Université Paris Sud XI Orsay THE LOCAL FLATTENING THEOREM. Master 2 Memoire by Christopher M. Evans. Advisor: Prof. Joël Merker


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1 Université Paris Sud XI Orsay THE LOCAL FLATTENING THEOREM Master 2 Memoire by Christopher M. Evans Advisor: Prof. Joël Merker August 2013
2 Table of Contents Introduction iii Chapter 1 Complex Spaces Sheaves and coherence Local properties of analytic sets Complex Spaces Chapter 2 Blowings up Blowing up plane curves Blowing up complex spaces The strict transform Existence of blowings up Further properties of blowings up Local blowings up Chapter 3 La voûte étoilée The category C(W ) Étoiles Further properties of étoiles Chapter 4 The local flattening theorem Flatness The local flattening theorem Choosing the sequence of blowings up Proof of the local flattening theorem Bibliography 62 ii
3 Introduction The aim of this text is to give a proof of the local flattening theorem of analytic geometry. The essential content of this result is that given a morphism of complex spaces f : V W and a compact subset L of a fibre f 1 (y) of f, we can pick a finite number of finite compositions of local blowings up σ α : W α W such that the strict transform of f by the σ α is flat at each point of W α corresponding to L, and the union of images of the σ α is a neighborhood of y in W. Thus the theorem provides a means of transforming f into to a finite set of mappings that are flat at the points corresponding to L, and which, up to a restriction of the domain, are the base change of f by a finite composition of local blowings up. This finds application for example in the study of subanalytic sets and subanalytic functions (those functions with subanalytic graphs), where the theorem allows to reduce the problem of rectilinearization of sets to the semianalytic case [1, 2]. The first proof of the local flattening theorem was given in [3] using generalized Newton polygon techniques. Hironaka published separately a different proof the same year [1]. Both proofs use the notion of an étoile over W : an étoile collects groups together some (though not all) of the finite compositions of local blowings up with a common point of W in their image. Etoiles provide a means of access to the topological requirements of the local flattening theorem, namely the requirement that the images of the σ α form a neighborhood of y in W. In order to complete the proof of the theorem, we then select from each étoile corresponding to the point y a wellchosen finite sequence of local blowings up. The key to this last choice is the notion of a flatificator. The flatificator for the given f and L of the local flattening theorem is a maximal locally closed subspace P of W containing the point y such that the restriction of f above P is flat. The second proof of Hironaka is the exposition that we follow for this step of the proof. A very brief outline of Hironaka s proof of the existence of the flatificator is as follows: first one reduces to the case when L = {z} is a single point of V ; the result then rests on a generalization of the Weierstrass division theorem, which when f is flat at z provides a decomposition of O V,z as a free O W,y module, and when f is not flat provides a system of generators for the flatificator for f and z. Once we have the existence of the flatificator for a given map, these locally closed subspaces then provide us with the centres of the local blowings up in the local flattening theorem. We can take the centres of the σ α asserted by the local flattening theorem to be nowhere dense subspaces. This is important in order that we capture all of the information given by the morphism f : V W. More precisely, a closed subspace of an open set U W that is dense in a neighborhood of some point contains an entire irreducible component of U, and when we blow up with centre equal to an entire component this component transforms via the blowing up to the empty set, so we lose information. Another important property is that we can also ensure that the centres of the blowings up are smooth. We do not discuss this somewhat deeper addition to the proof, which depends on desingularization of analytic sets. The exposition is organized as follows. In chapter 1 we briefly recall without proof some elementary definitions and results regarding coherent sheaves and complex spaces. All of these results can be found in [4, 5]; in general our use of complex spaces is closer to the more algebraic exposition of Grauert and Remmert. Further constructions on complex spaces are stated in later chapters as they are required; the same references apply in these cases. For the remainder of the text we follow the exposition of [1]. In chapter 2 we define the blowing up of a complex space along a closed subspace, establish existence of the blowing up, and look at the strict transform and a few further properties of this construction. In chapter 3 we define an étoile over a complex space, and we make precise the way in which an étoile captures only blowings up about a particular point. We also give a topological structure to the set of étoiles and use this to study the iii
4 INTRODUCTION iv mapping from an étoile to the common image point of the blowings up it contains. At this point we admit two results of Hironaka regarding étoiles; we hope our exposition remains sufficiently detailed to give a clear idea of how the notion of an étoile applies to the topological requirements of the local flattening theorem. Finally in chapter 4 we achieve the proof of the local flattening theorem following the procedure discussed above.
5 Chapter 1 Complex Spaces In this chapter we recall without proof some standard facts about coherent sheaves, local properties of analytic sets and complex spaces. 1.1 Sheaves and coherence Let X and Y be topological spaces, let F and G be sheaves on X and Y respectively, and let f : X Y be a continuous map. The direct image of F by f is f F : U F(f 1 (U)), U Y open, and the inverse image of G by f is f 1 G : U lim G(V ), V f(u) U X open. If A is a sheaf of rings on X and F, F are sheaves of Amodules (briefly, Amodules) then we define Hom A (F, F ) : U Hom A U (F U, F U ). We have the adjunction property: Hom(f 1 G, F) = Hom(G, f F). From now on, let A be a sheaf of rings on X and let F be an Amodule. We say that F is locally free of rank m if for all x X there exists an open neighborhood U of x such that F U A m U. We say that F is locally finitely generated if for all x X there exists an open neighborhood U of x and a finite number of sections s 1,..., s q F(U) such that for all y U, the germs s 1,y,..., s q,y generate the stalk F y. If U X is open and s 1,..., s q F(U) are any sections, then we define an A(U)module
6 2 R(s 1,..., s q )(U) to be the kernel of the morphism (g 1,..., g q ) A q x g 1 s 1,x + + g q s q,x x U. In this way we define a subsheaf of relations R(s 1,..., s q ) A q U. We say that F is coherent if F is locally finitely generated and for all U X and all sections s 1,..., s q F(U) the sheaf R(s 1,..., s q ) is locally finitely generated. Equivalently, F is coherent if for each x X there is an open neighborhood U of x and a finite presentation A q U Ap U F U 0. If we have an exact sequence of sheaves of Amodules 0 F G H 0 and any two of the sheaves F, G, H are coherent then the third is coherent. Moreover if F and G are two coherent subsheaves of a coherent sheaf H then the sheaves F G and F + G are coherent. A sheaf of rings A is coherent if it is coherent as an Amodule over itself. 1.2 Local properties of analytic sets First we introduce some general notation. If M is a complex manifold, we write O M for the sheaf of holomorphic functions on M. If U M is an open set, we write O M (U), or just O(U) for the Calgebra of holomorphic functions on U. Now let U C n be an open neighborhood of 0 C n. Let (z 1,..., z n ) be coördinates on C n. We write O n = O C n,0 = C{z 1,..., z n } for the ring of germs of holomorphic functions at zero, with the latter notation indicating that this is equal to the ring of power series convergent in some neighborhood of 0 C n. The fundamental theorem is the following: Theorem (Weierstrass preparation theorem). Let U be an open neighborhood of 0 C n, and let (z 1,..., z n ) = (z, z n ) be coördinates on C n. Let f O(U) be a nonzero holomorphic function. Then after a generic, Clinear change of coördinates there exists an integer s 0 such that lim z n 0 f(0, z n)/z s n (1.2.1) is nonzero and finite. For this choice of coördinates, after possibly shrinking the set U there exist u O(U) and a 1,..., a s O(U C n 1 {0})
7 3 with u nonvanishing on U and a 1 (0) = = a s (0) = 0 such that f(z) = u(z)(z s n + a 1 (z )z s 1 n + + a s (z )). (1.2.2) A consequence of this is: Theorem (Weierstrass division theorem). Let f O(U), and choose an open neighborhood U of 0 C n with coördinates (z 1,..., z n ) be coördinates on C n such that (1.2.1) and (1.2.2) hold for some s 0. Then for every bounded function g O(U) there exist unique q, r O(U) with r polynomial in z n of degree at most s 1 such that f = qg + r. Using these theorems it can be shown that O n is a noetherian unique factorization domain. Moreover the embedding O n 1 [z n ] O n is inert, in the sense that f O n 1 [z n ] is irreducible if and only if it is irreducible in O n. Now let U be any open neighborhood of 0 C n. We say that a subset A U is analytic if for each x A there exists an open neighborhood V of x in U and holomorphic functions f 1,..., f r O(V ) such that A V = {z V : f 1 (z) = = f r (z) = 0}. We define an equivalence relation on the power set of C n as follows: A B if and only if there exists a neighborhood V of 0 in C n such that A V = B V. We write (A, 0) for the equivalence class of a subset, called the germ of the subset at 0. We write (A, 0) (B, 0) if for some neighborhood V of 0 we have A V B V, and we define the union of germs (A, 0) (B, 0) to be the germ of the union (A B, 0). Let (A, 0) be a germ of an analytic subset of an open subset U C n. If f 0 O n then we say that f 0 vanishes on (A, 0) if there exists an open neighborhood V of 0 in C n such that f 0 and (A, 0) have representatives f and A respectively in V and f(z) = 0, z A V. We then associate to (A, 0) an ideal I A,0 O n, which is the ideal of all the germs in O n that vanish on (A, 0). In the other direction, if I O n is an ideal, then since O n is noetherian we can choose generators f 1,0,..., f r,0. Then there exists some neighborhood V of 0 in C n such that these germs have representatives f 1,..., f r in V, and we define an analytic subset of V by V (I) = {z V : f 1 (z) = = f r (z) = 0}. Now the germ (V (I), 0) does not depend on the choice of V or the generators of I. We call (V (I), 0) the zero locus of I. We call a germ of an analytic set (A, 0) irreducible if whenever we have an expression (A, 0) = (A 1, 0) (A 2, 0) there exists i {1, 2} such that (A, 0) = (A i, 0). Then every germ (A, 0) can be uniquely written as a finite unordered union of irreducible germs (A, 0) = r (A i, 0) i=1 provided we demand that (A i, 0) (A j, 0) for j i. These are called the components of (A, 0), or, if A is a
8 4 representative of (A, 0) in some open set, they are the local components of A at 0 (to be compared with the global components of a complex space below). A germ of an analytic set (A, 0) is irreducible if and only if the ideal I A,0 is prime. An analytic description of irreducible germs of analytic sets is given by the following: Theorem (Local parametrization theorem). Let A be an irreducible analytic subset of an open neighborhood of 0 in C n. After a nonsingular, Clinear change of coördinates on C n there exist d n, polydisks C d {0}, {0} C n d and a proper analytic subset S such that (i) A S := A (( \S) ) is a smooth, ddimensional complex manifold, dense in A ( ); (ii) The projection A S \S is a connected covering with a finite number of sheets. We also have the nullstellensatz: Theorem (Nullstellensatz). If I O n is an ideal and (A, 0) is the zero locus of I then the ideal of A is given by I A,0 = I, the radical of I. A corollary of local parametrization is that if A is an analytic subset of an open set U C n then the set of regular points, i.e. the set of points at which A is locally a complex manifold, is a open dense subset of A. In fact, the singular points of A, i.e. all the nonregular points, form an analytic subset of A. 1.3 Complex Spaces Topologically we define complex spaces by gluing together analytic subsets of open sets in C n. Our definition is that of [5]; what we call a complex space is the complex analytic scheme of [4]. That is, we allow for nilpotent elements in the structure sheaf of complex spaces. We suppose throughout that complex spaces are Hausdorff; this is mainly needed in chapter 3. First we fix our notation for ringed spaces. Definitions A ringed space is a pair (X, O X ) where X is a topological space and O X is a sheaf of rings on X. In the main text we usually write X in place of (X, O X ) and write X top for the topological space when we wish to make the distinction. We say that (X, O X ) is a locally ringed space if for all x X the stalk O X,x is a local ring, and a locally Cringed space if in addition the sheaf of rings O X is a sheaf of Calgebras and the residue field of O X,x is C. A morphism of ringed spaces is given by (f, f # ) : (X, O X ) (Y, O Y ) where f : X Y is a continuous map and f # : O Y f O X is a morphism of sheaves of rings. If moreover for each x X, if m X,x and m Y,f(x) are the maximal ideals of the local rings O X,x and O Y,f(x) respectively then f # x (m Y,f(x) ) m X,x where f x # denotes the morphism induced on stalks by f #, then (f, f # ) is a morphism of locally ringed spaces. In general we write f : X Y for a morphism of complex spaces, reserving the notation f # for when it is needed, and if x X we write f(x) for image point in Y. A morphism of ringed spaces between locally Cringed spaces is also a morphism of locally ringed spaces: the induced Calgebra homomorphisms are always homomorphisms of local rings. The basis of our construction of complex spaces is the following two theorems.
9 5 Theorem (Oka s coherency theorem). If M is a complex manifold, then the sheaf of rings O M coherent. is Theorem If U C n is an open set, and F is a coherent O U module, then supp(f) = {x X : F x 0} is an analytic subset of U. Let U C n be an open set, and let I O U be a coherent sheaf of ideals. Let A be the analytic set A = supp(o U /I). Then we call the locally Cringed space (A, (O U /I) A ) a local model of complex analytic space. Definition A complex space (X, O X ) is a ringed space over a Hausdorff topological space X such that there exists an open cover (Ω λ ) of X and isomorphisms of Cringed spaces Φ λ : (Ω λ, O X Ωλ ) (A λ, (O Uλ /I λ ) Aλ ) where (A λ, (O Uλ /I λ ) Aλ ) is a local model of complex analytic space. Note that a complex space is necessarily a locally ringed space. Locally a morphism of complex spaces is determined be the morphism of sheaves in the following sense. If φ : O Y,y O X,x is a Calgebra homomorphisms between local rings of complex spaces X and Y, then there exists an open set U X containing x and a morphism of complex spaces h : X U Y such that h # x = τ, and if g : X V Y is another morphism with this property then there is a neighborhood W U V of x such that h W = g W. In general the morphism of sheaves is not determined by the map of topological spaces unless the domain is reduced. From the coherency theorem of Oka above we deduce the general result: for any complex space X the structure sheaf O X is coherent. Let us state: Theorem Let X be a reduced complex space. The set of regular points X reg of X is a dense open subset of X, which is a disjoint union of connected complex manifolds: X reg = α X α If X α is the closure of X α in X then X = α X α and (X α ) is locally finite. We call the X α the global irreducible components of X. Further constructions on complex spaces are recalled in later chapters as they are encountered.
10 Chapter 2 Blowings up 2.1 Blowing up plane curves The blowing up X 0 of C 2 at 0 is defined by X 0 = {(p, l) : p l} C 2 P 1. We have a natural map π : X 0 C 2 given by the restriction of the projection. This is called the blowing up map. Let (x, y) be coördinates on C 2, and for (u, v) (0, 0) let us identify the line vx yu = 0 with the point [u : v] P 1. Then we see that: X 0 = {((x, y), [u : v]) C 2 P 1 : xv = yu}. Now we write P 1 = U 1 U 2 where U 1 = {[1 : v] : v C} and U 2 = {[u : 1] : u C}. Then we have: and X 1 := X 0 (C 2 U 1 ) = {((x, xv), [1 : v])} X 2 := X 0 (C 2 U 2 ) = {((yu, y), [u : 1])} are isomorphic to C 2 with coördinates (x, v) and (y, u) respectively. Moreover we see directly that π is an isomorphism outside π 1 (0) and E = π 1 (0) = {0} P 1 ; the latter is called the exceptional divisor. If C C 2 is a curve then the strict transform of C by π is C = π 1 (C\{0}). The idea is that the blowing up π pulls apart the directions through the origin, so if we take two lines with gradients a and b passing through 0, their strict transforms will be two disjoint lines meeting E at the points
11 7 ((0, 0), [1 : a]) and ((0, 0), [1 : b]) respectively. To see this, assume for simplicity that a 0, and consider π 1 ({y ax = 0 : x 0}). In the chart X 1 the set of points projecting to the line minus zero is: {((x, ax), [1 : a]) : x C\{0}} and in the chart X 2 it is: {((y/a, y), [1 : a]) : y C\{0}}. Therefore the strict transform C of C is a line intersecting E at the point [1 : a]. In this way blowing up allows us to desingularize curves. For example, take the curve C = {y 2 x 2 + x 3 = 0} C 2, with a double point at (0, 0). We factorize C near 0 into y = ±x 1 x, where 1 x is a fixed branch of the square root. We can locally invert each branch to give x = f + (y) and x = f (y) with f + and f holomorphic functions. Now, considering for example f + we have y = f + (y) 1 f + (y), and dividing through by y and letting y 0 we have that f + (y)/y 1 as y 0, since f + (0) = 0. Now in the chart X 2 we have: y 2 = x 2 x 3 π 1 X 2 (C\{0}) = {((f + (y), y), [f + (y)/y, 1]) : y 0}. Letting y 0 in this expression we see that the intersection of the strict transform of this branch with the exceptional divisor can be seen in the chart X 1. We see in the same way that this is also true for the branch x = f (y). Therefore we regard the chart X 1, where we have: π 1 X 1 (C\{0}) = {((x, ±x 1 x), [1, ± 1 x]) : x 0}. We see that C intersects E at two points, [1 : 1] and [1 : 1] (which is what we showed before in the chart X 2 ); viewing C as a union of two graphs, we have one point of intersection for each branch. With v = ± 1 x, in the (x, v)plane the curve C is transformed to the curve C = {x = 1 v 2 }, with the two points of intersection with E corresponding to x = 0, v = ±1, which are smooth points of C. In this example, the blowing up provides a mechanism by which we can realize the curve C as the shadow of a smooth curve in X 0, which is itself embedded in the higher dimensional space C 2 P Blowing up complex spaces Let X = { (z, l) C n P n 1 : z l }.
12 8 We find that X = {((z 1,..., z n ), [l 1 : : l n ]) : z i l j l i z j = 0, 0 i < j n}. ( ) n + 1 This is a set of equations, so X is a complex space. In fact X is a complex manifold of dimension 2 n. Let U j = {l j = 1} be an affine chart of P n 1. Then we have local trivializations X Uj Uj C (z, l) (l, z j ) (2.2.1) Here we describe a point of X by the direction of the line l, and the displacement along this line in the z j direction; by our choice of chart we cannot have z j constant in the direction l. Now let π : X C n be the restriction of the projection. Then π X\π 1 (0) : X\π 1 (0) C n \{0} is an isomorphism, and we have π 1 (0) = {0} P n 1. In the same way as for the two dimensional case, the exceptional divisor parametrizes the directions through the origin in C n. Definition Let X be a complex space, and let Y be a closed subspace with ideal sheaf I. Then π : X X is the blowing up of X with centre Y if (i) IO X is an invertible O X module; (ii) For any morphism of complex spaces f : T X, such that IO T exists a unique h : T X such that π h = f is an invertible O T module, there!h T X π f X Remark Here π 1 I is a π 1 O X module as a sheaf on X. Moreover there is a natural morphism π 1 O X O X, and we let IO X be the ideal generated by the image of π 1 I. Recall that π I is defined to be π 1 I π 1 O X O X. Now we have an inclusion I O X which gives a morphism π I π O X = O X such that IO X is the image of π I under this morphism. Now the map π I IO X is surjective by definition so if σ : X X is another morphism of complex spaces, σ 1 π I σ 1 IO X is surjective, since σ 1 is an exact functor. Tensoring over σ 1 O X we have a surjective map σ π I σ (IO X ). Now we find: (IO X )O X = im(σ (IO X ) O X )
13 9 = im(σ π I O X ) = im((π σ) I O X ) = IO X where in the last line IO X is defined via the map π σ. Thus this construction commutes with composition of mappings. Moreover we see easily that if I 1, I 2 are coherent sheaves of ideals of O X then: (I 1 I 2 )O X = (I 1 O X )(I 2 O X ). Indeed, both are locally generated as ideals of O X by products of the form π w 1 π w 2. If follows from the definition that if the blowing up exists, then it is unique up to unique isomorphism. We call π 1 (Y ) the exceptional divisor. From the definition of the blowing up π : X X with centre Y, it follows that if U X is an open subset, then π π 1 U : π 1 (U) U is the blowing up with centre Y U. Proposition The morphism π : X C n defined above is the blowing up with centre 0 = supp(o C n/(z 1,..., z n )). Proof. (i) The ideal of 0 in C n is I = (z 1,..., z n ), and we want to see that (z 1,..., z n ) O X is invertible (here π is just a projection, so we write z i also for the image of z i O C n in O X ). Since X is defined by the equations z i l j z j l i, in the chart X k = {l k = 1}, we have for each j k that z j = l j z k, and therefore I is generated by z k in X k. The isomorphism (2.2.1) shows that z k is a nonzerodivisor in O Xk. (ii) Let f : T C n any map such that IO T is an invertible O T module. Let f # : O C n f O T be the comorphism. At a given point w T, the ideal (IO T ) w in the local ring O T,w is generated by f # (z 1 ),..., f # (z n ). In a local ring, if a finite number of elements generate a principal ideal, then some element among this set of generators is a generator of the ideal. Therefore there exists i such that f # (z i ) generates (IO T ) w. By coherence, it follows that f # (z i ) generates IO T in a neighborhood of w. Thus we have an open cover U 1,..., U n of T such that f # (z i ) generates IO Ui. By uniqueness of the blowing up and the statement preceding this proposition, it suffices to establish existence of the required factorization on the sets U i and the intersections U i U j. In particular we have reduced to the case when there exists i such that f # (z i ) generates IO T. Now by hypothesis IO T is invertible, so there exist sections ξ ij of O T for each i, j with j i such that f # (z j ) = f # (z i )ξ ij. We define ξ ii = f # (z i ) and a map h : T X i = {l i = 1} by: h # (z j ) = f # (z j ), h # (l j ) = ξ ij. First we see that h # does indeed define a morphism of Calgebras: in X i we have the equations z j = l j z i, and h satisfies: h # (z j ) = f # (z j ) = f # (z i )ξ ij = h # (l j )h # (z i ). We deduce that: h # (l k )h # (z j ) = h # (l j )h # (l k )h # (z i ) = h # (l j )h # (z k )
14 10 in X i for each j and k. By construction, we have the identity: h # π # = f # since π # maps z j to the class of z j in O X for each j. This establishes existence of h. Now let g : T X satisfying the same property. We want to see that g = h. First we claim that the image of g is contained in X i. If not, then there is t T such that g # (l i )(t) = 0 and for some j, g # (l j )(t) 0, where we regard sections as functions via the morphism O W C W from the sheaf of rings to the sheaf of continuous functions defined for any complex space W. That is, g # (l j ) / m = m(o T,t ) and g # (l i ) m where m is the maximal ideal of O T,t. We have the relation g # (z i )g # (l j ) = g # (z j )g # (l i ) (2.2.2) which, since g # (z j ) = f # (z j ) for all j, shows that f # (z i ) f # (z j )m. Now f # (z i ) generates (f # (z 1 ),..., f # (z n ))O T,t, so this implies that f # (z j ) f # (z j )m, i.e. f # (z j ) = 0 by the Krull intersection theorem. Now by (2.2.2) this means that f # (z i )g # (l j ) = 0 in a neighborhood of t, and since g # (l j ) / m this means that also f # (z i ) = 0 in a neighborhood of t, which is impossible. Thus g maps into X i. Now the relations z j = z i l j in X i mean that g # (l j ) = ξ ij with f # (z j ) = f # (z i ) ξ ij so we have: f # (z i )(ξ ij ξ ij ) = 0. Now by assumption f # (z i ) generates the invertible ideal IO T, so this implies that ξ ij = ξ ij, and therefore g # = h #. Remark It follows from this proposition that the blowing up X C n constructed above is not dependent on the choice of coördinates on the linear space C n. Let us recall now the universal property for the fibre product of two complex spaces via morphisms φ 1 : X S, φ 2 : Y S. This is the complex space X S Y with canonical projections pr 1 : X S Y X, pr 2 : X S Y Y such that if Z is any complex space, and f : Z X, g : Z Y are two mappings such that φ 1 f = φ 2 g, then there exists a unique map h : Z X S Y such that f = pr 1 h and g = pr 2 h. Z f h g X S Y pr2 Y pr 1 φ 2 X φ 1 S We recall the following particular cases of fibre products: if S is a single point then the fibre product is the direct product. If Y = S is an open (resp. closed) subset of S and φ 2 is the inclusion then X S S is the inverse image φ 1 1 (S ) of S in X. For any X, Y, S, the fibre product X S Y is a closed subspace of the direct product X Y. Proposition If π : X X is the blowing up with centre Y, and V is any complex space, then π id V : X V X V is the blowing up with centre Y V.
15 11 Proof. Denote by pr 1 and pr 2 the canonical projections for X V, and pr 1, pr 2 those for X V. Let I be the ideal sheaf of Y in O X. The ideal IO X V is the ideal of Y V in the product space, so to verify the first part of the definition of the blowing up we want to see that (IO X V )O X V is invertible. We have pr 1 (π id V ) = π pr 1, so: (IO X V )O X V = (IO X )O X V. (2.2.3) By hypothesis IO X is invertible, and X V X is simply a projection, so the right hand side of (2.2.3) is invertible. To see this, we identify V locally with an analytic subset of a neighborhood of 0 C r. Then we can write the local ring of X V at the point x 0 as a quotient of O X,x{u 1,..., u r } such that O X,x injects into this quotient. Then the comorphism of the projection is the embedding O X,x O X,x{u 1,..., u r } followed by the canonical projection to the quotient defining X V at x 0. It is immediate that a nonzerodivisor maps to a nonzerodivisor via this mapping. Now let f : T X V be any morphism such that (IO X V )O T = IO T is invertible (here IO T is generated with respect to the morphism pr 1 f). Then by the universal property for the blowing up π : X X applied to pr 1 f, there exists a unique morphism g : T X such that pr 1 f = π g. By the universal property for the product X V, there is a unique h : T X V such that pr 1 h = g and pr 2 h = pr 2 f. That is, we have a diagram: T h g pr 2 f X V pr 1 X pr 2 V Now in the following diagram, noting that pr 1 f = π g, we see that placing both f and (π id) h along the arrow T X V in the following diagram make it commute: T h g pr 2 f X V pr2 V X pr π id 1 X V π X id V Therefore by the uniqueness statement of the universal property of X V we have (π id) h = f.
16 The strict transform Let us recall the definition of the inverse image of a complex space. If f : X X is a morphism of complex spaces, and Z U X is a locally closed subspace of X, then π 1 (Z) is the closed subspace of π 1 (U) defined by the coherent sheaf of ideals (IO X ) π 1 (U) = IO π 1 (U). This is compatible with the pointset theoretic inverse image under a mapping, and we have an induced morphism of complex spaces f : π 1 (Z) Z. Proposition Let π : X X be the blowing up with centre Y. Let Z be any locally closed complex subspace of X. Then there exists a unique smallest closed subspace Z π 1 (Z) such that Z \π 1 (Y ) = π 1 (Z)\π 1 (Y ). Moreover, π induces the blowing up π : Z Z with center Z Y. Definition The space Z in the proposition is called the strict transform of Z by the blowing up π. We will prove this proposition after a series of lemmas. The first two are coherency theorems for analytic sheaves. The third is a simple consequence of the nullstellensatz, a proof is indicated in [1]. Lemma Let X be a complex space, let O = O X, and let A be a coherent Omodule. Then there is a canonical homomorphism α : O Hom O (A, A) : f x [s x f x s x ] In particular we have: ker α = Ann (A) := x Ann (A x ), and Ann (A) is coherent. Proof. Let x X. An element s x Hom O (A, A) x is the germ of a section s Hom O (A, A)(U) for some neighborhood U of x. Since two such sections induce the same morphism A x A x we obtain a morphism τ : Hom O (A, A) x Hom Ox (A x, A x ). We show that τ is an isomorphism. Let φ : A U A U such that the germ φ x at x is zero. Since A is coherent, we have that φ is zero in a neighborhood of x, so τ is injective. Now let σ : A x A x be a homomorphism. Take a neighborhood U of x and sections s 1,..., s p that generate A U. Possibly shrinking U, let t 1,..., t p sections such that we have σ(s j ) = t j for j = 1,..., p. Now let R(s 1,..., s p ) be the sheaf of relations between the s j. By coherency, this is generated by relations: (f ij ) i=1,...,p O(U) p, j = 1,..., q where again we possibly shrink U. Now we claim that these generators belong to R(t 1,..., t p )(U) for some possibly smaller U. Indeed, at the point x, since σ is an O x homomorphism A x A x ( p ) f ij t i i=1 x ( p ) = σ f ij s i = σ(0) = 0 i=1 x
17 13 Thus we have that the homomorphism φ U : A U A U defined by φ y ( p i=1 a i s i ) y ( p ) = a i t i i=1 y This gives the required map, i.e. we have φ x = σ. This shows that τ is an isomorphism. Now we can show that Hom(A, A) is coherent. In a neighborhood of each point we have a resolution O p U Oq U A U 0. Then we have: 0 Hom(A U, A U ) Hom(O q U, A U ) Hom(O p U, A U ). Taking stalks we have that this is exact, since Hom O (A, A) x = Hom Ox (A x, A x ), so we may apply the result of commutative algebra to this effect. This sequence is: 0 Hom(A U, A U ) A q U Ap U. Therefore Hom(A, A) is coherent. It now follows that Ann (A) is coherent, as it is the kernel of a morphism of coherent sheaves. Lemma Let X be a complex space and let A be a coherent O X module. Let (A i ) i I be a directed family of coherent O X submodules of A, that is, such that for all i, j I, there exists k I such that Then A i + A j A k. B = i I A i is a coherent sheaf, locally isomorphic to some A i in a neighborhood of each point. This is a direct result of the strong noetherian property of A: every increasing sequence of coherent subsheaves of A is eventually stationary on every compact subset of X [4]. Lemma Let S be a closed subspace of a complex space X, with ideal sheaf I O X. Suppose further that Ann OX I = {0}. Then the following property is satisfied: If X is any closed subspace of X such that X \S = X\S, then X = X. Proof of proposition Let I be the ideal sheaf of Y in X. Let U X be open such that Z is closed in U, and let J O U be the ideal sheaf of Z. Let T = π 1 (Z), and: B = m 1 Ann OT (I m O T ). By the first two lemmas, this is a coherent sheaf. Let Z T be the closed subspace defined by B. First we show that T \π 1 (Y ) = Z \π 1 (Y ). Indeed, let y Z; if y / Y then we must have I = O X in a
18 14 neighborhood of y. We have by definition that J O π 1 (U) is the ideal sheaf of T, and by the above we have that IO T is the ideal generated by O X in O T in a neighborhood of each point of T \π 1 (Y ). In particular it is the unit ideal (1). Thus by the definition of B, the stalk of B is zero at each y T \π 1 (Y ) so we have T \π 1 (Y ) = Z \π 1 (Y ). Next we show that Ann OZ (IO Z ) is zero. Let j : Z T be the closed immersion of Z in T. The ideal IO Z is by definition the ideal generated by the image of j 1 (IO T ) = (IO T ) Z along the mapping: j 1 O T = (O T ) Z O Z = (O T /B) Z which on stalks is given by the canonical projection. Therefore the preimage of Ann OZ (IO Z ) in O T is contained in the union defining B, so Ann OZ (IO Z ) = (0). Now we apply the third lemma above (taking S = π 1 (Y ) Z ) to see that Z is the smallest subspace equal to T outside of π 1 (Y ). Now let us show that π : Z Z is the blowing up with centre Z Y. By hypothesis, IO X is locally principal, so mapping this to IO Z via Z T π 1 (U) X we have that IO Z is locally principal. We have just shown that Ann OZ (IO Z ) = 0, so IO Z is generated by a nonzerodivisor, i.e. it is invertible. Now let f : F Z be any morphism such that IO F is invertible. Composing f with Z U X we obtain g : F X such that π g = f. To conclude we show that g in fact maps into Z. Since f is a mapping to Z, we must have that g(f ) π 1 (Z) =: T. We want to factor g through the inclusion j : Z T, that is, we seek h making the following commute: Z j T π Z h g f F The ideal of the inverse image of Z is BO F so we want to see that BO F = 0 (for example the fibre product diagram of g : F T and Z T then gives the required factorization since we then have g 1 (Z ) = F ). But by hypothesis IO F is invertible, so Ann OF (I m O F ) = 0 for each m N, and we have m Ann OF (I m O F ) m Ann OT (I m O T )O F 2.4 Existence of blowings up First let n = d + m > d be an integer, and let C d = C d {0} C d C m = C n be the inclusion. Let π : U C m be the blowing up of C m at zero constructed above. By proposition 2.2.5, we have that Π = id π : C d U C n (2.4.1)
19 15 is the blowing up of C n at C d = C d {0}. Moreover as observed in remark 2.2.4, the blowing up is independent of the choice of coördinates, as is the product mapping id π, so this construction depends only on the inclusion C d C n as complex spaces. Now let us pass to the general case. We take a complex space X and a closed subspace Y. Since existence is a local question, we may assume that there is an open set Ω C d and f 1,..., f m O(Ω) such that X = {z Ω : f 1 (z) = = f m (z) = 0} and the ideal sheaf I Y = (f 1,..., f m ). Now we define an embedding X C n = C d C m as follows. Write z = (z 1,..., z n ) = ((z 1,..., z d ), (z d+1,..., z d+m )) = (z, z ). Let A = { (z, z ) C d C m : z X, z d+j = f j (z ), j = 1,..., m } and define the embedding of X as the composition of the inclusions X A and A C d C m. In this way we have Y = X C d, where we view all spaces as their embeddings in C n. Letting Π be the mapping (2.4.1), and X the strict transform of X by the blowing up Π. By proposition 2.3.1, we obtain that π = Π X : X X is the blowing up with center Y. We have nearly proved Theorem For every (Hausdorff) complex space X and every closed subspace Y of X, there exists a mapping π : X X with the following properties: (i) π is the blowing up of X with centre Y ; (ii) π is an isomorphism X \π 1 (Y ) X\Y ; (iii) π is proper; (iv) X is Hausdorff. Proof. It remains to prove (ii)(iv). For (ii), we recall that the existence of the blowing up is local on X, so we have that π X \π 1 Y is the blowing up of X\S with centre. Moreover the identity mapping also satisfies the universal property for the blowing up of X\S with centre, so we have that π is an isomorphism outside π 1 Y. To prove (iii) and (iv), we note that in the local construction preceding the theorem, the blowing up is a proper mapping from a Hausdorff space (since the blowing up of C n at a point is proper, so the product map (2.4.1) is proper, and therefore so is the restriction to a closed subspace). Since X is Hausdorff if and only if X is locally Hausdorff, (iv) follows. It remains therefore to show that π is proper, assuming the result in a neighborhood of each point. Let K X be compact. Choose finite open covers (U j ) and (V j ), j = 1,..., l of K such that U j V j for all j, the U j are relatively compact in the V j and π Vj is proper for each j. Then we can write π 1 (K) = l π 1 (K U j ) j=1
20 16 which is compact. Remark Note that by our construction, if the centre Y of the blowing up π : X X is the whole space X, then X is empty, since the smallest closed subspace Z of X such that Z \π 1 (X) = π 1 (X)\π 1 (X) = is of course the empty set. 2.5 Further properties of blowings up Lemma Let A be a local ring, and let I 1, I 2 be ideals in A. nonzero divisor in A, then the same is true for each of I 1, I 2. If I 1 I 2 is principal, generated by a Proof. Pick generators g ij, of I i, i = 1, 2. Then the g 1j g 2k generate I 1 I 2, and since A is local, we can choose g 1 g 2 among these elements that generates I 1 I 2. By our hypotheses g 1, g 2 are nonzerodivisors. We have I 1 I 2 = g 1 g 2 A g 2 I 1 I 1 I 2 so g 1 g 2 A = g 2 I 1, and therefore g 1 A = I 1, and similarly g 2 A = I 2. Corollary Let Y 1, Y 2 be closed subspaces of a complex space X, with ideal sheaves I 1 and I 2 respectively. Then (i) For each morphism f : T X, the ideal sheaf I 1 I 2 O T I 1 O T and I 2 O T are invertible as O T modules; is invertible as an O T module if and only if (ii) If Y 3 is the closed subspace defined by I 1 I 2 and π α : X α X is the blowing up with centre Y α for α {1, 2, 3} then we have a diagram: X 2 q 2 π 2 X 3 π 3 X q 1 X 1 π 1 Proof. (i) follows immediately from the lemma. (i) = (ii): By the definition of the blowing up, I 1 I 2 is invertible as an O X3 module, so by the lemma I 1 O X3 and I 2 O X3 are invertible O X3 modules. Now by the universal properties for the blowings up π 1 and π 2 we have mappings q 1 and q 2 making the diagram commute. Let us admit the following property relating the blowing up along a closed subspace to the blowing up along the inverse image of this subspace by a morphism, with the resulting complex space sitting inside the fibre product. A proof, in the algebraic case, is given in [6].
21 17 Proposition Let π : X X be the blowing up of a complex space X along a closed subspace Y. Let f : V X be a morphism of complex spaces. Let pr 1 : V X X V pr 2 : V X X X be the projections. Let Z = f 1 (Y ), and let V be a minimal complex subspace of V X X containing pr 1 1 (V \Z). Then τ : V V obtained by restricting pr 1 is the blowing up of V along Z. τ 1 (Z) = F V V X X pr 2 X τ pr 1 π Z V f X We will see below that the minimal complex subspace V in this proposition is unique. 2.6 Local blowings up Let U X be an open subset of a complex space X. Let E U be a closed subset of U. Let π : U U be the blowing up with centre E. Then the composition σ : U U X is called a local blowing up, and is denoted by the triple σ = (U, E, π). By a finite sequence of local blowings up we mean a composition: σ m 1 σ W m Wm 1 W 0 1 W0 where each σ i is a local blowing up. Now let f : V W be any morphism of complex spaces and let σ : W W be a local blowing up. We have a local blowing up σ : V V defined by (f 1 (U), f 1 (E), π ). We claim that there exists a unique f : V W making the following diagram commute: V σ V f f W σ W To see this, let I be the ideal sheaf of E in U. Then IO f 1 (U) is the ideal sheaf of f 1 (E). Since σ is the blowing up with centre f 1 (E), the ideal sheaf IO V is an invertible O V module. Therefore by the universal property for σ there exists a unique f : V W such that the diagram above commutes. We call f the strict transform of f by σ. If σ = σ 0 σ m 1 is a finite sequence of local blowingsup, σ i : W i+1 W i then the strict transform of f by σ is given by the sequence of strict transforms: σ m 1 σ V m 0 V 1 V = V 0 f m f 1 f=f 0 σ m 1 σ W m 0 W 1 W = W 0 where at each step we take the strict transform of f i by σ i to obtain f i+1, and if σ i = (U i, E i, π i ) then σ i 1 = (fi (U i ), f 1 i (E i ), π i ). Now by the universal property for the fibre product, we have a mapping κ m : V m V W W m such that:
22 18 V m f m σ 0 σ m 1 κ m V W W m W m σ V f W We claim that κ m is a closed immersion onto the smallest analytic subspace of V W W m containing V W (W \E), where E is the union of the exceptional divisors in W m. To see this, first suppose that σ = σ 0 = (U 0, E 0, π 0 ) is a single local blowing up. Since im(σ) U 0 we have V W W 1 = f 1 (U) U W 1, so to prove the result we may as well assume that σ 0 = π 0 is a global blowing up with centre E 0. Now by proposition 2.5.3, up to isomorphism V m = V 1 is any minimal complex subspace of V W W 1 containing pr 1 1 (V \f 1 (E 0 )) and σ 1, f 1 are the restrictions of the projections. Moreover if V W W 1 contains several isomorphic minimal closed subspaces containing pr 1 1 (V \f 1 (E 0 )) then the morphism κ m = κ 1 is not unique, so we may identify V 1 with the unique smallest closed subspace of the fibre product containing pr 1 1 (V \f 1 (E 0 )). Now we may take as κ 1 the inclusion V 1 V W W 1, so the result follows. The general case σ = σ 0 σ m 1 now follows by induction on m 1.
23 Chapter 3 La voûte étoilée 3.1 The category C(W ) First of all we define the notion of a strict morphism. This is a generalization of the notion of a finite sequence of local blowingsup, which has the advantage of a concrete definition that proves easier to work with. Definition A morphism f : V W of complex spaces if strict if there exists a closed complex subspace F V such that (i) f is an isomorphism outside the set F ; (ii) If V V is a closed subspace such that V \F = V \F, then V = V. The following property is central to our use of strict morphisms: Lemma Let Y X be a closed subspace of a complex space X. Let I be the ideal sheaf of Y in O X. Let π : X X be the blowing up with centre Y. If f : V X is any strict morphism of complex spaces then (i) there exists a most one morphism f : V X such that π f = f X π X f f V (ii) if such an f exists, then IO V is an invertible O V module and f is also strict Proof. First we note that by the universal property of the blowing up π, in order to show uniqueness of f, it suffices to show that IO V is an invertible O V module. First let us apply the definition of a strict morphism to f, denoting by F the closed subspace of V thus obtained. Let v V \F be any point outside F. By hypothesis f is an isomorphism in a neighborhood of v, so we can choose an open subset v U V \F such that f U : U f(u) is an isomorphism onto an open subset f(u) of X. We claim that f U is a locally
24 20 closed embedding. Since f(u) is open in X, M := π 1 (f(u)) is open in X. Moreover f U is proper: if K X is compact then π(k) is compact (a continuous image of a compact set is compact) and since f U is an isomorphism we have that f 1 π(k) is compact in U. Now (f ) 1 (K) (f ) 1 π 1 π(k) = f 1 π(k) U is a closed subset of a compact subset of U, hence compact. Therefore f U is proper. Now by Remmert s proper mapping theorem [5] we have that f (U) is an analytic subset of M = π 1 (f(u)); in particular it is closed. To show that f U is a locally closed embedding we want to show that f is a bijection onto its image. But we have that f 1 (z) = (f ) 1 π 1 (z), z f(u) is a single point; so f is a bijection onto its image. Next let us show that f (U) = M. We have f (U)\π 1 (Y ) = M\π 1 (Y ) since f is an isomorphism on U and π is an isomorphism outside the preimage of Y. Moreover the ideal sheaf IO X of π 1 (Y ) is locally generated by a nonzerodivisor since π is the blowing up with centre Y, so its annihilator is (0). Thus by lemma 2.3.5, we have f (U) = M. Therefore f is an isomorphism outside F, and F satisfies the minimality condition (ii) by assumption, so we have that f is strict. It remains to show that IO V is invertible. We know that IO V is locally principal, since this is true of IO X. Moreover IO V is invertible outside of V \F, since f is an isomorphism in this set. Therefore the annihilator sheaf Ann OV (IO V ) defines a closed subspace V V such that V \F = V \F, since Ann OV (IO V ) vanishes outside F. Now by the minimality assumption on F we have that V = V i.e. Ann OV (IO V ) = (0). Remark We saw in the proof of lemma that if f exists, then outside F, both f and f are isomorphisms. Therefore if π is not an isomorphism at any point of π 1 (Y ) then we have f 1 (Y ) F, i.e. Y f(f ). In particular if f is the blowing up with centre Y then Y Y. Corollary Let f : V X be a strict morphism of complex spaces. Let I be a coherent sheaf of ideals in O X, which is invertible as an O X module. Then IO V is an invertible O V module. Proof. Let Y X be the complex subspace defined by I. Since I is already invertible, the blowing up with centre Y is just id : X X. Therefore there exists a diagram as in the lemma with f = f: id X X f f V Now by the statement (ii) of the lemma we have that IO V is an invertible O V module. Proposition Let σ = σ 0 σ m 1 : W m W 0 be a finite sequence of local blowingsup with σ i : W i+1 W i. Then: (i) σ is strict; (ii) if f : V W 0 is any strict morphism, then there exists at most one f : V W m such that σ f = f; (iii) if f exists, then f is strict.
25 21 Proof. (i) Recall that if σ : X U X is a local blowing up then we write σ = (U, E, π), where π : X U is the blowing up with centre E. In the situation of the proposition, let σ i = (U i W i, E i U i, π i : W i+1 U i ). Let I i be the ideal sheaf of E i in U i, and let J = i (I io Wm ). Let F be the closed subspace of W m defined by J. Then clearly σ is isomorphic outside F since each σ i is an isomorphism outside E i, and F is the union of the inverse images of the E i. Moreover, I i O Wi+1 is invertible as an O Wi+1 module, and the σ i are each strict morphisms (the blowing up is strict, and composing with the inclusion does not change this) so by the corollary I i O Wm are invertible O Wm modules, and therefore so is J O Wm. This means that the annihilator of J O Wm is zero, so by lemma σ is strict. (ii) and (iii). Suppose that f : V W 0 is strict, and that there exists f : V W m such that σ f = f. We have a diagram: σ m 1 σ W m m 2 σ W m 1 1 σ 0 W 1 W 0 f f V Let f 1 : V W 1 be the morphism obtained as f 1 = σ 1 σ m 1 f. Then we have a commutative triangle W 1 σ 0 W 0 f 1 f V By commutativity of this diagram we have that f(v ) U 0 so we may apply lemma to the blowing up π 0 : W 1 U 1 W 0 to see that f 1 is strict and f 1 is unique assuming that f exists. Thus we have reduced the length of the diagram we must consider by 1: σ m 1 σ W m m 2 σ W m 1 1 W 1 f f 1 V Therefore we may repeat the process inductively with the morphisms f i = σ i σ m 1 f, i = 1,..., m 1, and at the last step we see that f is unique assuming f exists, and f is a strict morphism. An important corollary of this proposition is that if σ 1 : W 1 W and σ 2 : W 2 W are two finite sequences of local blowingsup, then there is at most one morphism q : W 2 W 1 such that σ 1 q = σ 2. Definition Let W be a complex space. We define a category C(W ) as follows. An object of C(W ) is a finite sequence of local blowingsup σ : W W, and a morphism q Hom(σ 1, σ 2 ) where σ 1 : W 1 W and σ 2 : W 2 W is a morphism q : W 1 W 2 of complex spaces such that σ 1 = σ 2 q. We therefore have that for any σ 1, σ 2 C(W ) that # Hom(σ 1, σ 2 ) 1. Our aim now is to show that products exist in the category C(W ). That is, given σ 1, σ 2 C(W ) we want to show that there exists σ 3 C(W ) together with q i Hom(σ 3, σ i ), i = 1, 2 such that if τ C(W ) and h i Hom(τ, σ i ), i = 1, 2 then there exists a unique h Hom(τ, σ 3 ) such that:
26 22 q q σ σ 3 σ 2 h 1 h h 2 τ is a commutative diagram. The picture becomes somewhat harder to read if we do not use the simplified notation of the category C(W ). Keeping the same notation suppose that σ i : W i W, i = 1, 2, 3, τ : V W, so the diagram is: W 1 h 1 σ 1 V h W 3 q 1 σ 3 W q 2 h 2 W 2 σ 2 where the morphism τ is not shown. The idea of the proof is quite simple: if σ 1 and σ 2 are each a single local blowing up, then we have already seen a construction of this kind in lemma In the general case we take the square formed by W 3, W 1, W 2 and W in the picture above for each blowing up in the composition, glue them together, and from this define a commutative lattice of such squares. That allows us to apply the case of a single local blowing up inductively to establish the result. Theorem Let σ i : W i W C(W ), i = 1, 2. Then there exists σ 3 : W 3 W such that: (i) there exists q i Hom(σ 3, σ i ), i = 1, 2; (ii) if f : V W is strict, and if h i : V W i such that f = σ i h i, i = 1, 2 then there exists a unique h 3 : V W 3 with q i h 3 = h i, i = 1, 2. Moreover q i C(W i ), i = 1, 2. Let us consider the case where σ 1, σ 2 each consist of a single local blowing up, say σ 1 = (U 1, E 1, π 1 : W 1 W ) and σ 2 = (U 2, E 2, π 2 : W 2 W ). Let I i be the ideal sheaf of E i in O Ui, i = 1, 2. Lemma Let U 3 = U 1 U 2 and let E 3 U 3 be the closed subspace defined by I 1 I 2. Let σ 3 = (U 3, E 3, π 3 : W 3 W ) be the local blowing up with centre E 3. Then σ 3 has the properties (i) and (ii) of the theorem. Proof. We apply lemma to the restrictions of π 1, π 2 to U 3 to obtain the following diagram: q 1 π 1 1 (U 2) = π1 1 (U 3) π 1 W 3 π 3 U 3 q 2 π2 1 (U 1) = π2 1 (U 3) π 2
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