Thursday, September 3, 2015

integration - Fresnel Integral?




Is $\int\cos\left(\frac{\pi}{2}\cdot x^2\right)\,dx$ a known integral ?




I found on the net something called Fresnel integral, but we didn't learn it, and it also somehow related to Euler, and we didn't touch the Euler stuff, so maybe I made a mistake before it while calculating the double integral:



$$\iint_D\sin\left(\frac{\pi x}{2y}\right)\,dxdy$$ where $D=\left\{(x,y)\in\mathbb R^2: x\le y\le \sqrt[3]{x}\wedge y\ge\frac{\sqrt{2}}{2}\right\}$




So I wrote the $D$ as a simple to $x$:



$\begin{cases}\frac{\sqrt{2}}{2}\le y\le 1\\y^3\le x\le y\end{cases}$



and then I did the integral by $x$ and got that troublesome integral.




So can I solve this problem without Fresnel integral or maybe i have some mistake on the way?




Answer



Yes, you can do without Fresnel integrals I give you the first step:
$$
\int_{y^3}^y \sin\Bigl(\frac{\pi x}{2y}\Bigr)\,dx = \frac{2}{\pi}y\cos\Bigl(\frac{\pi y^2}{2}\Bigr).
$$
Then integrate with respect to $y$ (if you do not see how to do that I give you the hint to change variable $u=y^2$).


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