Erdös Numbers
Paul Erdös is a 20th Century mathematician who has published so profusely that the world of mathematics has created the notion of an Erdös Number.
Erdös' own number is zero. Anyone who has published together with Erdös has a number 1. Anyone who has published with somone who has Erdös number 1 is assigned an Erdös number of 2, etc.
Because the academic world is so collaborative, almost anyone who has published a peer reviewed technical paper has a finite Erdös Number.
My own Erdös Number is 6. The traceback can be seen below.
Among some interesting personalities' Erdös Numbers are: Don Knuth (2), Carl Sagan (6), Alan Turing (5), Steven Hawking (4), Paul Horowitz ,W1HFA (6), Joe Taylor, K1JT (4), Paul J. Cohen, exKB6OLJ (SK) (5).
My Traceback

0.
Paul Erdös

1. Jacobus H. van Lint (1966)
Paul Erdös, J. H. van Lint "On the average ratio of the smallest and largest prime divisor of n." in Simon Stevin 40 (1966), 7376.
Web reference to paper.

2.
Elwyn R. Berlekamp (1971)
Elwyn R. Berlekamp, J. H. van Lint, J. J. Seidel, "A Strongly Regular Graph Derived from the Perfect Ternary Golay Code." In A Survey of Combinatorial Theory, Symp. Colorado State Univ., 1971 (Ed. J. N. Srivastava et al.) Amsterdam, Netherlands: North Holland, 1973.
Web reference to paper.

3.
Claude E. Shannon (1967)
Claude E. Shannon, R. G. Gallager, E. R. Berlekamp, "Lower bounds to error probability for coding on discrete memoryless channels. I." Information and Control 10 (1967) 65103; MR 35#1405.
Web reference to paper.

4.
Bernard "Barney" M. Oliver (1948)
Bernard M. Oliver, J. R. Pierce, C. E. Shannon, "The Philosophy of PCM." Proceedings of the IRE, Vol 36 November 1948.
Web reference to paper.

5. Ivan R. Linscott (1985)
D. Kent Cullers, I. R. Linscott, B. M. Oliver, "Signal processing in SETI." Communications of the ACM. 01/12/198512/1985.
Web reference to paper.

6.
Kok S. Chen (1985)
Allen M. Peterson, K. S. Chen, I. R. Linscott, "The Multichannel Spectrum Analyzer." IAU Symp. 112, 373384 (Ed. M.D. Papagiannis) 1985.
Web reference to paper.