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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]. 0
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 (list; graph; refs; listen; history; text; internal format)
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 * A099486 A047033 A017947

Adjacent sequences:  A211304 A211305 A211306 * A211308 A211309 A211310

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Apr 08 2012

STATUS

approved

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Last modified September 19 20:55 EDT 2020. Contains 337182 sequences. (Running on oeis4.)