a set of integers {1...n}, to be partitioned such that neighbours are in different subsets: {{1,3,5},{2,4}} {{1},{2,4},{3,5}} {{1,3},{2,4},{5}} {{1,3},{2,5},{4}} {{1,4},{2},{3,5}} {{1,4},{2,5},{3}} {{1,5},{2,4},{3}} {{1,3,5},{2},{4}} {{1},{2},{3,5},{4}} {{1},{2,4},{3},{5}} {{1},{2,5},{3},{4}} {{1,3},{2},{4},{5}} {{1,4},{2},{3},{5}} {{1,5},{2},{3},{4}} {{1},{2},{3},{4},{5}} this turns out to be Bell numbers, and by lengths they subttotal to StirlingS2: 1,7,6,1 but wasn't that "the number of ways of partitioning a set of n elements into m non-empty subsets" ? like 4 elements into 1..4 non-empty subsets? {{1,2,3,4}} {{1},{2,3,4}} {{1,2},{3,4}} {{1,3},{2,4}} {{1,4},{2,3}} {{1,2,3},{4}} {{1,2,4},{3}} {{1,3,4},{2}} {{1},{2},{3,4}} {{1},{2,3},{4}} {{1},{2,4},{3}} {{1,2},{3},{4}} {{1,3},{2},{4}} {{1,4},{2},{3}} {{1},{2},{3},{4}} Aaach, Math is *really* Fun ! Wouter =============================== 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.