thanks Shel, Rich's shortish answer helped me grok it in half a day (he must have seen it in half a second): ratio of successive central binomial coefficients = 4-2/(1+n) (running sum of seq ...), seq[n], seq[n+1] x + 1 = 4 (solve for x) (* same goes for catalans *) cute no? that's why I like this group. Wouter Meeussen email: wouter.meeussen@vandemoortele.com =============================== This email is confidential and intended solely for the use of the individual to whom it is addressed. If you are not the intended recipient, be advised that you have received this email in error and that any use, dissemination, forwarding, printing, or copying of this email is strictly prohibited. You are explicitly requested to notify the sender of this email that the intended recipient was not reached.