# Faster Way of Plotting a Multivariate Integral with No Closed Form and Maximized

I have a double integral that looks like this.

$$J=\int_0^2\int_{-2}^2 \sin(y\cdot B_z+z\cdot B_y+\phi_y+\phi_0)\,dy\,dz$$

$$\phi_y$$ I am defining as different functions of y, e.g. $$\phi_y=0$$, $$\phi_y=.5y$$, $$\phi_y= \left\{\begin{array}{ll} 0 & y\leq 0 \\ 1 & y\gt 0 \\ \end{array} \right.$$

$$B_y$$ and $$B_z$$ are the axes I am plotting over, while $$y$$ and $$z$$ are the real space coordinates of the surface I am integrating over.

So a simplified version of what I have written is

\*SubsuperscriptBox[$$\[Integral]$$, $$0$$, $$2$$]$$\*SubsuperscriptBox[\(\[Integral]$$, $$-2$$, $$2$$]Sin[\[Phi]0 +
y*Bz + z*By +  .2
\*SuperscriptBox[$$y$$, $$2$$]] \[DifferentialD]y \[DifferentialD]z\)\
\);
Plot[MaxValue[Limit[Ic, By -> 0], \[Phi]0], {Bz, -10, 10},
PlotRange -> All] // AbsoluteTiming I let $$\phi_y$$ be 0 to have the computation be as simple as possible and only plot a slice of $$J$$ at a given $$B_y$$. And I am maximizing $$J$$ with respect to $$\phi_0$$. As you can see it takes a minute and a half to plot. This is completely fine, but if I try more complicated functions the computation can take much longer. I need to try many different $$\phi_y$$ out, like $$\phi_y=\sin(y)$$, but these more complicated functions don't always have a closed form solution and might take an extremely long time to do. Additionally, I sometimes run into a problem where MaxValue gives errors that it does not evaluate to a numerical value, and I have not figured out exactly what causes those.

So my questions are..

1. What is a more efficient way of doing this process? I have thought about using NDSolve, approximations, or something similar, but haven't been able to get it to work yet.

2. Say I would like to tune $$\phi_y$$ with some parameter and watch the plot change using Manipulate or something similar. Is there a way to do that without running the computation over and over so I can see the evolution smoothly?

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• People here generally like users to post code as Mathematica code instead of just images or TeX, so they can copy-paste it. It makes it convenient for them and more likely you will get someone to help you. You may find this meta Q&A helpful – Michael E2 May 22 at 23:40
• My bad, I added a snippet of code to copy and paste. – user72882 May 23 at 2:53
• @user72882 Why do you use By->0? In that case we have 1D integral. Or it is just for simplification? Also don't use By since it is system symbol in Mathematica. Let put for instance B2 instead. – Alex Trounev May 23 at 23:35
• @AlexTrounev I am taking the limit as By->0 because it is easier for me to interpret what changing different parameters does. once I get a good grasp on that I will start visualizing the entire 2D surface. I did not realize I was typing the word 'By' either, thanks for pointing that out, I will change it to B2 in my code. – user72882 May 24 at 6:20