Dirichlet eta function

Etymology
Named after Johann Peter Gustav Lejeune Dirichlet (1805–1859), German mathematician.

Proper noun

 * 1)  The alternating sum of the Dirichlet series expansion of the Riemann zeta function: $$\eta(s) = \sum_{n=1}^{\infty}{(-1)^{n-1} \over n^s} = \frac{1}{1^s} - \frac{1}{2^s} + \frac{1}{3^s} - \frac{1}{4^s} + \cdots.$$