Nonlocal Problem with Integral Condition for Mixed-Type Equation with Bessel Operator

In a rectangular domain, a boundary value problem with an integral condition is considered for an equation of elliptic-hyperbolic type with singular coefficients in both spatial variables. The solution to the problem is constructed in explicit form—in the form of a Fourier–Bessel series of eigenfunctions of a one-dimensional spectral problem. A theorem for the uniqueness of a solution to the problem is proven when the condition of separability from zero of an expression containing the values of the region under consideration, one of the values of the equation parameter, and the eigenvalues of the spectral problem is satisfied. This problem of separability of the resulting expression from zero when justifying the correct solvability of the problem is called the problem of small denominators.