Willem de Sitter Sources

"Sitter Biography." MacTutor History of Mathematics. Web. 20 Feb. 2011. <http://www-history.mcs.st-andrews.ac.uk/Biographies/Sitter.html>.

A. Blaauw, "Sitter, Willem de (1872-1934)," in Biographical Dictionary of the Netherlands .                          <http://www.inghist.nl/Onderzoek/Projecten/BWN/lemmata/bwn2/sitter> [13/03/2008]

Lipanovich, Charles. "de Sitter, Willem" The Columbia Encyclopedia. Morningside Heights, New York.             Columbia University Press. 1940.