Flexagons yield a curious Catalan number identity

Details Flexagons yield a curious Catalan number identity Flexagons yield a curious Catalan number identity

2010-05-31

arxiv.org/abs/1005.5736v2

Mathematics

Combinatorics

4 pages, shorter method, suggested by Ira Gessel; other (minor) improvements

Scientific paper

Hexaflexagons were popularized by the late Martin Gardner in his first Scientific American column in 1956. Oakley and Wisner showed that they can be represented abstractly by certain recursively defined permutations called pats, and deduced that they are counted by the Catalan numbers. Counting pats by number of descents yields the curious identity Sum[1/(2n-2k+1)binom{2n-2k+1}{k}binom{2k}{n-k},k=0..n] = C(n), where only the middle third of the summands are nonzero.

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