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diff --git a/source/know/concept/partial-fraction-decomposition/index.md b/source/know/concept/partial-fraction-decomposition/index.md new file mode 100644 index 0000000..86e1e27 --- /dev/null +++ b/source/know/concept/partial-fraction-decomposition/index.md @@ -0,0 +1,61 @@ +--- +title: "Partial fraction decomposition" +date: 2021-02-22 +categories: +- Mathematics +layout: "concept" +--- + +**Partial fraction decomposition** or **partial fraction expansion** +is a method to rewrite quotients of two polynomials $g(x)$ and $h(x)$, +where the numerator $g(x)$ is of lower order than $h(x)$, +as sums of fractions with $x$ in the denominator: + +$$\begin{aligned} + f(x) = \frac{g(x)}{h(x)} = \frac{c_1}{x - h_1} + \frac{c_2}{x - h_2} + ... +\end{aligned}$$ + +Where $h_n$ etc. are the roots of the denominator $h(x)$. If all $N$ of +these roots are distinct, then it is sufficient to simply posit: + +$$\begin{aligned} + \boxed{ + f(x) = \frac{c_1}{x - h_1} + \frac{c_2}{x - h_2} + ... + \frac{c_N}{x - h_N} + } +\end{aligned}$$ + +The constants $c_n$ can either be found the hard way, +by multiplying the denominators around and solving a system of $N$ +equations, or the easy way by using this trick: + +$$\begin{aligned} + \boxed{ + c_n = \lim_{x \to h_n} \big( f(x) (x - h_n) \big) + } +\end{aligned}$$ + +If $h_1$ is a root with multiplicity $m > 1$, then the sum takes the form of: + +$$\begin{aligned} + \boxed{ + f(x) + = \frac{c_{1,1}}{x - h_1} + \frac{c_{1,2}}{(x - h_1)^2} + ... + } +\end{aligned}$$ + +Where $c_{1,j}$ are found by putting the terms on a common denominator, e.g. + +$$\begin{aligned} + \frac{c_{1,1}}{x - h_1} + \frac{c_{1,2}}{(x - h_1)^2} + = \frac{c_{1,1} (x - h_1) + c_{1,2}}{(x - h_1)^2} +\end{aligned}$$ + +And then, using the linear independence of $x^0, x^1, x^2, ...$, solving +a system of $m$ equations to find all $c_{1,1}, ..., c_{1,m}$. + + + +## References +1. O. Bang, + *Applied mathematics for physicists: lecture notes*, 2019, + unpublished. |