Thanks to Don Reble for the composite M(p), and to Richard Schroeppel for the gentle reminder of the (2p+1)|M(p) theorem. It's obviously been too long since I looked at my own notes of the '80s. Curious that M(M(61)) hasn't succumbed to factor-searches yet. There must be a good chance that it's prime. http://www.ltkz.demon.co.uk/ar2/mm61.htm may or may not still be active. "Last updated 2001" - mmm. Guy