The Erdős number describes the "collaborative distance" between mathematician Paul Erdős and another person, as measured by authorship of mathematical papers. An Erdős number of 3 indicates co-authorship of a paper with someone who co-authored a paper with a co-author of Paul Erdős
Peter Cameron → Colva Roney-Dougal → Tom Kelsey
Tractable Symmetry Breaking using Restricted Search Trees
C M Roney-Dougal, I P Gent, T W Kelsey, S A Linton
Proceedings of the 16th European Conference on Artificial Intelligence
Generating sets of finite groups
Cameron, Peter J., Lucchini, Andrea and Roney-Dougal, Colva M.
Trans. Amer. Math. Soc. 370 (2018), no. 9, 6751–6770.
On the number of sets of integers with various properties
Cameron, P. J. and Erdős, P.
Number theory (Banff, AB, 1988), 61–79, de Gruyter, Berlin