Ψψ

PSI

ψεῖ · [ps]

The 23rd letter of the Greek alphabet.

MATHEMATICS

  • Digamma function ψ(x) (derivative of log Γ).
  • Generic symbol in analysis.

PROBABILITY

  • Appears in moments/estimates involving Γ/Beta.
  • Shows up in Bayesian conjugacy contexts.

STATISTICS

  • Digamma ψ in MLE of Gamma/Dirichlet.
  • Used in EM/variational algorithms in some models.

PRONUNCIATION

Ancient Greek
sound [ps]
Modern Greek
ψι [psi] · sound [ps]

IN FORMULAS

  • ψ(x) = d/dx ln Γ(x)The digamma function, the derivative of the log of Γ.

HOW TO WRITE IT — click to copy

lowercase
uppercase

Note: notations vary between books/fields; these are the most common usages.