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Closed meanders with 3 components and 2n bridges.
(Formerly M4014)
1

%I M4014 #29 Sep 25 2019 07:46:15

%S 5,56,580,5894,60312,624240,6540510,69323910,742518832,8028001566,

%T 87526544560,961412790002,10630964761766,118257400015312,

%U 1322564193698320,14863191405246888,167771227744292160,1901345329566422790

%N Closed meanders with 3 components and 2n bridges.

%D S. K. Lando and A. K. Zvonkin "Plane and projective meanders", Séries Formelles et Combinatoire Algébrique. Laboratoire Bordelais de Recherche Informatique, Université Bordeaux I, 1991, pp. 287-303.

%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

%H P. Di Francesco, O. Golinelli and E. Guitter, <a href="http://arXiv.org/abs/hep-th/9506030">Meander, folding and arch statistics</a>, arXiv:hep-th/9506030, 1995.

%H S. K. Lando and A. K. Zvonkin, <a href="/A005316/a005316_1.pdf">Plane and projective meanders</a>, Séries Formelles et Combinatoire Algébrique. Laboratoire Bordelais de Recherche Informatique, Université Bordeaux I, 1991, pp. 287-303. (Annotated scanned copy)

%t A008828 = Import["https://oeis.org/A008828/b008828.txt", "Table"][[All, 2]];

%t a[n_] := A008828[[(n^2 - n + 6)/2]];

%t a /@ Range[3, 20] (* _Jean-François Alcover_, Sep 25 2019 *)

%Y Cf. A005315, A006657.

%Y A column of triangle A008828.

%K nonn

%O 3,1

%A _N. J. A. Sloane_

%E a(13)-a(20) from _Andrew Howroyd_, Nov 22 2015