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Can you please help me with this integral

$\int_{-1}^1 \frac{e^x}{\sqrt{1-x^2}}dx$

Thanks!

  • 0
    $\pi I_0(1) \approx 3.977 \ne 4.0$2012-04-09

2 Answers 2

2

$\int_{-1}^1 \frac{e^x}{\sqrt{1-x^2}}dx$ Substitution:$x=\sin\theta$

$I=\int_{-\pi/2}^{\pi/2}e^{\sin\theta}d\theta$

$e^{\sin\theta}=1+\frac{\sin\theta}{1!}+\frac{\sin^2\theta}{2!}+\frac{\sin^3\theta}{3!}.......$ The odd powers of sine when used in the integral will produce zero. Again for even values of n we have $\int_{-\pi/2}^{\pi/2}\sin^n\theta d\theta=2 \times \frac{n-1}{n}\frac{n-3}{n-2}\frac{n-5}{n-4}....\frac{3}{4}\frac{1}{2}\frac{\pi}{2}$ Therefore,

$I=\int_{-\pi/2}^{\pi/2}e^{\sin\theta}d\theta=\pi+2\times\Sigma\frac{1}{n!}[\frac{n-1}{n}\frac{n-3}{n-2}\frac{n-5}{n-4}....\frac{3}{4}\frac{1}{2}\frac{\pi}{2}]$

$=\pi+\pi\Sigma [\frac{1}{n(n-2)(n-4)......2}]^2$ ["n" running from two through even values ]

Writing n=2m we have,

\pi

[m running from 0 to infinity]

1

Let me try to sort out the previous takes. For this, we will have to take as a prerequisite the series for the zeroth-order modified Bessel function of the first kind

$I_0(x)=\sum_{k=0}^\infty \frac1{(k!)^2}\left(\frac{x^2}{4}\right)^k$

which can be derived from the modified Bessel differential equation

$x^2 y^{\prime\prime}+xy^\prime=x^2 y$

with initial conditions $y(0)=1,y^\prime(0)=0$.

Now,

$\begin{align*} \int_{-1}^1 \frac{\exp\,x}{\sqrt{1-x^2}}\mathrm dx&=\sum_{k=0}^\infty \frac1{k!}\int_{-1}^1 \frac{x^k}{\sqrt{1-x^2}}\mathrm dx\\ &=\sum_{k=0}^\infty \frac1{(2k)!}\int_{-1}^1 \frac{(x^2)^k}{\sqrt{1-x^2}}\mathrm dx\\ &=2\sum_{k=0}^\infty \frac1{(2k)!}\int_0^1 \frac{(x^2)^k}{\sqrt{1-x^2}}\mathrm dx\\ &=2\sum_{k=0}^\infty \frac1{(2k)!}\int_0^{\pi/2} \frac{\sin^{2k} u \; \cos\,u}{\sqrt{1-\sin^2 u}}\mathrm du\\ &=2\sum_{k=0}^\infty \frac1{(2k)!}\int_0^{\pi/2} \sin^{2k} u\mathrm du\\ &=\pi\sum_{k=0}^\infty \frac1{(2k)!}\frac{(2k)!}{4^k (k!)^2}\\ &=\pi\sum_{k=0}^\infty \frac1{(k!)^2}\left(\frac14\right)^k=\pi I_0(1)\\ \end{align*}$

where we used the odd/even properties of the moments in the second and third lines, and the Wallis formula in the fifth line.