MAT00001M Algebraic Geometry 2020/21
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MAT00001M
MMath and MSc Examinations 2020/21
Algebraic Geometry
1 (of 4). Let A = k[X1 , . . . , Xn], the polynomial algebra over k in n indeterminates, be the coordinate algebra on An .
Decide whether or not each of the following subsets of affine space is an algebraic set, fully justifying your answer in each case. Write the statement of any result you use clearly:
(a) {(t3 + 1, t2 ) I t ∈ k} ∈ A2 with k = C;
(b) Q, the set of rational numbers, considered as a subset of A1 with k = C;
(c) {(s, t) I Is + tI = 1} ∈ A2 , where k = C;
(d) the following subset of An :
&
ì(X1(d) 一 X2 );
d=1
(e) the principal open set Vf , where V = A2 and f := X1 + X2 ∈ k[X1 , X2].
2 (of 4). (a) Determine whether the following maps are morphisms, justifying your
answer in each case:
(i) V = A1 , W = A2 , with φ : V → W given by φ(t) = (t4 + 3t, t2 一 2), with k = C;
(ii) V = A1 = W , with φ : V → W given by φ(t) = 2t , with k = C; (iii) V = ì(X1X2X3 一 1) ∈ A3 where A3 has coordinate algebra A =
k[X1 , X2 , X3], W = A2 , with φ : V → W given by φ(a1 , a2 , a3 ) =
(1/a1 , 1/a2 ).
(b) Let V = A2 with coordinate algebra A = k[X, Y] and W = A3 with
coordinate algebra B = k[X1 , X2 , X3]. Define a map φ : V → W by the rule φ(a, b) := (ab, a2 + b3 , 0). Show that φ is a morphism and give the images of the generators X1 , X2 and X3 for B under the comorphism φ出 .
3 (of 4). (a) Determine if the following polynomials in k[X, Y, Z] are homogeneous.
For those that are homogeneous, give the degree of the polynomial, for those that are not homogeneous, indicate why.
(i) XY + Z2 ;
(ii) X 14 + Y7 Z7 + 14;
(iii) 4X4 Y + 34Y2 Z3 + Z5 .
(b) Let k = C, let V = A3 with coordinate algebra A = k[X1 , X2 , X3], and
let U be the following algebraic subset of V :
U := ì(X1(3) 一 X3X2 , X1(2)X3 一 X2 ).
(i) Show that U ζ ì(X2 (X1 一 X3(2))). [Hint: try to write X2 (X1 一 X3(2)) in terms of the defining polynomials for U .]
(ii) Show that ì(X1(3) 一 X3X2 , X1(2)X3 一 X2 , X2 ) = {(0, 0, t) I t ∈ C}. (iii) Show that ì(X1(3) 一X3X2 , X1(2)X3 一X2 , X1 一X3(2)) = {(t2 , t5 , t) I t ∈ C}. (iv) Deduce that U has two irreducible components. Give a brief justifi-
cation why the subsets you have written down are irreducible.
4 (of 4). (a) Let U = {(r, s, t) ∈ k3 I r s or rt 1} ζ A3 . Express U as the
union of two principal open subsets in A3 , and show explicitly that the
complement of U is an algebraic set.
(b) Let k = C, and let V = ì(4X2 + Y2 一 16) ∈ A2 , with coordinate algebra
A given by restriction of functions from the polynomial algebra k[X, Y]. You may assume V is irreducible.
Let X = XIV and Y = Y IV .
(i) Find a ∈ C such that if X0 = X + a ∈ A and Y0 = Y ∈ A, then 4X0(2) 一 16X0 + Y02 = 0.
(ii) Giving full justification, show that V is a rational curve.
SOLUTIONS: MAT00001M
1. (a) {(t3 + 1, t2 ) I t ∈ k} ∈ A2 : Clearly, we have {(t3 + 1, t2 ) I t ∈ k} ∈ ì(X2 一 2X 一 Y3 +1). On the other hand, if (a, b) ∈ ì(X2 一 2X 一 Y3 +1), then a2 一 2a 一 b3 + 1 = 0, and so b3 = a2 一 2a + 1 = (a 一 1)2 . Take c ∈ C such that c2 = b. Then (a 一 1)2 = b3 = c6 = (c3 )2 . Thus a is either c3 + 1 or 一c3 + 1. Therefore (a, b) is either (c3 + 1, c2 ) ∈ {(t3 + 1, t2 ) I t ∈ k} or (一c3 + 1, c2 ) = ((一c)3 + 1, (一c)2 ) ∈ {(t3 + 1, t2 ) I t ∈ k}. Therefore {t3 + 1, t2 ) I t ∈ k} = ì(X2 一 2X 一 Y3 + 1) is algebraic. f(伍)6 Marks
(b) Q, the set of rational numbers, is an infinite proper subset of A1 . As
proper algebraic subsets of A1 must be finite (as seen in the lecture), Q
is not an algebraic set. 6 Marks
(c) We will use a lemma from the lectures (Lemma 2.4). Write U := {(s, t) ∈
A2 I Is + tI = 1} and set U = {s ∈ A1 I (s, 0) ∈ U}. Then we have U = {s ∈ A1 I IsI = 1}.
This is an infinite proper subset of A1 . As proper algebraic subsets of A1 must be finite (as seen in the lecture), U is not an algebraic subset of A1 , and then by the Lemma, U is not an algebraic subset of A2 . f(伍)6 Marks
(d) Each of the sets ì(X1(d) 一 X2 ) are closed subsets of An in the Zariski topology. In a topology, any intersection of closed subsets is a closed subset, thus nì(X1(d) 一 X2 ) is a closed subset of An , that is, it is an algebraic set. Alternatively, one could show that nì(X1(d) 一 X2 ) = ì(X1 一 X2 , X1(2) 一 X2 ). f(伍)6 Marks
(e) We have seen in lectures that principal open sets are affine varieties
(Theorem 8.3). But the question here is if the given set is an algebraic subset. We will show that it is not.
Write U = Vf = {(a1 , a2 ) ∈ A2 I a1 + a2 = f(a1 , a2 ) 0}. We will once more use Lemma 2.4 from the lectures. Set U = {a1 ∈ A1 I (a1 , 0) ∈ U}.
Then we have
U = {a1 ∈ A1 I a1 + 0 0} = k \ 0.
It follows that U is an infinite proper subset of A1 . As proper algebraic
subsets of A1 must be finite (as seen in the lecture), U is not an algebraic subset of A1 , and then by the Lemma, U is not an algebraic subset of
A2 . f(伍)6 Marks
Remarks. Part (a), (b), and (c) consists of examples very similar to those seen in lectures and problem sheets. Part (d) is new. Part (e) is also new
and a bit tricky, as we have seen in class that principal open sets are affine varieties.
Total: 30 Marks
2. |
(a) |
(i) |
This is a morphism. In this case, say A = k[X] and B = k[X1 , X2]. It suffices to check the condition for the generators X1 and X2 of B . We have, for each t ∈ A1 , (X1 О φ)(t) = X1 (t4 + 3t, t2 一 2) = t4 + 3t, so X1 О φ = X4 + 3X ∈ k[X]. Similarly, X2 О φ = X2 一 2 ∈ k [X], so 伍 一f 、
|
(ii) Not a morphism. We need to show that there is no polynomial
f ∈ k[X] with f(t) = 2t for each t ∈ C. Any such f has no roots, so it is constant. But 2t is not a constant function, so no such f exists,
and φ cannot be a morphism. 6 Marks
(iii) This is a morphism. First note that the map given is well defined
since (a1 , a2 , a3 ) ∈ V will satisfy a1 a2 a3 一 1 = 0, hence a1 a2 a3 = 1 and so in particular a1 and a2 are non-zero. As V is a closed subset of A3 its coordinate algebra is A = k[Y1 , Y2 , Y3], where Yi = Xi IV is the restriction of the generators X1 , X2 , X3 of the coordinate algebra on A3 . Say B = k[Z1 , Z2] is the coordinate algebra on A2 . We need to check the condition on the generators Z1 and Z2 of B. We have for each (s, t, r) ∈ V ,
(Z1 О φ)(s, t, r) = Z1 (1/s, 1/t) = 1/s (Z2 О φ)(s, t, r) = Z2 (1/s, 1/t) = 1/t.
Note that for (s, t, r) ∈ V , we have str = 1, and so 1/s = tr and 1/t = sr. Thus (Z1 О φ)(s, t, r) = tr = Y2 Y3 (s, t, r) and (Z2 О φ)(s, t, r) = sr = Y1 Y3 (s, t, r). Thus, Z1 О φ = Y2 Y3 ∈ k[Y1 , Y2 , Y3]
and Z2 О φ = Y2 Y3 ∈ k[Y1 , Y2 , Y3] and we are done. 6 Marks
(b) We simply need to calculate
(X1 О φ)(a, b) = X1 (ab, a2 + b3 , 0) = ab = XY (a, b),
(X2 О φ)(a, b) = X2 (ab, a2 + b3 , 0) = a2 + b3 = (X2 + Y3 )(a, b), (X3 О φ)(a, b) = X3 (ab, a2 + b3 , 0) = 0 = 0(a, b),
for all (a, b) ∈ A2 . This shows that that X1 О φ = XY ∈ A, X2 О φ = X2 + Y3 ∈ A, and X3 О φ = 0 ∈ A. As X1 , X2 , X3 generate B, it follows that φ is a morphism and we have φ出 (X1 ) = XY , φ出 (X2 ) = X2 + Y3 and
φ出 (X3 ) = 0. f(伍)6 Marks
Remarks. Part (a) is similar to things done in lectures and problem sheets, the final part is a little more involved. Part (b) is also similar to problems from lectures and problem sheets.
Total: 24 Marks
3. (a) (i) The polynomial XY + Z2 is homogeneous of degree 2. f(伍)3 Marks
(ii) The polynomial X14 + Y7 Z7 + 14 is not homogeneous. The first two
terms have degree 14 and the last has degree 0. 3 Marks
(iii) The polynomial 4X4 Y + 34Y2 Z3 + Z5 is homogeneous of degree 5.
伍 一
3 Marks
f 、
(b) (i) We write X3 (X1(3) 一 X3X2 ) 一 X1 (X1(2)X3 一 X2 ) = X1X2 一 X2X3(2) =
X2 (X1 一 X3(2)). Hence any common solution of X1(3) 一 X3X2 = 0 and
X1(2)X3 一 X2 = 0 also satisfies X2 (X1 一 X3(2)). f(伍)3 Marks
(ii) Suppose (a1 , a2 , a3 ) is killed by the three given polynomials. Then
a2 = 0 because we have to be killed by X2 , but then X1(3) 一 X3X2
gives rise to a1 = 0 as well. Now note that any point (0, 0, t) does
satisfy the three given polynomials. 3 Marks
(iii) |
Suppose (a1 , a2 , a3 ) satisfies the given polynomials. Then the third polynomial gives a1 = a3(2), and then the second gives a2 = a3(5). Hence every point satisfying these polynomials has the given form. On the other hand, every point of the given form clearly satisfies the three 伍 一 f 、
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2022-01-11