Diophantus (lived AD 250)

Greek mathematician in Alexandria whose Arithmetica is one of the first known works on problem solving by algebra, in which both words and symbols are used.

His main mathematical study was in the solution of what are now known as ‘indeterminate’ or ‘Diophantine’ equations – equations that do not contain enough facts to give a specific answer but enough to reduce the answer to a definite type. These equations have led to the formulation of the theory of numbers, regarded as the purest branch of present-day mathematics.

In the solution of equations Diophantus was the first to devise a system of abbreviating the expression of his calculations by means of a symbol representing the unknown quantity. Because he invented only one symbol for the unknown, however, in equations requiring two or more variables his work can become extremely confusing in its repetition of that single symbol.