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On Sheaf Theory Lectures on Sheaf Theory by C.H. Dowker Tata Institute of Fundamental Research Bombay 1957 Lectures on Sheaf Theory by C.H. Dowker Notes by S.V. Adavi and N. Ramabhadran Tata Institute of Fundamental Research Bombay 1956 Contents 1 Lecture 1 1 2 Lecture 2 5 3 Lecture 3 9 4 Lecture 4 15 5 Lecture 5 21 6 Lecture 6 27 7 Lecture 7 31 8 Lecture 8 35 9 Lecture 9 41 10 Lecture 10 47 11 Lecture 11 55 12 Lecture 12 59 13 Lecture 13 65 14 Lecture 14 73 iii iv Contents 15 Lecture 15 81 16 Lecture 16 87 17 Lecture 17 93 18 Lecture 18 101 19 Lecture 19 107 20 Lecture 20 113 21 Lecture 21 123 22 Lecture 22 129 23 Lecture 23 135 24 Lecture 24 139 25 Lecture 25 143 26 Lecture 26 147 27 Lecture 27 155 28 Lecture 28 161 29 Lecture 29 167 30 Lecture 30 171 31 Lecture 31 177 32 Lecture 32 183 33 Lecture 33 189 Lecture 1 Sheaves. 1 onto Definition. A sheaf S = (S, τ, X) of abelian groups is a map π : S −−−→ X, where S and X are topological spaces, such that 1. π is a local homeomorphism, 2. for each x ∈ X, π−1(x) is an abelian group, 3. addition is continuous. That π is a local homeomorphism means that for each point p ∈ S , there is an open set G with p ∈ G such that π|G maps G homeomorphi- cally onto some open set π(G). Sheaves were originally introduced by Leray in Comptes Rendus 222(1946)p. 1366 and the modified definition of sheaves now used was given by Lazard, and appeared first in the Cartan Sem. 1950-51 Expose 14. In the definition of a sheaf, X is not assumed to satisfy any separation axioms. S is called the sheaf space, π the projection map, and X the base space. The open sets of S which project homeomorphically onto open sets of X form a base for the open sets of S . Proof. If p is in an open set H, there exists an open G, p ∈ G such that π|G maps G homeomorphically onto an open set π(G). Then H ∩ G is open, p ∈ H ∩G ⊂ H, and η|H ∩G maps H ∩G homeomorphically onto π(H ∩ G) open in π(G), hence open in X. 1 2 Lecture 1 2 π is a continuous open mapping Proof. Continuity of π follows from the fact that it is a local homeomor- phism, and openness follows from the result proved above. −1 The set S x = π (x) is called the stalk of S at x. S x is an abelian group. If π(p) , π(q), p + q is not defined. S x has the discrete topology. Proof. This is a consequence of the fact that π is a local homeomor- phism. Let S × S be the cartesian product of the space S with itself and let S + S be the subspace consisting of those pairs (p, q) for which π(p) = π(q). Addition is continuous means that f : S + S → S defined by f (p, q) = p+q is continuous. In other words, if p, q ∈ S and π(p) = π(q), then given an open G containing p + q, there exist open sets H, K with p ∈ H, q ∈ K such that if r ∈ H, s ∈ K and π(r) = π(s), then r + s ∈ G. We may write this as H + K ⊂ G. Proposition 1. Zero and inverse are continuous. (i) Writing Ox for the zero element of the group S x, zero is continuous means that f : X → S , where f (x) = Ox, is continuous. Proof. Let x ∈ X and let G be an open set containing f (x) = Ox. Then there is an open set G1 such that Ox ∈ G1 ⊂ G and π|G1 is a 3 homeomorphism of G1 onto open π(G1). Since Ox + Ox = Ox, and addition is continuous, there exist open sets H, K with Ox ∈ H, Ox ∈ K such that H + K ⊂ G1. Let L = G1 ∩ H ∩ K, then L is open, Ox ∈ L and π|L is a homeomorphism of L onto open π(L). Then x = π(Ox) ∈ π(L) and if y ∈ π(L) there exists q ∈ L with π(q) = y. Then q ∈ H, q ∈ K and hence q + q ∈ G1. But q ∈ G1, and π|G1 is 1−1; hence q = S y∩G1. Therefore, since q+q ∈ S y ∩G1, q+q = q, hence q = Oy. Thus if y ∈ π(L), f (y) = Oy ∈ L. Thus x is in open π(L), with f (π(L)) = L ⊂ G. Hence f is continuous. (Incidentally we have proved that each Ox is contained in an open set which Lecture 1 3 consists of zeros only and which projects homeomorphically onto an open set of X.) (ii) Writing-p for the inverse of p in the group S π(p) inverse is contin- uous means that g : S → S where g(p) = −p is continuous. Proof. Let p ∈ S , and let G be an open set containing-p. Then there exists open L containing Oπ(p) and consisting of zeros only. Since p+(−p) = Oπ(p) and addition is continuous, there exist open H, K, with p ∈ H, −p ∈ K and H + K ⊂ L. Hence if q ∈ H, r ∈ K, π(r) = π(q), then q + r = Oπ(q), i.e. r = −q. We may assume that π|H is a homeomorphism. Let 1 H1 = (π|H) (π(H) ∩ π(K ∩ G)), then p ∈ H1, and since π is open, continuous, H1 is open. Then if q ∈ H1, there exists r ∈ K∩G with π(r) = π(q); then r = −q = g(q). 4 Thus H1 is open and g(H1) ⊂ G. Hence g is continuous. Corollary 1. Subtraction is continuous. i.i. f : S + S → S , where f (p, q) = p − q is continuous. Corollary 2. The set of all zeros is an open set. Example 1. If X is a topological space, and G is an abelian group fur- nished with the discrete topology, let S = (X × G,π, X) where π(x, g) = x and (x, g1) + (x, g2) = (x, g1 + g2). Each stalk S x is isomorphic to G. Axioms a) and c) are easily verified. This sheaf is called the constant sheaf associated with G. Example 2. Form the constant sheaf (A × Z,π, A) where A is the square {(x, y) : 0 ≤ x ≤ 1, 0 ≤ 0 ≤ y ≤ 1}, and Z is the group of integers. Then identify (x, 0) with (1 − x, 1) in A to get a M¨obius band X, and identify (x, 0, n) with (1 − x, 1, −n) in A × Z to get S . The resulting sheaf S = (S,π, X) is the sheaf of “twisted integers” over the M¨obius band. Each S x is isomorphic to the group of integers. 4 Lecture 1 Example 3. Let X be the sphere of complex numbers. Let S x be the additive group of function elements at x, each function element being a power series converging in some neighbourhood of x. Let S = ∪xS x 5 and define π : S → X by π(S x) = x. If p is a function element, a neighbourhood of p in S is defined by analytic continuation. Then S = (S,π, X) is the sheaf of analytic function elements. Each component (maximal connected subset) of S is a Riemann surface without branch points. The sheaf space S is Hansdorff. Lecture 2 Example 4. Let S : {(x, y) : x2 + y2 = 1, x < 1} together with the group 6 of integers Z}. The topology on the former set is the usual induced topology, and the neighbourhoods for an integer n ∈ Z are given by 2 2 2 2 Ga(n) = {n, (x, y) : x + y = 1, a < x < 1}; X : {(x, y) : x + y = 1} and let π : S → X be defined by π(n) = (1, 0), π(x, y) = (x, y). π−1(1, 0) is the group Z, for other pints (x, y) ∈ X, π−1(x, y) = (x, y) is regarded as the zero group. It is easily verified that S = (s,π, X) is a sheaf. Here S is locally euclidean, and has a countable base. The set of all zeros is open (Corollary 2) and compact but not closed; its closure is not compact, S is T1 but not Hausdorff though X is Hausdorff. Exercise. If X is a To or a T1 space, space, so is S So far we have only defined sheaves of abelian groups. It is now quite clear how we can extend the definition to the case where the stalks are any algebraic systems. A sheaf of rings is a local homeomorphism π : S → X such that each π−1(x) is a ring and addition and multiplication are continuous, i.e f : S + S → S where f (p, q) = p + q, g : S + S → S where g(p, q) = p · q, are continuous. The sheaf of function elements (Example 3) where multiplication 7 of two function elements in the same stalk is defined to be the usual multiplication of power series is a sheaf of rings. In the sheaf of twisted integers (Example 2) each S x is isomorphic to the ring Z, but this sheaf is not a sheaf of rings. 5 6 Lecture 2 A sheaf of rings with unit is a local homeomorphism π : S → X −1 such that each π (x) is a ring with unit element 1x and addition, multi- plication and unit are continuous; i,e., f : S + S → S, f (p, q) = p + q, g : S + S → S, f (p, q) = p · q, h : X −→ S,h(x) = 1x are continuous.
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