"Complex number" , Encyclopedia of Mathematics , Springer , ISBN   978-1-55608-010-4 , http://www.encyclopediaofmath.org/index.php?title=p/c024140   Introduction to Complex Numbers from Khan Academy Imaginary Numbers on In Our Time at the BBC . Euler's work on Complex Roots of Polynomials at Convergence. MAA Mathematical Sciences Digital Library. John and Betty's Journey Through Complex Numbers