In algebra, a module homomorphism is a function between modules that preserves the module structures. Explicitly, if M and N are left modules over a ring R, then a function is called a module homomorphism or an R-linear map if for any x, y in M and r in R,
If M, N are right R-modules, then the second condition is replaced with
The pre-image of the zero element under f is called the kernel of f. The set of all module homomorphisms from M to N is denoted by HomR(M, N). It is an abelian group (under pointwise addition) but is not necessarily a module unless R is commutative.
The composition of module homomorphisms is again a module homomorphism. Thus, all the (say left) modules together with all the module homomorphisms between them form the category of modules.
A module homomorphism is called an isomorphism if it admits an inverse homomorphism; in particular, it is a bijection. One can show a bijective module homomorphism is an isomorphism; i.e., the inverse is a module homomorphism. In particular, a module homomorphism is an isomorphism if and only if it is an isomorphism between the underlying abelian groups.
The isomorphism theorems hold for module homomorphisms.
A module homomorphism from a module M to itself is called an endomorphism and an isomorphism from M to itself an automorphism. One writes for the set of all endomorphisms between a module M. It is not only an abelian group but is also a ring with multiplication given by function composition, called the endomorphism ring of M. The group of units of this ring is the automorphism group of M.
Schur's lemma says that a homomorphism between simple modules (a module having only two submodules) must be either zero or an isomorphism. In particular, the endomorphism ring of a simple module is a division ring.
- The zero map M → N that maps every element to zero.
- A linear transformation between vector spaces.
- For a commutative ring R and ideals I, J, there is the canonical identification
- given by . In particular, is the annihilator of I.
- Given a ring R and an element r, let denote the left multiplication by r. Then for any s, t in R,
- That is, is right R-linear.
- For any ring R,
- as rings when R is viewed as a right module over itself. Explicitly, this isomorphism is given by the left regular representation .
- through for any left module M. (The module structure on Hom here comes from the right R-action on R; see #Module structures on Hom below.)
- is called the dual module of M; it is a left (resp. right) module if M is a right (resp. left) module over R with the module structure coming from the R-action on R. It is denoted by .
- Given a ring homomorphism R → S of commutative rings and an S-module M, an R-linear map θ: S → M is called a derivation if for any f, g in S, θ(f g) = f θ(g) + θ(f) g.
- If S, T are unital associative algebras over a ring R, then an algebra homomorphism from S to T is a ring homomorphism that is also an R-module homomorphism.
Module structures on Hom
In short, Hom inherits a ring action that was not used up to form Hom. More precise, let M, N be left R-modules. Suppose M has a right action of a ring S that commutes with the R-action; i.e., M is an (R, S)-module. Then
has the structure of a left S-module defined by: for s in S and x in M,
It is well-defined (i.e., is R-linear) since
Similarly, is a ring action since
Note: the above verification would "fail" if one used the left R-action in place of the right S-action. In this sense, Hom is often said to "use up" the R-action.
Similarly, if M is a left R-module and N is an (R, S)-module, then is a right S-module by .
A matrix representation
The relationship between matrices and linear transformations in linear algebra generalizes in a natural way to module homomorphisms. Precisely, given a right R-module U, there is the canonical isomorphism of the abelian groups
obtained by viewing consisting of column vectors and then writing f as an m × n matrix. In particular, viewing R as a right R-module and using , one has
which turns out to be a ring isomorphism (as a composition corresponds to a matrix multiplication).
Note the above isomorphism is canonical; no choice is involved. On the other hand, if one is given a module homomorphism between finite-rank free modules, then a choice of an ordered basis corresponds to a choice of an isomorphism . The above procedure then gives the matrix representation with respect to such choices of the bases. For more general modules, matrix representations may either lack uniqueness or not exist.
In practice, one often defines a module homomorphism by specifying its values on a generating set. More precisely, let M and N be left R-modules. Suppose a subset S generates M; i.e., there is a surjection with a free module F with a basis indexed by S and kernel K (i.e., one has a free presentation). Then to give a module homomorphism is to give a module homomorphism that kills K (i.e., maps K to zero).
If and are module homomorphisms, then their direct sum is
and their tensor product is
Let be a module homomorphism between left modules. The graph Γf of f is the submodule of M ⊕ N given by
which is the image of the module homomorphism M → M ⊕ N, x → (x, f(x)).
The transpose of f is
If f is an isomorphism, then the transpose of the inverse of f is called the contragredient of f.
A short sequence of modules
consists of modules A, B, C, and homomorphisms f, g. It is exact if the image of any arrow is the kernel of the next one; that is, f is injective, the kernel of g is the image of f and g is surjective. A longer exact sequence is defined in a similar way. A sequence of modules is exact if and only if it is exact as a sequence of abelian groups.
Any module homomorphism f defines an exact sequence
where K is the kernel of f, and C is the cokernel, that is the quotient of N by the image of f.
are exact, where the subscript means the localization at a maximal ideal .
If are module homomorphisms, then they are said to form a fiber square (or pullback square), denoted by M ×B N, if it fits into
Example: Let be commutative rings, and let I be the annihilator of the quotient B-module A/B (which is an ideal of A). Then canonical maps form a fiber square with
Endomorphisms of finitely generated modules
Let be an endomorphism between finitely generated R-modules for a commutative ring R. Then
- is killed by its characteristic polynomial relative to the generators of M; see Nakayama's lemma#Proof.
- If is surjective, then it is injective.
See also: Herbrand quotient (which can be defined for any endomorphism with some finiteness conditions.)
An additive relation from a module M to a module N is a submodule of  In other words, it is a "many-valued" homomorphism defined on some submodule of M. The inverse of f is the submodule . Any additive relation f determines a homomorphism from a submodule of M to a quotient of N
where consists of all elements x in M such that (x, y) belongs to f for some y in N.
A transgression that arises from a spectral sequence is an example of an additive relation.