Definition:Partial Fractions Expansion

Definition

Let $\map R x = \dfrac {\map P x} {\map Q x}$ be a rational function, where $\map P x$ and $\map Q x$ are expressible as polynomial functions.

Let $\map Q x$ be expressible as:

$\map Q x = \ds \prod_{k \mathop = 1}^n \map {q_k} x$

where the $\map {q_k} x$ are themselves polynomial functions of degree at least $1$.

Let $\map R x$ be expressible as:

$\map R x = \map r x \ds \sum_{k \mathop = 0}^n \dfrac {\map {p_k} x} {\map {q_k} x}$

where:

$\map r x$ is a polynomial function which may or may not be the null polynomial, or be of degree $0$ (that is, a constant)

each of the $\map {p_k} x$ are polynomial functions

the degree of $\map {p_k} x$ is strictly less than the degree of $\map {q_k} x$ for all $k$.

Then $\map r x \ds \sum_{k \mathop = 0}^n \dfrac {\map {p_k} x} {\map {q_k} x}$ is a partial fractions expansion of $\map R x$.

$\dfrac 1 {x \paren {x + 1}^2} = \dfrac 1 x - \dfrac 1 {\paren {x + 1} } - \dfrac 1 {\paren {x + 1}^2}$