Gross, E. P. Unified theory of interacting bosons. Phys. Rev. 106, 161–162 (1957). Article ADS CAS MATH Google Scholar Andreev, A. F. & Lifshitz, I. M. Quantum theory of defects in crystals. Zhur. Eksper. Teoret. Fiziki 29, 1107–1113 (1969). MATH Google Scholar Leggett, A. J. Can a solid be ‘superfluid’? Phys. Rev. Lett. 25, 1543–1546
