A module is a mathematical object in which things can be added together commutatively by multiplying coefficients and in which most of the rules of manipulating vectors hold. A module is abstractly very similar to a vector space, although in modules, coefficients are taken in rings that are much more general algebraic objects than the fields used in vector spaces. A module taking its coefficients in a ring is called a module over , or a R-module.
Modules are the basic tool of homological algebra. Examples of modules include the set of integers , the cubic lattice in dimensions , and the group ring of a group.
is a module over itself. It is closed under addition and subtraction (although it is sufficient to require closure under subtraction). Numbers of the form for and a fixed integer form a submodule since, for all ,
and is still in .
Given two integers and , the smallest module containing and is the module for their greatest common divisor, .