CM Values of Higher Green's Functions

Higher Green functions are real-valued functions of two variables on the upper half plane which are bi-invariant under the action of a congruence subgroup, have logarithmic singularity along the diagonal, and satisfy the equation $\Delta f = k(1 - k)f$, where $\Delta$ is a hyperbolic Laplace operator and $k$ is a positive integer. Such functions were introduced in the paper of Gross and Zagier "Heegner points and derivatives of $L$-series"(1986). Also it was conjectured in this paper that higher Green's functions have "algebraic" values at CM points. In many particular cases this conjecture was proven by A. Mellit in his Ph. D. thesis. In this note we present a proof of the conjecture for any pair of CM points lying in the same quadratic imaginary field.