1. Continuity conditions for a homogeneous or heterogeneous PN approximation;Davis;J. Nucl. Sci. and Engng.,1967
2. On different PN-approximations of the method of spherical harmonics;Gryn;Zh. vychisl. Mat. mat. Fiz.,1985
3. Finite approximations of the method of spherical harmonics with different orders in spatial subdomains;Kutov;Zh. vychisl. Mat. mat. Fiz.,1986