Hi Athanasios,
Check that there a only numbers inside the NDEigenvalues call
sol = NDEigensystem[{-u''[x] + ( x^2 + Sin[6 \[Pi] x]) u[x],
u[-1] == 0, u[1] == 0}, u[x], {x , -1, 1}, 5 ]
The Interpolation of the eigenvalues gives a parabola
egv = Interpolation[sol[[1]]]
Plot[egv[x], {x, 1, 5}]
In[39]:= Array[Differences[sol[[1]], #] &, {4}]
Out[40]= {{7.55424, 12.3676, 17.288, 22.2322}, {4.81333, 4.92039,
4.94427}, {0.107064, 0.0238753}, {-0.0831883}}
Plot[ sol[[2]], {x, -1, 1}]
Regards Roland
