# Neumann and Dirichlet eigenvalue problem - Eigenfunctions on the square [closed]

I don't know if I am in a good place to ask my question, but I'd like someone to create a visual support for a problem of mathematics. I was wondering what is the fundamental different between a Neumann eigenvalue problem and Dirichlet eigenvalue problem. I know that for DEP, we just fix the boundary (e.g. a drum), but what about the NEP. Now, consider a rectangle $\Omega = [0,l] \times [0,m]$. Separate variables using cartesian coordinates $x$ and $y$. That is, look for solution of the form $\varphi(x,y)=f(x)g(y)$

- Dirichlet boundary condition $\varphi| \partial \Omega=0$.

The eigenfunction are $$\varphi_{j,k}(x,y)= \sin(\frac{j \pi}{l}x) \sin (\frac{k \pi}{m}y) \text{ for } j,k \geq 1$$ and have eigenvalues $$\lambda_{j,k} = (\frac{j \pi}{l})^2 + (\frac{k \pi}{m})^2 \text{ for } j,k \geq 1$$

- Neumann boundary conditions $\partial_{\nu} \varphi | \partial \Omega = 0$

The eigenfunction are $$\varphi_{j,k}(x,y)= \cos(\frac{j \pi}{l}x) \cos (\frac{k \pi}{m}y) \text{ for } j,k \geq 0$$ and have eigenvalues $$\lambda_{j,k} = (\frac{j \pi}{l})^2 + (\frac{k \pi}{m})^2 \text{ for } j,k \geq 0$$

With these informations, does someone could show me, in 3-D, some eigenfunctions on the square for DEP and NEP (with Mathematica)?

## closed as off-topic by MarcoB, user9660, Yves Klett, xzczd, JensJun 16 '16 at 18:39

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "The question is out of scope for this site. The answer to this question requires either advice from Wolfram support or the services of a professional consultant." – MarcoB, Community, Yves Klett, xzczd, Jens
If this question can be reworded to fit the rules in the help center, please edit the question.

• @Jens I don't have Mathematica on my computer, so I don't try anything. – Sharpie Jun 16 '16 at 1:28
• I posted a quick answer, but if you want more pictures you will have to get Mathematica and change the parameters j and k in the plots on your own. – Jens Jun 16 '16 at 1:31

Her is a start:

Clear[ψD];
ψD[j_, k_][x_, y_] := Sin[j Pi x] Sin[k Pi y]

Clear[ψN];
ψN[j_, k_][x_, y_] := Cos[j Pi x] Cos[k Pi y]

ContourPlot[ψD[1, 2][x, y], {x, y} ∈
Polygon[{{0, 0}, {1, 0}, {1, 1}, {0, 1}}],
PlotPoints -> 100, AspectRatio -> Automatic, PlotRange -> All]


ContourPlot[ψN[2, 2][x, y], {x, y} ∈
Polygon[{{0, 0}, {1, 0}, {1, 1}, {0, 1}}], PlotPoints -> 100,
AspectRatio -> Automatic, PlotRange -> All]


• Could you tell me how I can get Mathematica for free on Mac? – Sharpie Jun 16 '16 at 1:32
• You may be able to get Mathematica through a site license at the place where you study or work, but I can't say anything else. – Jens Jun 16 '16 at 1:33
• Good idea, thanks! – Sharpie Jun 16 '16 at 1:35
• @Sharpie, or you could try stuff out in the cloud. – J. M. will be back soon Jun 16 '16 at 1:53
• Use Plot3D[] instead of ContourPlot[], @Sharpie. – J. M. will be back soon Jun 16 '16 at 2:41