Cardano 

He is best known now for his work on algebra. He published the solutions to the cubic equation and quartic equation in his 1545 book Ars Magna. The solution to one particular case of the cubic, $x^3+ax=b$ (in modern notation), was communicated to him by Niccolo Fontana Tartaglia (who later claimed that Cardano had sworn not to reveal it, and engaged Cardano in a decadelong feud), and the quartic equation was solved by Cardano's student Lodovico Ferrari. Both were acknowledged in the foreword of the book, as well as in several places within its body. In his exposition, he acknowledged the existence of what are now called imaginary numbers, although he did not understand their properties.