Burkert, Jeffrey; Johnson, Peter (2011), "Szlam's lemma: mutant offspring of a Euclidean Ramsey problem from 1973, with numerous applications", Ramsey theory, Progr. Math., vol. 285, Birkhäuser/Springer, New York, pp. 97–113, doi:10.1007/978-0-8176-8092-3_6, MR2759046. See also Soifer (2008), Problem 40.26, p. 496.
Burkert, Jeffrey; Johnson, Peter (2011), "Szlam's lemma: mutant offspring of a Euclidean Ramsey problem from 1973, with numerous applications", Ramsey theory, Progr. Math., vol. 285, Birkhäuser/Springer, New York, pp. 97–113, doi:10.1007/978-0-8176-8092-3_6, MR2759046. See also Soifer (2008), Problem 40.26, p. 496.