We will show n is an upper bound on the size of any A-linearly independent subset of M. Let f : A n M be the linear map where f(e i ) = x i for all i. (By e 1,..., e n we mean the standard basis of A n.) Let y 1,..., y k be linearly independent in M, so their A-span is isomorphic to A k. Write y j = n a ijx i with a ij A. We pull the y j s back to A n by setting v j = (a 1j,..., a nj ), so f(v j ) = y j. A linear dependence relation on the v j s is transformed by f into a linear dependence relation on the y j s, which is a trivial relation by their linear independence. Therefore v 1,..., v k is A-linearly independent in A n, hence -linearly independent in n. By linear algebra over fields, k n. rom the bound k n, there is a linearly independent subset of M with maximal size, say t 1,..., t d. Then d j=1 At j = A d by identifying t j with the jth standard basis

3 IDEAL CLASSES AND RELATIVE INTEGERS 3 vector in A d. We will find a scalar multiple of M inside d j=1 At j. or any x M, the set {x, t 1,..., t d } is linearly dependent by maximality of d, so there is a nontrivial linear relation a x x + d a it i = 0, necessarily with a x 0 in A. Thus a x x d j=1 At j. Letting x run through the spanning set x 1,..., x n, we have ax i d j=1 At j for all i where a = a x1 a xn 0. Thus am d j=1 At j. Multiplying by a is an isomorphism of M with am, so we have the sequence of A-linear maps M am d At j A d, j=1 where the first and last maps are A-module isomorphisms. In the above composite map, t j M is mapped to ae j in A d, so this composite map is an embedding M A d such that M meets each standard coordinate axis of A d in a nonzero vector. Compose this linear map with projection A d A onto the last coordinate in the standard basis: a 1 e a d e d a d. Denote the restriction of this to a map M A as ϕ, so a := ϕ(m) is a nonzero ideal in A. With ϕ we get a surjective map M a, so Lemma 5 (the first time we need A to be a Dedekind domain, not just an integral domain) tells us M = a ker ϕ. Obviously ker ϕ A d 1 0 = A d 1, so ker ϕ is a finitely generated (and torsion-free) A-module with at most d 1 A-linearly independent elements. Using induction on the largest number of linearly independent elements in the module, ker ϕ is a direct sum of ideals in A. Remark 7. Using equations rather than isomorphisms, Theorem 6 says M = M 1 M d where each M i is isomorphic to an ideal in A. Those ideals need not be principal, so M i need not have the form Am i. If M is inside a vector space over the fraction field of A, then M = d a ie i for some linearly independent e i s, but be careful: if a i is a proper ideal in A then e i is not in M since 1 a i. The e i s are not a spanning set for M as a module since their coefficients are not running through A. The decomposition of the integers of Q( 6, 3) as a module over Z[ 6] in Example 3 illustrates this point. How much does a direct sum a 1 a d, as a module, depend on the individual a i s? Lemma 8. Let A be a Dedekind domain. A-module isomorphism a b = A ab. or fractional A-ideals a and b, there is an Proof. Both sides of the isomorphism are unchanged up to A-module isomorphism when we scale a and b, so without loss of generality a and b are nonzero ideals in A. We can further scale so a and b are relatively prime. Indeed, let a 1 a 0 where a 0 A. Using the Chinese remainder theorem in A, there is a nonzero ideal c such that a 0 c is principal and gcd(c, b) = (1). Since c a 1 0 a, we can replace a by c without changing a b or A ab up to A-module isomorphism. The linear map f : a b a + b = A given by f(a, b) = a b is surjective and ker f = {(a, a) : a a b} = a b, which is ab since gcd(a, b) = (1). Applying Lemma 5 to the fractional A-ideal A, a b = A ker f = A ab. Example 9. or A = Z[ 5], let p = (, 1 + 5), so p is not principal but p = A is principal. Then there is an A-module isomorphism p p = A p = A A. That is intriguing: p does not have an A-basis but p p does! Working through the proof of

4 4 KEITH CONRAD Lemma 8 will show you how to write down a basis of p p explicitly. In a similar way, p p in Example 3 is a free Z[ 6]-module since p is principal. Theorem 10. Let A be a Dedekind domain. or fractional A-ideals a 1,..., a d, there is an A-module isomorphism a 1 a d = A d 1 a 1 a d. Proof. Induct on d and use Lemma 8. Corollary 11. Let E/ be a finite extension of number fields with [E : ] = n. As an O -module, O E n 1 a for some nonzero ideal a in O. Proof. Since O E is a finitely generated Z-module it is a finitely generated O -module and obviously has no torsion, so Theorems 6 and 10 imply O E d 1 a for some d 1 and nonzero ideal a in O. Letting m = [ : Q], both O and a are free of rank m over Z, while O E is free of rank mn over Z. Computing the rank of O E and O d 1 a over Z, mn = m(d 1) + m = md, so d = n. Thus O E is almost a free O -module. If a is principal then O E is free. As an O -module up to isomorphism, O n 1 a only depends on a through its ideal class, since a and any xa (x ) are isomorphic O -modules. Does O n 1 a, as an O -module, depend on a exactly through its ideal class? That is, if O n 1 a n 1 b as O -modules, does [a] = [b] in Cl( )? The next two theorems together say the answer is yes. Theorem 1. Let A be a domain with fraction field. or fractional A-ideals a and b in, a = b as A-modules if and only if a = xb for some x. Here is any field, not necessarily a number field. Proof. ( ): Multiplication by x is an A-module isomorphism from b to a. ( ): Suppose f : a b is an A-module isomorphism. We want an x such that f(t) = xt for all t a. or this to be possible, f(t)/t has to be independent of the choice of nonzero t. Then we could define x to be this common ratio, so f(t) = xt for all t in a (including t = 0). or any nonzero t 1 and t in a, f(t 1 )? f(t ) = t f(t 1 ) =? t 1 f(t ). t 1 t You may be tempted to pull the t and t 1 inside on the right, confirming the equality, but that is bogus because f is A-linear and we don t know if t 1 and t are in A (they are just in ). This is easy to fix. Since a is a fractional A-ideal, it has a denominator: da A for some nonzero d A. Then dt 1, dt A, so t f(t 1 )? = t 1 f(t ) dt f(t 1 )? = dt 1 f(t ) f(dt t 1 ) = f(dt 1 t ). Theorem 13. or nonzero ideals a 1,..., a m and b 1,..., b n in a Dedekind domain A,we have a 1 a m = b1 b n as A-modules if and only if m = n and [a 1 a m ] = [b 1 b n ] in Cl(A). Proof. The if direction follows from Theorems 10 and 1: a 1 a m = A m 1 a 1 a m, so if m = n and [a 1 a m ] = [b 1 b m ] in Cl(A) then A m 1 a 1 a m = A m 1 b 1 b m = b1 b m.

5 IDEAL CLASSES AND RELATIVE INTEGERS 5 Turning to the only if direction, we show m is determined by the A-module structure of a 1 a m : it is the largest number of A-linearly independent elements in this module. Picking a nonzero a i a i, the m-tuples (..., 0, a i, 0,... ) for 1 i m are easily A-linearly independent, so m a i has m linearly independent members. Since a 1 a m m, where is the fraction field of A, any set of more than m members of a 1 a m has a nontrivial -linear relation in m, which can be scaled to a nontrivial A-linear relation in m a i by clearing a common denominator in the coefficients. Therefore if a 1 a m = b 1 b n as A-modules we must have m = n by computing the maximal number of A-linearly independent elements in both modules. To show m a i = m b i a 1 a m and b 1 b m are scalar multiples, we can collect ideals by multiplication into the last summands: it is enough to show A m 1 a = A m 1 b a and b are scalar multiples. Let ϕ: A m 1 a A m 1 b be an A-module isomorphism. Viewing A m 1 a and A m 1 b as column vectors of length m with the last coordinate in a or b, ϕ can be represented as an m m matrix of A-linear maps (ϕ ij ), where ϕ ij has domain A or a and target A or b. The proof of Theorem 1 shows any A-linear map from one fractional A-ideal to another (not necessarily injective or surjective) is a scaling function. Therefore ϕ is described by an m m matrix of numbers, say M, acting in the usual way on column vectors. or any α a, let D α = diag(1,..., 1, α) be the diagonal m m matrix with α in the lower right entry. Then D α (A m 1 A) A m 1 a, so MD α maps A m 1 A to A m 1 b. This means the bottom row of MD α has all entries in b, so det(md α ) b. Since D α has determinant α and α is arbitrary in a, det(m)a b. In the same way, det(m 1 )b a, so det(m)a = b. Example 14. Let s return to Example 3: = Q( 6), E = ( 3), and O E p, where p = (3, 6). We can show O E is not a free O -module: if it were free then O E, so O p O as O -modules. Then Theorem 13 implies p as O -modules, so p is principal, but p is nonprincipal. This is a contradiction. We can now associate to any finite extension of number fields E/ a canonical ideal class in Cl( ), namely [a] where O E n 1 a as O -modules. Theorem 13 assures us [a] is well-defined. Since the construction of [a] is due to Steinitz (191), [a] is called the Steinitz class of E/. Example 15. By Example 14, when = Q( 6) the nontrivial member of Cl( ) is the Steinitz class of the quadratic extension ( 3)/. Since the ideal class group of a number field is finite, as E varies over all extensions of with a fixed degree n the different O E s have finitely many possible O -module structures, in fact at most h( ) of them. There are infinitely many nonisomorphic extensions of with degree n, so it is natural to ask if each ideal class is realized among them: for any [a] Cl( ) and integer n is there some extension E/ of degree n whose Steinitz class is [a], i.e., O E n 1 a as O -modules? The answer is yes for n = and n = 3, and in fact Kable and Wright [1] have proved an equidistribution of the E s with degree or 3 over in terms of their Steinitz classes in Cl( ). In particular, each ideal class in Cl( ) is a Steinitz class for infinitely many nonisomorphic degree n extensions of when n = or 3. The extension to general degrees is still an open problem in general. We have focused on the description of a single finitely generated torsion-free module over a Dedekind domain. What if we want to compare such a module and a submodule? If A

6 6 KEITH CONRAD is a PID, M is a finite free A-module and M is a submodule then we can align M and M in the sense that there is a basis e 1,..., e n of M and nonzero scalars a 1,..., a m in A (where m n) such that M = n Ae i and M = m j=1 Ae j. This has an analogue over Dedekind domains, but it doesn t use bases. If A is a Dedekind domain, M is a finitely generated torsion-free A-module, and M is a submodule of M, then we can align M and M in the sense that we can write n m M = M i and M = a j M j where m n, each M i is isomorphic to a nonzero ideal in A, and each a j is a nonzero ideal in A. It is generally false that such an alignment is compatible with an isomorphism M = A n 1 a. That is, such an isomorphism need not restrict to M to give an isomorphism M = A m 1 a with a a, even for A = Z. Consider, for instance, M = Z Z and M = az az for a > 1 (so m = n = ). We can t write M = Ze 1 Ze and M = Ze 1 bze for some integer b since that would imply M/M = Z/bZ is cyclic, whereas M/M = (Z/aZ) is not cyclic. References [1] A. C. Kable and D. J. Wright, Uniform distribution of the Steinitz invariants of quadratic and cubic extensions, Compositio Math. 14 (006), j=1

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