1. O. V. Besov, V. P. Il'in, and C. M. Nikol'ski i, Integral Representations and Embedding Theorems [in Russian], Nauka, Moscow (1975).
2. L. R. Volevich, “Local properties of solutions to quasi-elliptic systems,” Mat. Sb.,59, 3–52 (1962).
3. L. R. Volevich, and S. G. Gindikin, “On a certain class of hypoelliptic polynomials,” Mat. Sb.,75, No. 3, 400–416 (1968).
4. E. Giusti, “Equazioni quasi-ellittiche e spazi $$\mathfrak{L}^{p,\theta } (\Omega ,\delta )$$ . I,” Ann. Mat. Pura Appl. (4),75, 313–353 (1967).
5. E. Giusti, “Equazioni quasi-ellittiche e spazi $$\mathfrak{L}^{p,\theta } (\Omega ,\delta )$$ . II,” Ann. Scuola Norm. Sup. Pisa Cl. Sci. (4),21, No. 3, 354–372 (1967).