# Quotient module

(Redirected from Factor module)

In abstract algebra, a branch of mathematics, given a module and a submodule, one can construct their quotient module. This construction, to be described below, is analogous to how one obtains the ring of integers modulo an integer n, see modular arithmetic. It is the same construction used for quotient groups and quotient rings.

Given a module A over a ring R, and a submodule B of A, one consideres the quotient space A/B defined by the equivalence relation

a ~ b iff ba is in B,

for any a and b in A.

One defines the addition operation for two equivalence classes as the equivalence class of the sum of two representatives from these classes; and in the same way for multiplication by elements of R. In this way A/B becomes itself a module over R, called the quotient module.

## Examples

Consider the ring R of real numbers, and the R-module A=R[X], that is the polynomial ring with real coefficients. Consider the submodule

B=(X2+1) R[X]

of A, that is, the submodule of all polynomials divisible by X2+1. It follows that the equivalence relation determined by this module will be

P(X) ~ Q(X) if and only if P(X) and Q(X) give the same remainder when divided by X2+1.

Therefore, in the quetient module A/B one will have X2+1 be the same as 0, and such, one can view A/B as obtained from R[X] by setting X2+1=0. It is clear that this quotient module will be isomorphic to the complex numbers, viewed as a module over the real numbers R.

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