Locally free sheaf
A sheaf of modules that is locally isomorphic to the direct sum of several copies of the structure sheaf. More precisely, let be a ringed space. A sheaf of modules over is said to be locally free if for every point there is an open neighbourhood , , such that the restriction of to is a free sheaf of modules over , that is, it is isomorphic to the direct sum of a set of copies of the structure sheaf . If is connected and is finite, for example consisting of elements, then does not depend on the point and is called the rank of the locally free sheaf . Let be a vector bundle of rank on and let be the sheaf of germs of its sections. Then is a locally free sheaf of rank . Conversely, for every locally free sheaf of rank there is a vector bundle of rank on such that is the sheaf of germs of its sections (see , ); hence there is a natural one-to-one correspondence between the isomorphy classes of locally free sheaves of rank and the isomorphy classes of vector bundles of rank on .
Example. Let be a smooth connected algebraic variety of dimension . Then the sheaf of regular differential forms is a locally free sheaf of rank .
Let , a connected affine scheme, be the spectrum of the commutative ring (cf. Spectrum of a ring), let be a locally free sheaf of rank and let be the -module of its global sections. Then the -module is projective and the mapping establishes a one-to-one correspondence between the set of classes (up to isomorphisms) of locally free sheaves of rank and the set of classes (up to isomorphisms) of projective -modules of rank (see ).
|||R. Godement, "Topologie algébrique et théorie des faisceaux" , Hermann (1958)|
|||R. Hartshorne, "Algebraic geometry" , Springer (1977)|
Locally free sheaf. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Locally_free_sheaf&oldid=15760