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I need to solve either of the following integrals, they are related to each other by integration by parts:

$$I_1 = \int_{0}^{\infty} e^{-ax} \frac{x}{\sqrt{1+x^2}}dx$$

or

$$I_2 = \int_{0}^{\infty} e^{-ax} \sqrt{1+x^2}dx$$

I have not been able to find them in integral tables and I wonder if it is possible to solve either of these integrals.

Any ideas on how to calculate $I_1$ or $I_2$?

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    $\begingroup$ Are you confident with Bessel and Struve functions? $\endgroup$ – Jack D'Aurizio Sep 23 '16 at 14:15
  • $\begingroup$ put $x=\sinh(t)$ then you obtain $$ I_1=\int_0^{\infty}\exp(-a\sinh(t))\sinh(t) $$ then have a look here dlmf.nist.gov/11.5 and here dlmf.nist.gov/11.4 $\endgroup$ – tired Sep 23 '16 at 14:17
  • $\begingroup$ No. I know how to solve it in terms of these, but I would like to have a function as a solution, not another integral or infinite sum @JackD'Aurizio $\endgroup$ – Mencia Sep 23 '16 at 14:17
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    $\begingroup$ @Mencia: $Y_1(z)$ is a function. It also equals an infinite series, just like $e^z$ equals $\sum_{n\geq 0}\frac{z^n}{n!}$. $\endgroup$ – Jack D'Aurizio Sep 23 '16 at 14:19
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    $\begingroup$ @tired yes, I mentioned they are related by integration by parts. $\endgroup$ – Mencia Sep 23 '16 at 14:31
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If you notice it, you will see that this is the form of Laplace transformation. Thus the first integration can be written as the Laplace transformation of (x/root(x^2 +1)) The second integral can be written by integration by parts Which is e^-ax * root(x^2 +1) - integrate(-ae^-at * root(x^2 +1) dx) Which is then equals to e^-ax * root(x^2 +1) + a*laplace of (root(x^2 +1))

Hope this helped

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  • $\begingroup$ how does this answer helps the op? $\endgroup$ – tired Sep 23 '16 at 14:51

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