Euler Four-Square Identity -- from Wolfram MathWorld

The amazing polynomial identity communicated by Euler in a letter to Goldbach on April 12, 1749 (incorrectly given as April 15, 1705--before Euler was born--in Conway and Guy 1996, p. 232). The identity also follows from the fact that the norm of the product of two quaternions is the product of the norms (Conway and Guy 1996).

Polynomial Identity -- from Wolfram MathWorld

Complex Numbers as Matrices: Euler's Identity – IB Maths Resources

Euler's Four-square Identity

PDF) Taylor's series expansions for real powers of two functions

Leonhard Euler's Contributions in Mathematics – StudiousGuy

PDF) Sums of four squares and Waring's Problem

How to plot a square root on a complex plane - Quora

mathrecreation: Triangular Numbers and Euler's Number Triangle

Full article: Prospective mathematics teachers learning complex

Maths Ed Ideas: On Descartes Numbers

Numerical nonlinear frequency analysis of pre-damaged curved