In mathematics, the digamma function is defined by

where D is the differential operator.

The digamma function, often denoted also ψ₀(x) or even ψ⁰(x), is related to the harmonic numbers in that

ψ(n) = H_{n - 1} - γ

where H_n−1 is the (n−1)th harmonic number, and γ is the well-known Euler-Mascheroni constant.

Also see

• Gamma function
• Polygamma function
• Harmonic number