Ramanujan theta function

Etymology
Named after mathematician.

Proper noun

 * 1)  A theta function that generalizes the form of the Jacobi theta functions while capturing their general properties. It is defined as: $$f(a,b) = \sum_{n=-\infty}^\infty a^\frac{n(n+1)}{2} \; b^\frac{n(n-1)}{2} $$ for |ab| < 1.