It was the Indians in the seventh century AD who successfully developed a theory of negative numbers. In particular, Brahmagupta deduced some of the important mathematical properties of these numbers: for example, that ‘a debt multiplied by a debt is a fortune’, or minus times minus is plus. (Interestingly, this isn’t a rule, it’s a consequence of the axioms of mathematics. It is a fun challenge to prove why this must be so.) It took Europeans till the fifteenth century to be convinced that there were numbers that could solve these sorts of equations. Indeed, the use of negative numbers was
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