# PROBLEMS, MATH 214A. Affine and quasi-affine varieties

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1 PROBLEMS, MATH 214A k is an algebraically closed field Basic notions Affine and quasi-affine varieties 1. Let X A 2 be defined by x 2 + y 2 = 1 and x = 1. Find the ideal I(X). 2. Prove that the subset in A 2 consisting of all points of the form (t 2, t 3 ), t k is closed. 3. Let X and X be subsets of A n. Show that I(X) I(X ) if and only if X X. 4. Let S and S be subsets of k[t 1,..., T n ]. Show that Z(S) Z(S ) if and only if S S. 5. Prove that a subset X A n is quasi-affine if and only if X = Z 1 \ Z 2, where Z 1 and Z 2 are closed subsets of A n. 6. Prove that the intersection of two quasi-affine subsets is quasi-affine. 7. Let Z = Z(x) and U = A 2 \ Z(y) be respectively closed and open subsets of A 2 with the coordinates (x, y). Prove that the set Z U is not quasi-affine. 8. Let X be a quasi affine subset of A n. Prove that every closed or open subset of X is quasi-affine. 9. Prove that every quasi-affine subset of A 1 is either open or closed. 10. Give an example of a quasi-affine subset of A n, n 2 that is neither open nor closed. 11. Prove that every quasi-affine set (in particular, an open set!) is compact in Zariski topology. Regular functions and maps 12. Let X A 2 be defined by x(y 2 x) = 0. Show that the function f : X k defined by { 0 if x = 0, f(x, y) = y otherwise is not regular. Prove that f(x, y) 2 is regular. 13. Prove that every open subset of A 1 is principal. 14. Show that the open subset A 2 \ (0, 0) is not principal in A Suppose that X consists of n points. Prove that the ring k[x] is isomorphic to the direct product of n copies of k. 1

2 2 PROBLEMS, MATH 214A 16. Establish a bijection between an closed set X and the set of all k-algebra homomorphisms k[x] k. 17. Let f : A 2 A 2 be given by f(x, y) = (xy, y). Show that the image of f is not quasi-affine. 18. Let f : X Y be a regular maps of quasi-affine sets and let I k[y ], J k[x] be two ideals. Prove that and f 1 (Z(I)) = Z(f (I)) f(z(j)) = Z(f 1 (J)). 19. Prove that two curves X and Y given in A 2 by equations xy = 0 and x 2 = x respectively are not isomorphic. 20. Let x be a point in A n and let X = A n \ {x}. Prove that if n 2, then k[x] = k[a n ]. 21. Let f : X A 1 = k be a regular function. Prove that the image of f is either closed or open in A Let X be a quasi-affine set such that k[x] = k. Prove that X is a one point set. 23. Let S be a subset of k[x], X a closed set. Prove that the principal open sets D(f), for all f S, cover X if and only if the ideal generated by S is equal to k[x]. 24. Let X be a point and let Y = A 2 \ {(0, 0)}. Show that the natural map Mor(X, Y ) Hom k alg (k[y ], k[x]) is not surjective. (Mor(X, Y ) is the set of all regular maps X Y.) 25. Let X be a quasi-affine set and Y be a closed set. Prove that the natural map Mor(X, Y ) Hom k alg (k[y ], k[x]) is a bijection. 26. Prove that A 1 \ {0, 1} and A 1 \ {a, b} are isomorphic for all a b k. 27. Prove that A 1 is not isomorphic to any proper quasi-affine subset of A Prove that the curves Z(x 2 + y 2 1) and Z(xy 1) in A 2 are isomorphic (assume char k 2). Are these curves isomorphic to A 1? Categories. Products and coproducts 29. Let A be a category and let X, Y Ob(A). The product of X and Y in A is an object X Y together with two morphisms p : X Y X and q : X Y Y such that for every two morphisms f : Z X and g : Z Y there exists a unique morphism h : Z X Y with f = ph and g = qh. Prove that the product is unique up to canonical isomorphism. Determine products in the categories of sets, rings, commutative rings, (left) modules over a ring, closed sets, quasi-affine sets.

3 PROBLEMS, MATH 214A Formulate the dual notion of the coproduct in a category. Determine coproducts in the categories of sets, rings, commutative rings, (left) modules over a ring, closed sets, quasi-affine sets. 31. An initial (resp. final) object of a category A is an object X Ob(A) such that for any object Y Ob(A) there is a unique morphism X Y (resp. Y X). Prove that initial and final objects are unique up to canonical isomorphism. Determine initial and final objects in the categories of sets, rings, commutative rings, (left) modules over a ring, closed sets, quasi-affine sets. 32. Prove that for quasi-affine sets X and Y, the canonical homomorphism k[x] k[y ] k[x Y ] is an isomorphism. Irreducible varieties 33. Let X 1, X 2,..., X n be the irreducible components of a Noetherian topological space X. a) Let U X be an open subset. Prove that the nonempty intersections U X i are the irreducible components of U. b) Prove that an open subset U X is dense if and only if U X i for every i. c) Prove that every irreducible subset of X is contained in the X i for some i. d) Prove that the irreducible components of X can be defined as maximal elements in the set of all closed irreducible subsets of X. 34. Let f : X Y be a continuous map of topological spaces and let Z X be an irreducible subset. Prove that f(z) is also irreducible. 35. Find irreducible components of the variety given by x 2 = yz, xz = x in A Find irreducible components of the variety given by xy = z 3, xz = y 3 in A Let X be a quasi-affine variety and let Y X be a closed irreducible subset. Prove that I(Y ) is a prime ideal in k[x]. 38. Show that every nonempty quasi-affine variety admits a cover by open affine dense subsets. 39. Prove that any quasi-affine variety is isomorphic to a dense open subset of an affine variety. 40. Prove that the algebra k[x] for a quasi-affine variety X is isomorphic to a subalgebra of a finitely generated k-algebra. 41. Let X be an affine variety, X = Z 1 Z 2, where Z i are closed and disjoint. Show that there is a function e k[x] such that e Z1 = 0 and e Z2 = 1. Prove that k[x] is isomorphic to the product k[z 1 ] k[z 2 ]. 42. Let X be an affine variety and let e k[x] be a function such that e 2 = e. Prove that X = Z 1 Z 2, where Z i are closed, disjoint and e Z1 = 0, e Z2 = Prove that an quasi-affine variety X is connected if and only if the ring k[x] has no idempotents other than 0 and 1.

4 4 PROBLEMS, MATH 214A 44. Prove that a Noetherian topological space is Hausdorf if and only if it is finite with discrete topology. Rational functions 45. Find the domain of definition of the rational function (1 y)/x on the curve given by x 2 + y 2 = 1 in A Prove that the variety A 1 satisfies the following property: If f k(a 1 ) and f 2 k[a 1 ], then f k[a 1 ]. Does the variety Z(y 2 x 2 x 3 ) A 2 satisfy this property? 47. Prove the the map α : A 2 A 2 given by α(x, y) = (x, xy) is a birational isomorphism. Find domain of definition of α Prove that the field k(x, 1 x 2 ) is purely transcendental over k. 49. Prove that the plane curves given by the equations y 2 = x 3 and y 2 = x 3 + x 2 respectively are rational varieties. 50. Let X be sphere given in A n by the equation x x x 2 n = 1 (n 2). Prove that X is a rational variety. Local ring of a subvariety 51. Let X and X be quasi-affine varieties and let Y X and Y X be closed irreducible subvarieties. Prove that the local rings O X,Y and O X,Y are isomorphic as k-algebras if and only if there are neighborhoods U and U of Y and Y in X and X respectively and an isomorphism f : U U such that f(u Y ) = U Y. 52. Let Y and Z be closed irreducible subvarieties of X such that Y Z. Let P O X,Y be the prime ideal corresponding to Z. Prove that O X,Y /P O Z,Y and (O X,Y ) P O X,Z. 53. The completion ˆR of a local commutative ring R with maximal ideal M is the inverse limit of the rings R/M i over all i 1. Show that ˆR is a local ring. Let X and Y be plane curves given by equations xy = 0 and x 3 + y 3 + xy = 0 respectively. Let z = (0, 0) be the origin. Prove that the local rings O X,z and O Y,z are not isomorphic but have isomorphic completions. Quasi-projective varieties Basic notions 54. Let I k[s 0, S 1,..., X n ] be an ideal. Prove that the following are equivalent: (i) I is generated by homogeneous polynomials; (ii) If F 0 +F 1 + +F d I, where F i is a homogeneous polynomial of degree i, then F i I for all i. 55. Let F be a homogeneous polynomial. Prove that every divisor of F is also homogeneous.

5 PROBLEMS, MATH 214A Let I k[s 0, S 1,..., X n ] be a homogeneous ideal. Prove that the ideal I is also homogeneous. Regular functions and maps 57. Prove that the image the Veronese map is closed. 58. Is there a surjective regular map A 1 P 1? 59. Let x P 2. Prove that P 2 \ {x} is neither projective nor quasi-affine variety. 60. Determine all regular maps P 1 A Let F k[s 0, S 1,..., S n ] be a homogeneous polynomial of degree d and let U be the principal open set D(F ) in P n. Prove that the k-algebra k[u] is canonically isomorphic to the k-algebra k[s 0, S 1,..., S n ] (F ) of all rational functions G, where G is a homogeneous polynomial of degree md. F m 62. Prove that every regular map f : P n P m is of the form x [F 0 (x) : F 1 (x) : : F m (x)], where F 0, F 1,..., F m k[s 0, S 1,..., S n ] are homogeneous polynomials of the same degree having no nontrivial common zeros. Prove that every regular map P n P m with m < n is constant. 63. Prove that a regular map f : X Y of quasi-projective varieties is an isomorphism if and only if f is a homeomorphism and for any point x X the induced local ring homomorphism is an isomorphism. Rational functions and maps f : O Y,f(y) O X,x 64. Find the domain of definition of the regular function f = S 1 /S 0 on P Let f : P 2 P 2 be the rational map defined by f([s 0 : S 1 : S 2 ]) = [S 1 S 2 : S 0 S 2 : S 0 S 1 ]. Find the domain of definition of f. Prove that f is a birational isomorphism and f 2 = id. 66. Prove that every rational map P 1 P n is regular. 67. Let f : X Y be a rational map of quasi-projective varieties with X irreducible. Prove that there is a regular map g : X X for some quasiprojective X such that g is a birational isomorphism and the composition f g is regular. Product of quasi-projective varieties 68. Prove that (X Y ) Z X (Y Z). 69. Prove that P n P m is not isomorphic to P n+m. 70. Prove that A 1 P 1 is not isomorphic to P 1 P Let f : X S and g : Y S be two regular maps. Show that the set {(x, y) X Y : f(x) = g(y)} is a quasi-projective variety.

6 6 PROBLEMS, MATH 214A Proper maps 72. Prove the if f 1 : X 1 Y 1 and f 2 : X 2 Y 2 are proper maps, then f 1 f 2 : X 1 X 2 Y 1 Y 2 is proper. 73. Prove that the map X pt is proper if and only if X is a projective variety. A morphism f : X Y is proper if and only if f factors as X Y P n Y for some n with the first map a closed embedding and the second one the projection. 74. Let f : X Y and g : Y Z be two regular maps. Prove that if g f is proper, then f is proper. 75. Prove that a map f : X Y is proper if and only if Y can be covered by open subsets U i such that f 1 (U i ) U i is proper for each i.

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