%I
%S 1,4,14,52,192,716,2680,10080,38064,144288,548864,2094640,8017760,
%T 30774656,118422080,456752512,1765443584,6837128064,26525842176,
%U 103080123392,401172719104,1563451421696,6100771536896,23833507144448,93208099595776,364876084582400,1429651650049024,5606325999663104
%N a(n) = number of nlettered words in the alphabet {1, 2, 3, 4} with as many occurrences of the substring (consecutive subword) [1, 2] as of [2, 1].
%H Shalosh B. Ekhad and Doron Zeilberger, <a href="http://arxiv.org/abs/1112.6207">Automatic Solution of Richard Stanley's Amer. Math. Monthly Problem #11610 and ANY Problem of That Type</a>, arXiv preprint arXiv:1112.6207, 2011. See subpages for rigorous derivations of g.f., recurrence, asymptotics for this sequence.
%K nonn
%O 0,2
%A _N. J. A. Sloane_, Apr 08 2012
