POTHEA: A PROGRAM FOR COMPUTING EIGENVALUES AND EIGENFUNCTIONS AND THEIR FIRST DERIVATIVES WITH RESPECT TO THE PARAMETER OF THE PARAMETRIC SELF-ADJOINED 2D ELLIPTIC PARTIAL DIFFERENTIAL EQUATION Article Gusev A.A., Chuluunbaatar O., Vinitsky S.I., Abrashkevich A.G. Computer Physics Communications. Elsevier Science Publishers B.V., Amsterdam. Vol. Vol 185. 2014.