This is a verification case of an electromagnetic port model to obtain
a propagation parameter. A waveguide having a rectangular cross section of
size a x 2a and consisting of two materials is modelled. This problem has been
discussed for example in

[ZR18] A. Zdunek and W. Rachowicz. Inhomogeneous lossy waveguide mode
       analysis. Computers & Mathematics with Applications, 75(3):798-808,
       2018.

Here a dimensionless form of the model is used and the second-order
approximation is applied. The four eigenvalues having
the smallest real part give the following values for a scaled propagation
parameter (see [ZR18])

            1.27576
            0.97154
            0.72865
            0.59390

which are in good agreement with the results given in the literature.