%I A032351
%S A032351 1,2,6,22,88,366,1552,6652,28696,124310,540040,2350820,10248248,
%T A032351 44725516,195354368,853829272,3733693872,16333556838,71476391800,
%U A032351 312865382004,1369760107576,5998008630244,26268304208032
%N A032351 Number of permutations of length n which avoid the patterns 2143, 1324 
               (smooth permutations); or avoid the patterns 1342, 2431; etc.
%D A032351 R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see 
               Problem 6.47.
%D A032351 A. Woo and A. Yong, "When is a Schubert variety Gorenstein?", preprint 
               2004.
%H A032351 Miklos Bona, The permutation classes equinumerous to the smooth class, 
               <a href="http://www.combinatorics.org/">Electron. J. Combin.</a>, 
               5 (1998), no. 1, Research Paper 31, 12 pp.
%H A032351 M. Bousquet-Melou and S. Butler, <a href="http://arXiv.org/abs/math.CO/
               0603617">Forest-like permutations</a>
%H A032351 A. Woo and A. Yong, <a href="http://www.math.berkeley.edu/~ayong/Goren.ps">
               When is a Schubert variety Gorenstein?</a>.
%F A032351 G.f.: A(x)=\frac{1-5x+3x^2+x^2\sqrt{1-4x}}{1-6^x+8x^2-4x^3}
%Y A032351 Cf. A053617.
%Y A032351 Sequence in context: A165534 A165535 A165536 this_sequence A165537 A165538 
               A165539
%Y A032351 Adjacent sequences: A032348 A032349 A032350 this_sequence A032352 A032353 
               A032354
%K A032351 nonn,easy,nice
%O A032351 1,2
%A A032351 Miklos Bona (bona(AT)math.ufl.edu)
%E A032351 More terms from Erich Friedman (erich.friedman(AT)stetson.edu).