The Dirichlet lambda function λ(x) is the Dirichlet L-series defined by λ(x) | congruent | sum_(n = 0)^∞ 1/(2n + 1)^x | = | (1 - 2^(-x)) ζ(x), where ζ(x) is the Riemann zeta function. The function is undefined at x = 1. It can be computed in closed form where ζ(x) can, that is for even positive n. The Dirichlet lambda function is implemented in the Wolfram Language as DirichletLambda[x].

Dirichlet beta function | Dirichlet eta function | Dirichlet L-series | Legendre's chi-function | Riemann zeta function | zeta function