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Dedicated to Raytcho Lazarov on the occasion of his 60th birthday.Abstract -A boundary-value problem with a nonlocal integral condition is considered for a two-dimensional elliptic equation with constant coefficients and a mixed derivative. The existence and uniqueness of a weak solution of this problem are proved in a weighted Sobolev space. A difference scheme is constructed using the Steklov averaging operators. It is proved that the difference scheme converges in discrete W

The optical absorption, photoconductivity, and short-circuit photocurrent spectra of structurally perfect Cd 1x Zn x As 2 ( x = 0.03, 0.05, 0.06) single crystals are studied for the first time near the intrinsic edge in the range 80-300 K. The results demonstrate that the intrinsic edge in the solid solutions is due to indirect transitions involving the formation of excitons for both the E || c and E ⊥ c polarizations. The indirect gaps of the solid solutions are determined. In the range 80-300 K, the data for x = 0-0.06 and both polarizations are well fitted by ( x ) = (0) + 0.0866 x + 2.35 x 2 . The introduction of 6 mol % ZnAs 2 into CdAs 2 increases its by Ӎ 14 meV.