Oscillation of a kind of two-variables functional equation

In this paper, we consider the functional equation of the form where a, b, c are positive constants, k, l are positive integers, function P : I × I → R+ = (0, ∞), I (0, ∞) is an unbounded set, τ, σ : I → I, τ(x) ≠ x, σ(y) ≠ y, and limx→∞ τ(x) = ∞, limy→∞ σ(y) = ∞, x, y ε I. Some oscillation criteria of this equation are obtained.