BibTeX: 
@article{SumSquaresAFP,
author = {Roelof Oosterhuis},
title = {Sums of Two and Four Squares},
journal = {Archive of Formal Proofs},
month = aug,
year = 2007,
note = {\url{https://isaafp.org/entries/SumSquares.html},
Formal proof development},
ISSN = {2150914x},
}
