“…This approach is called Euclidean due to its conceptual analogy with the Euclidean quantum field theory. Its further development was conducted in the papers [2,3,4,5,8,7,11,12,13,14,16,48,49,50,52,54,55,66,67]. Among the achievements one has to mention the settlement in [3,5,6] of a long standing problem of the influence of quantum effects on structural phase transitions in quantum anharmonic crystals, which first was discussed in [77], see also [67,86,87].…”