517.95We study the well-posedness of the problem with general nonlocal boundary conditions in the time variable and conditions of periodicity in the space coordinates for partial differential equations unsolved with respect to the higher time derivative. We establish the conditions of existence and uniqueness of the solution of the considered problem. In the proof of existence of the solution, we use the method of divided differences. We also prove metric statements on the lower bounds of small denominators appearing in constructing the solution of the problem.Boundary-value problems for equations unsolved with respect to the higher time derivative are encountered in the study of small vibrations of ideal [1, 2] and viscous [3] fluids in rotating vessels, in the problems of filtration of fluid in cracked rocks [4], in the analysis of small vibrations of exponentially stratified fluid in gravitational fields [5,6], etc. In the cited papers and in [7 -9], the Cauchy problem and mixed problems are studied for differential and differential-operator equations unsolved with respect to the higher derivative.The multipoint and Dirichlet-type problems for equations and systems of equations unsolved with respect to the higher time derivative are studied in [10,11].The problems with nonlocal conditions for partial differential equations unsolved with respect to the higher time derivative are studied in [12,13]. For differential-operator equations, these problems are investigated in [14].In the present paper, we consider the problem with general nonlocal boundary conditions in the time variable and periodic conditions in the space coordinates for partial differential equations unsolved with respect to the higher time derivative. In deducing the conditions of well-posedness of the problem, we use divided differences and obtain new metric statements for the lower bounds of small denominators of a certain type.