the keeper wrote:Does that mean I have an Erdos number of 6?, or because he wasn't a co-author my number is still undefined?

Still undefined. The AMS website has an Erdos-calculator, but it looks at only a small set of magazines (e.g. Phys Rev Letters not included after a certain year) that are "hardcore maths" --- giving strange results where some of my definitely more mathematical friends score undefined/infinite where very biological ones have 3 and 4.

Your supervisor having 5 is relatively high... he might have a lower one in an unexpected way.

@StigHemmer: it gives you 3, not 4! The MathSciNet says so!

I note that however few co-authors I have, I'm 5 in 4 different ways (and 4 in one way)... that is, recognized by the AMS calculator only.

Edit: hm, I'm not 4 because that was a chapter in a book --- the chapter is not jointly written, I had counted myself as co-author of the editors.