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6. Projective Varieties I: Topology 46 Andreas Gathmann 6. Projective Varieties I: Topology In the last chapter we have studied (pre-)varieties, i. e. topological spaces that are locally isomorphic to affine varieties. In particular, the ability to glue affine varieties together allowed us to construct compact spaces (in the classical topology over the ground field C) as e. g. P1, whereas affine varieties themselves are never compact unless they consist of only finitely many points (see Exercise 2.34 (c)). Unfortunately, the description of a variety in terms of its affine patches and gluing isomorphisms is quite inconvenient in practice, as we have seen already in the calculations in the last chapter. It would therefore be desirable to have a global description of these spaces that does not refer to gluing methods. Projective varieties form a very large class of “compact” varieties that do admit such a global de- scription. In fact, the class of projective varieties is so large that it is not easy to construct a variety that is not (an open subset of) a projective variety — in this class we will certainly not see one. In this chapter we will construct projective varieties as topological spaces, leaving their structure as ringed spaces to Chapter7. To do this we first of all need projective spaces, which can be thought of as compactifications of affine spaces. We have already seen P1 as A1 together with a “point at infinity” in Example 5.5 (a); other projective spaces are just generalizations of this construction to higher dimensions. As we aim for a global description of these spaces however, their definition looks quite different from the one in Example 5.5 (a) at first. n Definition 6.1 (Projective spaces). Let n 2 N. We define projective n-space over K, denoted PK or n n+ simply P , to be the set of all 1-dimensional linear subspaces of the vector space K 1. Notation 6.2 (Homogeneous coordinates). Obviously, a 1-dimensional linear subspace of Kn+1 is uniquely determined by a non-zero vector in Kn+1, with two such vectors spanning the same linear subspace if and only if they are scalar multiples of each other. In other words, we have n n+1 P = (K nf0g)= ∼ with the equivalence relation ∗ (x0;:::;xn) ∼ (y0;:::;yn) :, xi = lyi for some l 2 K and all i; where K∗ = Knf0g is the multiplicative group of units of K. This is usually written as n n+1 ∗ n P = (K nf0g)=K , and the equivalence class of (x0;:::;xn) will be denoted by (x0 : ··· :xn) 2 P (the notations [x0 : ··· :xn] and [x0;:::;xn] are also common in the literature). So in the notation n (x0 : ··· :xn) for a point in P the numbers x0;:::;xn are not all zero, and they are defined only up to a common scalar multiple. They are called the homogeneous coordinates of the point (the reason for this name will become obvious in the course of this chapter). Note also that we will usually label n the homogeneous coordinates of P by x0;:::;xn instead of by x1;:::;xn+1. This choice is motivated n n by the following relation between A and P . n Remark 6.3 (Geometric interpretation of P ). Consider the map n n f : A ! P ; (x1;:::;xn) 7! (1:x1 : ··· :xn): n n+1 As in the picture below on the left we can embed the affine space A in K at the height x0 = 1, and then think of f as mapping a point to the 1-dimensional linear subspace spanned by it. 6. Projective Varieties I: Topology 47 (1 : a1 : a2) x0 x0 2 1 2 1 A (a1;a2) A x(t) x2 x1 bb x1 The map f is obviously injective, with image U0 := f(x0 : ··· :xn) : x0 6= 0g. On this image the inverse of f is given by −1 n x1 xn f : U0 ! A ; (x0 : ··· :xn) 7! ;:::; ; x0 x0 n n+ it sends a line through the origin to its intersection point with A embedded in K 1. We can thus n n x1 xn n think of as a subset U0 of . The coordinates ;:::; of a point (x0 : ··· :xn) 2 U0 ⊂ are A P x0 x0 P called its affine coordinates. n The remaining points of P are of the form (0:x1 : ··· :xn); in the picture above they correspond to lines in the horizontal plane through the origin, such as e. g. b. By forgetting their first coordinate n− (which is zero anyway) they form a set that is naturally bijective to P 1. Thinking of the ground field we can regard them as points at infinity: consider e. g. in n a parametrized line C AC x(t) = (a1 + b1 t;:::;an + bn t) for t 2 C n n for some starting point (a1;:::;an) 2 C and direction vector (b1;:::;bn) 2 C nf0g. Of course, there is no limit point of x(t) in n as t ! ¥. But if we embed n in n as above we have AC AC PC 1 a a x(t) = (1:a + b t : ··· :a + b t) = : 1 + b : ··· : n + b 1 1 n n t t 1 t n in homogeneous coordinates, and thus (in a suitable topology) we get a limit point (0:b1 : ··· :bn) 2 n n P nA at infinity for x(t) as t ! ¥. This limit point obviously remembers the direction, but not the C C n n position of the original line. Hence we can say that in P we have added a point at infinity to A in n n each direction. In other words, after extension to P two distinct lines in A will meet at infinity if and only if they are parallel, i. e. point in the same direction. n n Usually, it is more helpful to think of the projective space P as the affine space A compactified n− by adding some points (parametrized by P 1) at infinity, rather than as the set of 1-dimensional n+ n linear subspaces in K 1. In fact, after having given P the structure of a variety we will see in n n− Proposition 7.2 and Exercise 7.3 (b) that with the above constructions A and P 1 are open and n closed subvarieties of P , respectively. n n Remark 6.4 (P is compact in the classical topology). In the case K = C one can give P a standard C n C (quotient) topology by declaring a subset U ⊂ P to be open if its inverse image under the quotient map p : n+1nf0g ! n is open in the standard topology. Then n is compact: let C P PC n+1 2 2 S = f(x0;:::;xn) 2 C : jx0j + ··· + jxnj = 1g n+ be the unit sphere in C 1. This is a compact space as it is closed and bounded. Moreover, as every n n point in P can be represented by a unit vector in S, the restricted map pjS : S ! P is surjective. n Hence P is compact as a continuous image of a compact set. Remark 6.5 (Homogeneous polynomials). In complete analogy to affine varieties, we now want to n define projective varieties to be subsets of P that can be given as the zero locus of some polynomials in the homogeneous coordinates. Note however that if f 2 K[x0;:::;xn] is an arbitrary polynomial, it does not make sense to write down a definition like n V( f ) = f(x0 : ··· :xn) : f (x0;:::;xn) = 0g ⊂ P ; because the homogeneous coordinates are only defined up to a common scalar. For example, if 2 1 f = x1 − x0 2 K[x0;x1] then f (1;1) = 0 and f (−1;−1) 6= 0, although (1:1) = (−1: − 1) in P . To 48 Andreas Gathmann get rid of this problem we have to require that f is homogeneous, i. e. that all of its monomials have the same (total) degree d: in this case d ∗ f (lx0;:::;lxn) = l f (x0;:::;xn) for all l 2 K ; and so in particular we see that f (lx0;:::;lxn) = 0 , f (x0;:::;xn) = 0; n so that the zero locus of f is well-defined in P . So before we can start with our discussion of projective varieties we have to set up some algebraic language to be able to talk about homogeneous elements in a ring (or K-algebra). Definition 6.6 (Graded rings and K-algebras). (a)A graded ring is a ring R together with Abelian subgroups Rd ⊂ R for all d 2 N, such that: • Every element f 2 R has a unique decomposition f = ∑d2N fd such that fd 2 Rd for all d 2 N and only finitely many fd are non-zero. In accordance with the direct sum L notation in linear algebra, we usually write this condition as R = d2N Rd. • For all d;e 2 N and f 2 Rd, g 2 Re we have f g 2 Rd+e. For f 2 Rnf0g the biggest number d 2 N with fd 6= 0 in the decomposition f = ∑d2N fd as above is called the degree deg f of f . The elements of Rdnf0g are said to be homoge- L neous (of degree d). We call f = ∑d2N fd and R = d2N Rd as above the homogeneous decomposition of f and R, respectively. (b) If R is also a K-algebra in addition to (a), we say that it is a graded K-algebra if l f 2 Rd for all d 2 N and f 2 Rd.
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