Primitive of Error Function
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Theorem
- $\ds \int \map \erf x \rd x = x \map \erf x + \frac 1 {\sqrt \pi} e^{-x^2} + C$
where $\erf$ denotes the error function.
Proof
By Derivative of Error Function, we have:
- $\dfrac \d {\d x} \paren {\map \erf x} = \dfrac 2 {\sqrt \pi} e^{-x^2}$
So:
\(\ds \int \map \erf x \rd x\) | \(=\) | \(\ds \int 1 \times \map \erf x \rd x\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds x \map \erf x - \frac 2 {\sqrt \pi} \int x e^{-x^2} \rd x\) | Integration by Parts | |||||||||||
\(\ds \) | \(=\) | \(\ds x \map \erf x + \frac 1 {\sqrt \pi} \int \paren {-2 x e^{-x^2} } \rd x\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds x \map \erf x + \frac 1 {\sqrt \pi} \int e^u \rd u\) | substituting $u = -x^2$ | |||||||||||
\(\ds \) | \(=\) | \(\ds x \map \erf x + \frac 1 {\sqrt \pi} e^u + C\) | Primitive of Exponential Function | |||||||||||
\(\ds \) | \(=\) | \(\ds x \map \erf x + \frac 1 {\sqrt \pi} e^{-x^2} + C\) | substituting back for $u$ |
$\blacksquare$