# Definition:Linear Combination

## Definition

Let $R$ be a ring.

### Linear Combination of Sequence

Let $M$ be an $R$-module.

Let $\sequence {a_n} := \sequence {a_j}_{1 \mathop \le j \mathop \le n}$ be a sequence of elements of $M$ of length $n$.

An element $b \in M$ is a linear combination of $\sequence {a_n}$ if and only if:

$\ds \exists \sequence {\lambda_n} \subseteq R: b = \sum_{k \mathop = 1}^n \lambda_k a_k$

### Linear Combination of Subset

Let $M$ be an $R$-module.

Let $\O \subset S \subseteq G$.

Let $b \in M$ be a linear combination of some sequence $\sequence {a_n}$ of elements of $S$.

Then $b$ is a linear combination of $S$.

### Linear Combination of Empty Set

Let $G$ be an $R$-module.

$b = e_G$

## Also see

An integer combination is also called a linear combination. The definition is compatible with the one on this page.