Keldysh problem for mixed type equation with strong characteristic degeneration and singular coefficient

For a mixed-type equation of the second kind with a singular coefficient with the use of spectral expansions, we establish a uniqueness criterion for solving the problem with incomplete boundary data. The solution is constructed in the form of the Fourier–Bessel sums. In substantiation of the uniform convergence we establish an estmate on separation from zero of small denominator with the corresponding asymptotic behavior that allowed to substantiate the convergence of the series in the class of regular solutions.