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In this setup, for example, one can define a tensor field on a smooth manifold M as a (global or local) section of the tensor product (called tensor bundle) () where O is the sheaf of rings of smooth functions on M and the bundles , are viewed as locally free sheaves on M.
If F is an O-module, then the direct image sheaf is an O '-module through the natural map O ' →f * O (such a natural map is part of the data of a morphism of ringed spaces.) If G is an O '-module, then the module inverse image of G is the O-module given as the tensor product of modules:
The tensor product is the direct product of objects, and any terminal object (empty product) is the unit object. The category of bimodules over a ring R is monoidal (using the ordinary tensor product of modules), but not necessarily symmetric. If R is commutative, the category of left R-modules is symmetric monoidal. The latter example class ...
The tensor product of two vector spaces is a vector space that is defined up to an isomorphism.There are several equivalent ways to define it. Most consist of defining explicitly a vector space that is called a tensor product, and, generally, the equivalence proof results almost immediately from the basic properties of the vector spaces that are so defined.
0 (A, B) ≅ A ⊗ R B for any right R-module A and left R-module B. Tor R i (A, B) = 0 for all i > 0 if either A or B is flat (for example, free) as an R-module. In fact, one can compute Tor using a flat resolution of either A or B; this is more general than a projective (or free) resolution. [5] There are converses to the previous statement ...
Mitchell's embedding theorem states every abelian category arises as a full subcategory of the category of modules over some ring. Projective limits and inductive limits exist in the categories of left and right modules. [4] Over a commutative ring, together with the tensor product of modules ⊗, the category of modules is a symmetric monoidal ...
A -module is called a flat module if the tensor product functor is exact. In particular, every projective module is flat. free A free module is a module that has a basis, or equivalently, one that is isomorphic to a direct sum of copies of the scalar ring .
Functors are often defined by universal properties; examples are the tensor product, the direct sum and direct product of groups or vector spaces, construction of free groups and modules, direct and inverse limits. The concepts of limit and colimit generalize several of the above.