LECTURE Affine Space & the Zariski Topology. It is easy to check that Z(S)=Z((S)) with (S) denoting the ideal generated by elements of S.

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1 LECTURE Affine Space & the Zariski Topology Definition 1.1. Let k a field. Take S a set of polynomials in k[t 1,..., T n ]. Then Z(S) ={x k n f(x) =0, f S}. It is easy to check that Z(S)=Z((S)) with (S) denoting the ideal generated by elements of S. Definition 1.2. Y k n Z(S) =Y. is an (affine) algebraic set if S k[t 1,..., T n ] = A such that Example 1.3. (1) Consider the ideal I =(xy) k[x, y]. Then Z(I) ={(x, y) k 2 xy = 0} = {x =0} {y =0}. (2) Consider the ideal I =(x 2 y 3 ) k[x, y]. Then Z(I) ={(x, y) k 2 x 2 y 3 =0}. Proposition 1.4. The following are true: (1) The union of a finite collection of algebraic sets is algebraic. (2) Arbitrary intersections of algebraic sets are algebraic. (3) and A n are algebraic. Proof. (1) It suffices to show this for the union of two sets. The general case can then be established by induction. Let Y 1 = Z(T 1 ) and Y 2 = Z(T 2 ). We claim that Y 1 Y 2 = Z((T 1 )(T 2 )). For the forward inclusion, let x Y 1 Y 2, so without loss of generality, assume x Y 1. This implies f(x) = 0 for all f (T 1 ). Since (T 1 )(T 2 ) (T 1 ), we have f(x) = 0 for every f (T 1 )(T 2 )sox Z((T 1 )(T 2 )). For the reverse inclusion, let x Z((T 1 )(T 2 )) so h(x) = 0 for all h (T 1 )(T 2 ). Assume x / Z((T 2 )) so f (T 2 ) such that f(x) 0. Take g (T 1 ). Then gf (T 1 )(T 2 ) which implies (gf)(x) =0 g(x)f(x) = 0. But f 0sog(x) = 0 for every g. Thus x Z((T 1 )) = Y, and we have equality of sets. Date: Revised 09/15/

2 2 LECTURE 10 (2) Let {Z(I λ )} λ Λ. We claim that λ Λ Z(I λ )=Z( λ I λ ) where the latter expression is a sum over a finite subset of Λ. Beginning with reverse containment, let x Z( I λ ) f(x) = 0 for all f I λ. In particular, f(x) = 0 for f I λ,sox Z(I λ ) for all λ. For forward containment, let x Z(I λ ) for every λ. Then f(x) = 0 for every f I λ for all λ. Then for g I λ, we have g = h h n with h i I λi. This implies that g(x) = 0 because h i (x) = 0 for all i. Hence x Z( I λ ), and we have equality of sets. (3) = Z(1) and A n = Z(0). Remark 1.5. The above Proposition shows that the collection of algebraic sets defines a topology on k n where the closed sets are the algebraic sets. This topology will be called Zariski topology and k n endowed with this topology will be denoted A n. Remark 1.6. Is the Zariski topology Haussdorf? In general no. For an example, in the special case A 1 with k = k, every f k[t ] has finitely many zeroes, so closed sets in the Zariski topology have a finite number of points. We may ask if A 1 is Hausdorff by taking x, y A 1, and assuming that U x,u y open disjoint neighborhoods of x and y, respectively. But this would be equivalent to having closed sets that cover A 1, a contradiction since A 1 is infinite The Ideal-Variety Correspondence. Definition 1.7. Let Y A n, and A = k[t 1,..., T n ]. Then I(Y )={f A f(p )=0 Y k n }. Also, I(Y ) is an ideal of A. This ideal is in fact radical. P Proposition 1.8. Let A = k[t 1,..., T n ]. Then the following are true: (1) If A 1 A 2 A, then Z(A 1 ) Z(A 2 ) in A n. (2) If Y 1 Y 2 A n, then I(Y 1 ) I(Y 2 ) in A. (3) If Y 1,Y 2 A n,i(y 1 Y 2 )=I(Y 1 ) I(Y 2 ). (4) If I,J are ideals in A, then Z(I) Z(J) =Z(IJ). Also, Z( S j )= Z(S j ), for any family of subsets {S j } of A n. (5) If S A n and J ideal of A, then S Z(I(S)) and J I(Z(J)). (6) If V is an algebraic set then V = Z(I(V )). IfJ is an ideal of A of the form J = I(S), then I(Z(J)) = J. Corollary 1.9. The functions Z( ) defined on the family of ideals of the form I(S) for some S A n and I( ) defined on algebraic sets in A n are inverses to each other. Theorem (1) If k = k, then I(Z(J)) = Rad(J) for all J A = k[t 1,...T n ]. This is known as the Hilbert Nullstellensatz.

3 LECTURE 10 3 (2) Z(I(Y )) = Y for all Y A n. Proof. The statement of (1) can be restated as f I(Z(J) f(p ) = 0 for all P Z(J), where Z(J) ={x A n g(x) =0 g J}. The statement implies that if f vanishes where J vanishes then h such that f h J. This only follows when k = k. (2) We start with forward inclusion. Let Y Z(I(Y )) which implies Y Z(I(Y )). For the reverse, let W be a closed superset of Y. Then W = Z(J), which gives Z(J) Y. Examine the ideals corresponding to these sets, and we get I(Y ) I(Z(J)), so J I(Y ). Now go to the sets corresponding to the ideals, and we get Z(I(Y )) Z(J) =W, so any closed set containing Y contains Z(I(Y )). This statement applied to W = Y gives that Y Z(I(Y )). Corollary Assume that k is an algebraically closed field. The maps Z( ) and I( ) are inverses to each other and establish a one-to-one correspondence between the family of algebraic sets in A n and radical ideals of A. Corollary In this correspondence, a point (a 1,...,a n ) A n corresponds to the maximal ideal (T 1 a 1,...,T n a n ) of A. Proof. Let I =(T 1 a 1,...,T n a n ) which is a maximal (hence radical) ideal. The Corollary follows at once since Z(I) =(a 1,...,a n ). The correspondence implies that I(a 1,...,a n )=(T 1 a 1,...,T n a n ) which is a non-trivial statement (and which may not be true if k is not algebraically closed). Definition Let Y X, with X a topological space. Then Y is irreducible if Y is not a union of two proper closed subsets of Y. An example of a reducible set in A 2 is the set of points satisfying xy = 0 which is the union of the two axis of coordinates. Definition We call Y an affine algebraic variety if Y is an irreducible algebraic set. Corollary Let Y be algebraic variety. Then I(Y ) is prime. Conversely, I(Y ) is prime implies that Y is an algebraic variety. Therefore, in our 1-1 correspondence, varieties (irreducible algebraic sets) correspond to prime ideals. Proof. Take Y = Z(I) irreducible. Let fg I(Y ), so (fg)(y) = 0 for all y Y. This implies Y Z(fg)=Z(f) Z(g). Then Y =(Y Z(f)) (Y Z(g)). Note that both sets in our union

4 4 LECTURE 10 are closed in the subspace topology. But Y is irreducible, so the sets Y Z(f) and Y Z(g) are not simultaneously proper. Assume, without loss of generality, that Y Z(f) = Y which implies that Y Z(f),sof I(Y ) by definition. This gives f I(Y ) and hence I(Y )is prime. Conversely, let Y = Y 1 Y 2 with Y i = Z(I i ),I i ideals in A, for i = 1, 2. Assume, for a contradiction, that Y 1,Y 2 are strictly contained in Y. First note that I(Y ) I(Y i ) since equality would give Z(I(Y i )=Z(I(Y )), for i =1, 2. But Z( ),I( ) are inverses to each other when restricted to the set of ideals of algebraic sets and, respectively, algebraic sets, hence Y i = Y, false. Now, let f i I(Y i ) \ I(Y ). Then f 1 f 2 I(Y 1 )I(Y 2 ) I(Y 1 ) I(Y 2 ) I(Y 1 Y 2 )=I(Y ). Bt I(Y ) is prime. Therefore, either f 1 I(Y )orf 2 I(Y 2 ). This is a contradiction. Definition If A = k[t 1,..., T n ], and Y A n is an algebraic set, then k[y ]=A/I(Y ) is called the coordinate ring of functions of Y. Definition A map between two algebraic sets φ : V A n W A m is called a morphism (or regular map) if there are polymomials F 1,...F m such that φ(a 1,...,a n )=(F 1 (a 1,...,a n ),...,F m (a 1,...,a n )), for all (a 1,...,a n ) V. A morphism φ is called isomorphism between V and W if there exists a morphism ψ : W V inverse to φ. Let φ : V A n W A m be a morphism. This morphism induces a natural map φ : k[w ] k[v ]byφ ( ˆf) = f ˆ φ. Indeed, if f g I(W ) then f(w) =g(w) for all w W,sof(φ(v)) = g(φ(v) for all v V, since φ(v) W. This means that f φ g φ I(V ) and hence φ is well defined. It is a routine check that Φ is in fact a k-algebra homomorphism. Moreover, every k-algebra homomorphism Φ : k[w ] k[v ] is induced by a unique φ, that is Φ=φ. The morphism φ is an isomorphism if and only if φ is an isomorphism of k-algebras. Given Φ : k[w ] k[v ], k-algebra homomorphism, let us construct φ:

5 LECTURE 10 5 Let Φ( ˆT i )= ˆF i, for all i =1,...,m and F i k[t 1,...,T n ]. Then φ =(F 1,...,F m ) defines a morphism between A n and A m. Let us show that it maps V to W. Let g I(W )sog(w) = 0 for all w W. Moreover g( ˆT 1,..., ˆ Tm )= k[w ], since g I(W ). Therefore Φ(g( ˆT 1,..., ˆ T m )=0ink[V ] since a homomorphims maps 0 to 0. g(t1,...,t m )=ˆ0 in But Φ is a k-algebra homomorphism, so g(φ( ˆT 1 ),...,Φ( T ˆ m )) = 0 which is equivalent to g( ˆF 1,..., Fˆ m )=0ink[V ], or g(f 1,...,F ˆ m )=0ink[V ]. This gives g(f 1,...,F m ) I(V )orin other words, g(f 1,...,F m )(v) = 0 for all v V, i.e. g(f 1 (v),...,f m (v)) = 0 for all v V. Since φ(v)=(f 1 (v),...f m (v)) we see that g(φ(v)) = 0 for all v V and so g I(W ) implies that φ(v) Z(g). In other words, φ(v) Z(I(W )). But Z(I(W )) = W, since W is an algebraic set, and so φ(v ) W. Note that φ ( ˆT i )= T i φ = ˆF i =Φ(ˆT i ) for all i =1,...,m. Since ˆT i are k-algebra generators for k[w ]wegetφ =Φ. Proposition Every nonempty affine algebraic set V may be uniquely written in the form V = V 1 V n where each V i is an algebraic variety and V i V j for all j i. (These V i s are called irreducible components on V ). 2. Dimension Definition 2.1. Let V A n be an algebraic set. The supremum over all n such that there exists a chain V 0 V 1 V n of distinct irreducible algebraic sets in V is called the dimension of V. Definition 2.2. Let P be a prime ideal in a ring A. The supremum over all n such that there exists a chain of distinct prime ideals P 0 P 1 P n contained in P is called the height of P and it is denoted by ht(p ). The Krull dimension of A, dim(a), is the supremum of all ht(p ) over all P Spec(A). If I is an arbitrary ideal of A, we let the height of I, ht(i), equal the infimum of all ht(p ) over all prime ideals P containing I.

6 6 LECTURE 10 Theorem 2.3. Let V be an algebraic set in A n k where k is algebraically closed. Then dim(v )=dim(k[v ]). Proof. This follows at once since irreducible algebraic sets in V correspond to prime ideals containing I(V ) in our correspondence.

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