1. Bruijn, N. G. de: e-mail to A. Soifer, May 26, 2004
2. Bruijn, N. G. de: e-mail to A. Soifer, June 1, 2004
3. Bruijn, N. G. de: e-mail to A. Soifer, June 15, 2004
4. Camus, A.: [The Artist and His Age], Lecture at Uppsala University, December 14, 1957, in the anthology Speaking Out: Lectures and Speeches 1937–1958. Vintage International, New York (2022)
5. Delahaye, J.-P.: Coloriages irréals. Pour la Science, Février, 88–93 (2005). English translation: Imaginary coloring, Geombinatorics XV(3), 101–119 (2006)