1. Helgason, S., Differential Geometry and Symmetric Spaces, New York: Academic, 1962. Translated under the title Differentsial’naya geometriya i simmetricheskie prostranstva, Moscow: Mir, 1964.
2. Alimov, Sh.A., Il’in, V.A., and Nikishin, E.M., Questions on the Convergence of Multiple Trigonometric Series and Spectral Expansions. I, Uspekhi Mat. Nauk, 1976, vol. 31, no. 6, pp. 27–82.
3. G?arding, L., Eigenfunction Expansions Connected with Elliptic Differential Operators, in 12 Skand. Math. Kongr., 1953, pp. 44–55.
4. Il’in, V.A., Theorem on the Possibility of Expansion of a Piecewise Smooth Function in the Eigenfunctions of an Arbitrary Two-Dimensional Region, Dokl. Akad. Nauk SSSR, 1956, vol. 109, pp. 442–445.
5. Maslov, V.P., Absolute Convergence Properties of Multidimensional Fourier Series from the Point of View of the Geometry of the Weak Discontinuities of the Functions Being Expanded, Dokl. Akad. Nauk SSSR, 1970, vol. 191, no. 2, pp. 275–278.