# $\int\text{e}^{-ax^2 } \text{erf}\left(bx + c\right) dx$

I'm hoping to find a closed expression for the following integral. $$\int\text{e}^{-ax^2 } \text{erf}\left(bx + c\right) dx$$ One can find a solution for a family of products between exponentials and error functions. None of which apparently have the offset term in the error function.

I have tried tackling the problem with two approaches.

Approach #1: Expanding the error function hoping to find nice cancelations leading to the maclerin series of some known elementary function. Following a similar approach by Alex:

\begin{aligned} \int\text{e}^{-ax^2 } \text{erf}\left(bx + c\right) dx &= \frac{2}{\sqrt{\pi}} \sum_{n=0}^\infty \frac{(-1)^n }{n!(2n+1)} \int(bx+c)^{2n +1} \text{e}^{-ax^2} dx \\ & = \frac{2}{\sqrt{\pi}} \sum_{n=0}^\infty \frac{(-1)^n }{n!(2n+1)} \int \sum_{k=0}^{2n+1} {{2n+1}\choose{k}} (bx)^{k} c^{2n+1-k} \text{e}^{-ax^2} dx \\ & = \frac{2}{\sqrt{\pi}} \sum_{n=0}^\infty \frac{(-1)^n }{n!(2n+1)} \sum_{k=0}^{2n+1} {{2n+1}\choose{k}} c^{2n+1-k} b^k\int x^{k} \text{e}^{-ax^2} dx = \\ &\frac{2}{\sqrt{\pi}} \sum_{n=0}^\infty \frac{(-1)^n }{n!(2n+1)} \sum_{k=0}^{2n+1} {{2n+1}\choose{k}} c^{2n+1-k} b^k \left(-\frac{1}{2}a^{-\frac{k+1}{2}} \Gamma\left(\frac{k+1}{2},ax^2\right)\right) \\ &= -\frac{1}{\sqrt{a}\sqrt{\pi}} \sum_{n=0}^\infty \frac{(-1)^n }{n!(2n+1)} \sum_{k=0}^{2n+1} {{2n+1}\choose{k}} c^{2n+1-k} \left(\frac{b}{\sqrt{a}}\right)^k \Gamma\left(\frac{k+1}{2},ax^2\right) \end{aligned}

I have used the binomial expansion for $(bx + c)^{2n+1}$ and that $\int x^k \text{e}^{-ax^2}dx = -\frac{1}{2}a^{-\frac{k+1}{2}} \Gamma\left(\frac{k+1}{2} ,ax^2\right)$ where $\Gamma(,)$ is the incomplete gamma function. Too bad, the last term can not be combined again in the form of a binomial expansion.

Approach #2: Instead of expanding the error function, I tried writing it in terms of the cumulative CDF function (Q-Function) as $\text{erf}(x) = 2\Phi(\sqrt{2} x) - 1$. However, the following can be shown to be true using integration under the integral sign with respect to $\mu$. [Section 2.4, and ref] $$\frac{1}{\sqrt{2 \pi} \sigma}\int_{-\infty}^{\infty}\Phi(\lambda x) \text{e}^{-\frac{(x - \mu)^2}{2 \sigma^2}}dx =\Phi\left(\frac{\lambda \mu}{\sqrt{1+\lambda^2\sigma^2}}\right)$$

Now with some change of variables and rescaling we are instead interested in the following integral: $$\int\text{e}^{a_1 x^2 + a_2 x} \text{erf}\left(x\right) dx = \underbrace{2\int \text{e}^{a_1 x^2 + a_2 x} \Phi(\sqrt{2} x) dx}_{I} - \underbrace{\int \text{e}^{a_1 x^2 + a_2 x} dx}_{easy}$$

However, what I'm not certain of if I can use the trick of integration under integral sign for the indefinite integral labeled I. Can I, with some change of variables, use the result deduced for the definite integral case as $\Phi\left(\frac{\lambda \mu}{\sqrt{1+\lambda^2\sigma^2}}\right) + C(x)$?

EDIT: It seems that the problem in had has no closed form solution as pointed out by user90369. Also, user90369 has pointed out that the following more general case have no closed form solution. $$\int x^{2n} \text{e}^{-ax^2} \text{erf}(bx+c) dx$$ I was wondering, if there are any good approximations that I can use here. By good, I mean refer to an error that is $|e(x)| \leq 10^{-5} \forall x$. For starter, I was looking at the high accuracy approximations in here for the erf function. Unfortunately, none of these approximations result into an integral that inherits a closed form solution. I, however, have the following suggested approach with the use of the following identity. $$\text{erf}(bx+c) = 2 \Phi\left(\sqrt{2} (bx+c)\right) - 1$$ This results into the following: \begin{aligned} \int\text{e}^{-ax^2 } \text{erf}\left(bx + c\right) dx = 2\int \text{e}^{-ax^2 } \Phi\left(\sqrt{2} (bx+c)\right) dx - \int \text{e}^{-ax^2 } dx \end{aligned} Now, one can use the approximation of the $\Phi$-function that results from applying Chernof's bound. Link $$\Phi(x) \approx \frac{1}{12} \text{e}^{-\frac{x^2}{2}} + \frac{1}{4} \text{e}^{-\frac{2}{3} x^2}$$ I'd like to take a suggestion of how good is this approximation after computing the integral. Or maybe if there are other better approximations/recommendations that result into a manageable integral afterwards.

• @user90369 Thanks. Would you care suggesting an approximation? For instance, all the approximations provided here people.math.sfu.ca/~cbm/aands/page_299.htm are not useful in solving the resulting integral. I was considering approximating the $\Phi$ function with an exponential from Chernof's bound. en.wikipedia.org/wiki/Q-function – Adel Bibi Jul 5 '17 at 12:17
• @user90369 Also, there appears to be some hope for the definite integral case. math.stackexchange.com/questions/2236490/… – Adel Bibi Jul 5 '17 at 12:27
• There is a very big difference between $\int$ and $\int_{-\infty}^\infty$ . Maybe someone can give you an useful answer if you decide where your focus is here. – user90369 Jul 5 '17 at 12:38
• If you use the method in math.stackexchange.com/questions/2236490/… (the link you've mentioned) you will get a formula for $\int\limits_{-\infty}^\infty e^{-ax^2} \text{erf}(bx+c)dx$ . One must substitute $x$ by $(x-c)/b$ and then do the derivation of the integral with respect to $c$ . – user90369 Jul 5 '17 at 15:45
• You haven't defined what a "good" approximation is here (for you) but anyway you can make numerical tests with some values and see whether they meet your expectations. To approximate $Q$ and therefore at the end $\,\text{erf} \,$ by sums of $e^{-a(x+b)^2}$ is of course a good idea. – user90369 Jul 5 '17 at 16:07

I don't know if this helps for an useful approximation but maybe it's better than nothing. :-)

For $$\,v\in\mathbb{N}_0\,$$ we get

$$\int x^{2v+1}e^{-ax^2}dx= -\frac{v!e^{-ax^2}}{2a^{v+1}}\sum\limits_{j=0}^v\frac{(ax^2)^j}{j!} + C_{2v+1}$$

and

$$\int x^{2v}e^{-ax^2}dx= \frac{(2v)!\sqrt{a\pi}\text{erf}(\sqrt{a}x)}{2^v v!(2a)^{v+1}}-e^{-ax^2}\sum\limits_{j=0}^{v-1}\frac{(v-j)!(2v)!x^{2v-2j-1}}{2^j v!(2v-2j)!(2a)^{j+1}} + C_{2v}$$

and it follows:

\begin{align} & \hphantom{ {}={}} \int e^{-ax^2} \text{erf}(bx+c)dx \\ &= \sum\limits_{k=0}^\infty\frac{(-1)^k}{k!(2k+1)}\int (bx+c)^{2k+1} e^{-ax^2} dx \\ &= \sum\limits_{k=0}^\infty \frac{(-1)^k}{k!(2k+1)}\sum_{v=0}^{2k+1}\binom {2k+1} v b^v c^{2k+1-v}\int x^v e^{-ax^2} dx \\ &= \sum\limits_{k=0}^\infty \frac{(-1)^k}{k!(2k+1)}\sum_{v=0}^k\binom {2k+1} {2v} b^{2v} c^{2k+1-2v}\int x^{2v} e^{-ax^2} dx \\ &\hspace{5mm} +\sum\limits_{k=0}^\infty \frac{(-1)^k}{k!(2k+1)}\sum_{v=0}^k\binom {2k+1} {2v+1} b^{2v+1} c^{2k-2v}\int x^{2v+1} e^{-ax^2} dx \\ &= \sum\limits_{k=0}^\infty\frac{(-1)^k}{k!(2k+1)} \sum\limits_{v=0}^k \binom {2k+1} {2v} b^{2v}c^{2k-2v+1} \\ &\hspace{3cm} \cdot \left( \frac{(2v)!\sqrt{a\pi}\text{erf}(\sqrt{a}x)}{2^v v!(2a)^{v+1}}-e^{-ax^2}\sum\limits_{j=0}^{v-1}\frac{(v-j)!(2v)!x^{2v-2j-1}}{2^j v!(2v-2j)!(2a)^{j+1}} \right) \\ &\hspace{5mm} - \sum\limits_{k=0}^\infty\frac{(-1)^k}{k!(2k+1)} \sum\limits_{v=0}^k \binom {2k+1} {2v+1} b^{2v+1}c^{2k-2v} \frac{v!e^{-ax^2}}{2a^{v+1}}\sum\limits_{j=0}^v\frac{(ax^2)^j}{j!} + C \\ &= \sqrt{a\pi}\text{erf}(\sqrt{a}x)\sum\limits_{k=0}^\infty\frac{(-1)^k}{k!(2k+1)}\sum\limits_{v=0}^k \binom {2k+1} {2v} \frac{(2v)!b^{2v}c^{2k-2v+1}}{2^v v!(2a)^{v+1}} \\ &\hspace{5mm} -e^{-ax^2}\sum\limits_{k=0}^\infty\frac{(-1)^k}{k!(2k+1)}\sum\limits_{v=0}^k \binom {2k+1} {2v} b^{2v}c^{2k-2v+1}\sum\limits_{j=0}^{v-1}\frac{(v-j)!(2v)!x^{2v-2j-1}}{2^j v!(2v-2j)!(2a)^{j+1}} \\ &\hspace{5mm} -e^{-ax^2}\sum\limits_{k=0}^\infty\frac{(-1)^k}{k!(2k+1)}\sum\limits_{v=0}^k \binom {2k+1} {2v+1} b^{2v+1}c^{2k-2v} \frac{v!}{2a^{v+1}}\sum\limits_{j=0}^v\frac{(ax^2)^j}{j!} + C \end{align}

• Thanks for the approach. Not sure really if that helps alot. I was looking into some more simpler forms as the result of the required integral will be later integrated too over some other parameter. I have tried the approximation mentioned in the OP; however, this approximation is not continuous. Resulting in a disjunction in the final solution that is broken into pieces depending on the region of integration. This is because $\Phi(x) \approx \frac{1}{2} e^{-\frac{1}{2}x^2} \forall x \ge 0$. – Adel Bibi Jul 11 '17 at 8:23
• @AdelBibi : The series is the exact result. For simplification it's necessary to specify additional decision criteria, e.g. the value ranges ​​of the parameters. – user90369 Jul 11 '17 at 10:10
• I have looked at it more now. I think that should be a good approximation. However, may you detail the integral. Also, I don't quite see where did the error function disappear after the second line. Also, are there a missing equal signs from third and forth row? Could you add some details to it so that I can officially accept it as an answer? – Adel Bibi Jul 12 '17 at 9:23
• @AdelBibi: Sorry, yes, you are right. I have add some steps. --- The error function doesn't disappear, it's a part of the solution. --- Missing equal signs ? No, but the terms and sums are partly too long for one line. (Maybe your screen display is different to mine ? Or do I misunderstand you ?) – user90369 Jul 12 '17 at 9:53
• @AdelBibi : You are welcome (and sorry for the inconvenience because of a too short explanation first). :-) – user90369 Jul 13 '17 at 8:45