1. E. Gagliardo, “Ulteriori proprietà di alcune classi di funzioni in pi\‘u variabili,” Ricerche Mat. 8, 24–51 (1959).
2. L. Nirenberg, “On elliptic partial differential equations,” Ann. Scuola Norm. Sup. Pisa Cl. Sci. (3) 13, 115–162 (1959).
3. O. V. Besov, V. P. Il’in, and S. M. Nikol’skii, Integral Representations of Functions and Embedding Theorems (Nauka, Moscow, 1996) [in Russian].
4. H. Triebel, Interpolation Theory, Function Spaces, Differential Operators (Birkhäuser, Berlin, 1977).
5. L. Hörmander, Estimates for Translation-Invariant Operators in $$L^p$$ Spaces (Uppsala, 1960).