Chatting to colleagues at work the other day I boasted that my Erdös number had dropped to 5 (see previous post on this), and Dan asked what an Erdös number was. I explained, and he obviously went back to his office to see if he could work out his, because he returned a few minutes later to say that his was 3, and I was 4. This was because he and Josh Salomon had co-authored a paper with Adrian Raftery (Erdös 2), and I have published several papers with Josh. Here is the full path:

1 Erdös, P.; Babu, G. Jogesh; Ramachandra, K. An asymptotic formula in additive number theory.   Acta Arith. 28 (1976) no. 4, 405-412.

2 Mukherjee, S., Feigelson, E.D., Babu, G.J., Murtagh, F., Fraley, C. and Raftery, A.E. Three types of gamma ray bursts. Astrophysical Journal 1998; 508, 314-327.

3 Le Bao, Josh A Salomon, Tim Brown, Adrian E Raftery, Daniel R Hogan. Modelling national HIV/AIDS epidemics: revised approach in the UNAIDS Estimation and Projection Package 2011. Sexually transmitted infections 2012, 88 (Suppl 2), i3-i10.

4 Colin D Mathers, Ritu Sadana, Josh Salomon, Christopher JL Murray, Lopez AD. Healthy life expectancy in 191 countries, 1999. The Lancet 2001, Vol 357: 1685-1691.

And this gives me another path for Einstein number of 6, since Einstein has Erdös number 2 (through Ernst G Strauss). Through Josh, most of my colleagues of the early days in WHO will also have an Erdös number of 4, as will the many hundreds of his co-authors on the recent GBD 2010 papers (Lancet December 2012).  Presumably this has also happened in physics, where the particle physics papers have had huge numbers of authors for decades now.