A211307 a(n) = number of n-lettered words in the alphabet {1, 2, 3, 4} with as many occurrences of the substring (consecutive subword) [1, 2] as of [2, 1].

1, 4, 14, 52, 192, 716, 2680, 10080, 38064, 144288, 548864, 2094640, 8017760, 30774656, 118422080, 456752512, 1765443584, 6837128064, 26525842176, 103080123392, 401172719104, 1563451421696, 6100771536896, 23833507144448, 93208099595776, 364876084582400, 1429651650049024, 5606325999663104

Offset

0,2

Links

Table of n, a(n) for n=0..27.

Shalosh B. Ekhad and Doron Zeilberger, Automatic Solution of Richard Stanley's Amer. Math. Monthly Problem #11610 and ANY Problem of That Type, arXiv preprint arXiv:1112.6207, 2011. See subpages for rigorous derivations of g.f., recurrence, asymptotics for this sequence.

Crossrefs

Sequence in context: A165813 A253199 A198279 * A369577 A099486 A047033

Adjacent sequences: A211304 A211305 A211306 * A211308 A211309 A211310

Keyword

nonn

Author

N. J. A. Sloane, Apr 08 2012

Status

approved