George is right;

Hero of Alexandria was the first who conceived the number,

but Bombelli was the first who set down the rules for complex numbers,

the concept appeared also in works of Cardano and Descartes,

thanks to Euler and Gauss complex numbers became widely accepted.

Greetings, Tadeusz

George is right;

Hero of Alexandria was the first who conceived the number,

but Bombelli was the first who set down the rules for complex numbers,

the concept appeared also in works of Cardano and Descartes,

thanks to Euler and Gauss complex numbers became widely accepted.

Greetings, Tadeusz

Gaussss

It is Augustin Louis Cauchy who used the famous "i"  to express sqt(-1) as a mathematical tool. There is a book written in French dedicated to his manuscripts.

 As what said Tadeusz Ostrowski, the following links gave an history of the imagenary number i.

https://en.wikipedia.org/wiki/Imaginary_number

http://rossroessler.tripod.com/