 Suppose that "ƒ" is an analytic function in a region in the complex plane which contains the closed disk D of radius "r" about the origin, "a"1, "a"2, ..., "a""n" are the zeros of "ƒ" in the interior of D repeated according to multiplicity, and "ƒ"(0) ≠ 0.Jensen's formula states that

This formula establishes a connection between the moduli of the zeros of the function "ƒ" inside the disk D and the average of log |"f"("z")| on the boundary circle |"z"| = "r", and can be seen as a generalisation of the mean value property of harmonic functions.Namely, if "f" has no zeros in D, then Jensen's formula reduces to
which is the mean-value property of the harmonic function formula_3.