Born in Northwest India, I am often referred to as Bhillamalacarya (i.e. the teacher from Bhillamala).
While the head of the astronomical observatory at Ujjain, I wrote four texts on mathematics and astronomy, the most famous being The Brahmasphutasiddhanta. (Corrected Treatise of Brahma).
This book is written in the style of the "elliptic verse" common to Indian mathematics at that time...including a "poetic ring" and no proofs.
I was the first to use zero as a number (and not just a placeholder), and offered rules for computing with it...and now people point out that I erred in my interpretation of 0/0 and N/0.
My fame is tied to an eponymic formula in geometry: Given the side lengths of any cyclic quadrilateral, the formula computes its area.
But, I also showed how to solve a general linear equation, found sums of series of squares and cubes, found the equivalent of Pythagorean Triples, gave a recurrance relation for solving Pell's equation, gave an approximate value for pi, created a sine table...among other things.
Brahmagupta (598 - 668)