Prove that this expression is equal to $\pi$

calculusimproper-integralsintegrationreal-analysisrecreational-mathematics

Today, on the auspicious $\pi$ day, I saw on a local chat group $$\pi=4\phi^2\left(\phi^2+2\sqrt{\phi}\right)\left(\int_{\ 0}^{\ \infty\ }e^{x^2}\frac{\ \sin\left(x^2\right)\ }{x^2}dx\right)^2$$ I wonder how one can prove this.

My first thoughts were to evaluate the term involving the golden ratio. I calculated it approximately with the help of wolframaplha but you see that I can't prove it with mathematical methods. And for the integral I've no idea as I don't know how to solve improper integrals.

This is what I saw enter image description here

Edit: I confirmed this query with the Original poster. Indeed it should be $e^{{-x}^2}.$ Surprised that wolfram got it wrong…

Best Answer

As said in my first comment above, the integral is divergent (so: wolframaplha is wrong). Indeed, let $a_k:=\frac\pi2+2k\pi$ with $\Bbb N\ni k\to+\infty$ then $$\int_{\sqrt{a_k-\frac\pi4}}^{\sqrt{a_k+\frac\pi4}}e^{x^2}\frac{\sin(x^2)}{x^2}dx=\frac12\int_{a_k-\frac\pi4}^{a_k+\frac\pi4}e^y\frac{\sin y}{y^{3/2}}dy $$ $$\ge\frac{e^{a_k-\frac\pi4}}{2\sqrt2(a_k+\frac\pi4)^{3/2}}\to+\infty$$

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