Catalan's constant

Etymology
Named after French and Belgian mathematician Eugène Charles Catalan.

Proper noun

 * 1)  The number $$G = \beta(2) = \sum_{n=0}^{\infty} \frac{(-1)^{n}}{(2n+1)^2} = \frac{1}{1^2} - \frac{1}{3^2} + \frac{1}{5^2} - \frac{1}{7^2} + \frac{1}{9^2} - \cdots$$, where $β$ is the Dirichlet beta function, with a value of approximately 0.915965594177…