Srinivasa Iyengar Ramanujan (1887-1920) was a Indian mathematician and autodidact who, with almost no formal training in pure mathematics, made extraordinary contributions to mathematical analysis, number theory, infinite series and continued fractions. He compiled about 3900 results that were original and highly unconventional. The Ramanujan prime and the Ramanujan theta function have led to a tremendous further research. A few major discoveries entered the mathematical mainstream a bit slowly. After his death, his formulae were found useful in string theory and crystallography.