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I have to solve the following equation using Mathematica.

$\frac{8 \pi \sqrt{a^2+b^2}\ e^{-a \pi^2}}{b}(b\ \cos b\ \pi^2+a\ \sin b \ \pi^2)\int\limits_0^1 e^{-a\ \pi^2 u^2}\cos b\ \pi^2 u^2 ~du=1$.

For each $ \frac{1}{2}\le a \le 2$, there exists many $b$'s satisfying the equation. I want to find the first or minimum value of $b$ satisfying the above equation. I tried NSolve but it is not working. I tried FindRoot, it's working but I need to guess the root. Since I need many points to make list plot so it is very hard.

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  • $\begingroup$ Are you looking for real solutions b? In which range? $\endgroup$
    – Ulrich Neumann
    Jul 20, 2018 at 8:38
  • $\begingroup$ There are many $b$s for a fixed valued of $a$ satisfying the equation. I want the minimum value of $b$ corresponding to given value of $a$. Then making list plot. $\endgroup$
    – Shabbir
    Jul 20, 2018 at 9:01

2 Answers 2

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In order to guess the location of the zeroes, try the following:

ContourPlot[Log[Abs[a + b - 1]], {a, 1/2, 2}, {b, -1, 1}]

Instead of a + b, type the right hand side of your formula. The main idea is to exaggerate the location of the zeroes by converting them into singularities. Use those locations in FindRoot.

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Try 

gl = (8 Pi Sqrt[a^2 + b^2 ] Exp[-a Pi^2])/b (b Cos[b Pi^2] + a Sin[b Pi^2]) Integrate[Exp[-a Pi^2 u^2] Cos[b Pi^2 u^2], {u, 0, 1} ] == 1

ContourPlot  [gl // Evaluate, {a, -1/2, 2}, {b, -1, 1},MaxRecursion -> 5, FrameLabel -> {a, b}]

In the plot you can see the possible solutions of your equation. It looks like there are no solutions in the parameter region 1/2<a<2&&-1<b<1 and several solutions in the range -1/2<a<1/2&&-1<b<1!

enter image description here

A minimal solution can be evaluated with NMinimize

NMinimize[{b^2 + a^2, gl, -1/2 <= a <= 1/2}, {a, b}]
(*{0.00758855, {a -> -0.0871123, b -> -2.97896*10^-7}} *)
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  • $\begingroup$ Thats simple to draw....I have this figure. My main problem is to find a pair $(a, min(b))$ satisfying the equation and plot it. $\endgroup$
    – Shabbir
    Jul 20, 2018 at 9:16
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    $\begingroup$ Why didn't you show your efforts? $\endgroup$
    – Ulrich Neumann
    Jul 20, 2018 at 9:18
  • $\begingroup$ I dnt know how to write Mathematica commands here? Just copy and paste from Mathematica nb? $\endgroup$
    – Shabbir
    Jul 20, 2018 at 9:25
  • $\begingroup$ Yes, copy and paste into the box. The buttons at the top let you do formatting (the one with a pair of braces { } marks highlighted text as code, the $\alpha\beta$ button in the top right converts Mathematica Greek letters to unicode letters. $\endgroup$
    – SPPearce
    Jul 20, 2018 at 9:31
  • $\begingroup$ For a = 1, you have the value of b that is complex. Is this expected ? Or are you expecting only real values of b ? $\endgroup$
    – Lotus
    Jul 20, 2018 at 9:35

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