# Density plot for eigenvalues of a matrix

I am trying to make a density plot of $$|min~[Im(v)]|$$ (where $$v$$ are the eigenvalues of a matrix) as function of $$\gamma$$ (horizontal axis) and $$T$$(vertical), The code upto now is

    ListDensityPlot[
Table[Min[
Im /@ Eigenvalues[
With[{k = 2, p = Pi/2},
With[{a = -2 Cos[p], b = -2 Cos[p] - \[Gamma]/(1 + Abs[T]^2)^2,
c = (\[Gamma]*T^2*E^(2 I*p))/(1 + Abs[T]^2)^2, n = 2*k + 1},
mat = SparseArray[{Band[{1, 1}] ->
Join[ConstantArray[a, n - (k + 1)], {b},
ConstantArray[a, n - (k + 1)]],
Band[{2, 1}] -> ConstantArray[1, 2 k],
Band[{1, 2}] -> ConstantArray[1, 2 k],
Band[{n + 1, n + 1}] ->
Join[ConstantArray[-a, n - (k + 1)], {-b},
ConstantArray[-a, n - (k + 1)]],
Band[{n + 1, n + 2}] -> ConstantArray[-1, 2 k],
Band[{n + 2, n + 1}] ->
ConstantArray[-1, 2 k], {n + k + 1, k + 1} -> -c, {k + 1,
n + k + 1} -> c}, {2 n, 2 n}];
mat]]]], {\[Gamma], -30, 30, 0.1}, {T, 0., 5, 0.1}],
PlotRange -> All, DataRange -> {{-30, 30}, {0, 5}}]


which gave me

However, my required output should be close to the following [![enter image description here][2]][2]

The required output is for k=49 case, however it takes longer to get an output with $$198\times 198$$ matrix ($$k=49$$), so I thought one could get atleast something similar for a smaller matrix. Therefore I used $$k=2$$ just to see if I get any close.

Seems to me like you've already gotten the correct result, just rotated a bit. I've switched $$T$$ and $$\gamma$$ in your code.

    ListDensityPlot[
Table[Min[
Im /@ Eigenvalues[
With[{k = 2, p = Pi/2},
With[{a = -2 Cos[p], b = -2 Cos[p] - \[Gamma]/(1 + Abs[T]^2)^2,
c = (\[Gamma]*T^2*E^(2 I*p))/(1 + Abs[T]^2)^2, n = 2*k + 1},
mat = SparseArray[{Band[{1, 1}] ->
Join[ConstantArray[a, n - (k + 1)], {b},
ConstantArray[a, n - (k + 1)]],
Band[{2, 1}] -> ConstantArray[1, 2 k],
Band[{1, 2}] -> ConstantArray[1, 2 k],
Band[{n + 1, n + 1}] ->
Join[ConstantArray[-a, n - (k + 1)], {-b},
ConstantArray[-a, n - (k + 1)]],
Band[{n + 1, n + 2}] -> ConstantArray[-1, 2 k],
Band[{n + 2, n + 1}] ->
ConstantArray[-1, 2 k], {n + k + 1, k + 1} -> -c, {k + 1,
n + k + 1} -> c}, {2 n, 2 n}];
mat]]]], {T, 0, 5, 0.1}, {\[Gamma], -30, 30, 0.1}],
PlotRange -> All, DataRange -> {{-30, 30}, {0, 5}}]


$\gamma$ and $$T$$ ">

Also, the Y-axis in your bottom picture is reversed. You can use ScalingFunctions for that.

    ListDensityPlot[
Table[Min[
Im /@ Eigenvalues[
With[{k = 2, p = Pi/2},
With[{a = -2 Cos[p], b = -2 Cos[p] - \[Gamma]/(1 + Abs[T]^2)^2,
c = (\[Gamma]*T^2*E^(2 I*p))/(1 + Abs[T]^2)^2, n = 2*k + 1},
mat = SparseArray[{Band[{1, 1}] ->
Join[ConstantArray[a, n - (k + 1)], {b},
ConstantArray[a, n - (k + 1)]],
Band[{2, 1}] -> ConstantArray[1, 2 k],
Band[{1, 2}] -> ConstantArray[1, 2 k],
Band[{n + 1, n + 1}] ->
Join[ConstantArray[-a, n - (k + 1)], {-b},
ConstantArray[-a, n - (k + 1)]],
Band[{n + 1, n + 2}] -> ConstantArray[-1, 2 k],
Band[{n + 2, n + 1}] ->
ConstantArray[-1, 2 k], {n + k + 1, k + 1} -> -c, {k + 1,
n + k + 1} -> c}, {2 n, 2 n}];
mat]]]], {T, 0, 5, 0.1}, {\[Gamma], -30, 30, 0.1}],
PlotRange -> All, DataRange -> {{-30, 30}, {0, 5}}, ScalingFunctions -> {Identity, "Reverse"}]


When using Reverse in ScalingFunctions, you have to adjust the ticks also. I wasn't able to figure that out yet, this answer might help.

You can change the colors by adding a ColorFunction. For example, adding ColorFunction -> "TemperatureMap". Though to get colors like in your picture, you'd have to write a specific function.

Note: This was done in Mathematica 11.0, earlier versions may not support ScalingFunctions on ListDensityPlot.

• Thanks. So you mean that my x-axis was $T$, instead of $\gamma$? What is on the x-axis and y axis in your answer now? Secondly, the scale in the required figure appears to be different>? on the y-axis..? One more thing, is it possible to sharpen the colors a bit? – AtoZ Oct 26 '18 at 2:33
• @AtoZ Yes, $T$ and $\gamma$ were switched around. I made an edit to the answer. – Valrog Oct 26 '18 at 5:07
• Thanks. So in yourlast update, $\gamma$ is on x-axis and $T$ on y-axis? – AtoZ Oct 26 '18 at 6:43
• @AtoZ Indeed, $\gamma \in [-30, 30]$ on x-axis and $T \in [0, 5]$ on y-axis, just as you defined. – Valrog Oct 26 '18 at 6:45