By G. Kempf
During this publication, Professor Kempf offers an creation to the speculation of algebraic forms from a sheaf theoretic viewpoint. by way of taking this view he's in a position to provide a fresh and lucid account of the topic, that allows you to be simply obtainable to all beginners to algebraic types.
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Un wbich define a morphism f: X_. P' as before such that X is isomorphic to its image which is a subvariety of P". Such l is just ample if r,®n is very ample for some positive n. 9. mple if X is an affine variety. 4. Let X be a product X1 x X 2. mple invertible sheaves on X 1 and X 2. Then 7r:k,L1®1t':k 3 L2 is ample on X. 5. Let l, and M be invertible sheaves on a variety X. e. there are sections <11, ... , un of l, such that zeroes(u1, ... , un)= O. ®M®" is ample for some n ~ O. 6. Let J:, be an invertible sheaf on a projective variety X C pn.
O. Applying - we see that M =o. Hence M = r(X,O) =o. Tbis shows that ( b) is true. 5. Let X be any variety. r ..... g be an Oxhomomorphiam between two quaai-coherent O x -modulea. Then Ker( t/J) and Cok(t/1) are quaai-coherent Ox-modulea. Proof. The statement is loe~ X so we may assume th~ is afline. Then by the above Ker(t/1) = Ker(r(X, t/1)) and Cok(t/J) = Cok(r(X,t/l)). o Proof. The "if" part is clear. For the converse wc may assume that we ~ave a finite open cover X = UD(/¡) where Mio(/;) = Ñ¡ where N¡ IS a k(X)u,,-module of finite type.
Are in M which span the k(X)u,,-module M(/¡)· Let M1 be the k(X)sub-module generated by the finitely many m¡,;. + M but a is locally surjective by construction. Hence a and consequently r(X,a): Mi-+ Misan isomorphism. 9.! P~ve that a quasi-coherent Ox-module on a variety X wh1ch 1s contamed m a coherent one is also coherent. 1"' on Y. By definition F_:(-)= F(Xn-) with the obvious restrictions and multiplication J ·a= (a /)·a. r~ at x in X. Thus F' is said to be aupported by the set X. ffine.