OFFSET
1,2
COMMENTS
In a 1-1 mapping with permutations that avoid the patterns (1423, 4213, 2314, 2431, 2413, <3142,{2},{2}>) (the fourth pattern is bivincular).
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..1000
Gadi Aleksandrowicz, Andrei Asinowski and Gill Barequet, A polyominoes-permutations injection and tree-like convex polyominoes, Journal of Combinatorial Theory, Series A, Volume 119, Issue 3, April 2012, Pages 503-520
A. Goupil, H. Cloutier, and F. Nouboud, Enumeration of inscribed polyominos, FPSCA 2010 (San Francisco) DMTS proc. AN 2010, 737-748, eq. (10)
Index entries for linear recurrences with constant coefficients, signature (6,-14,16,-9,2).
FORMULA
G.f.: (x*(1-4*x+8*x^2-6*x^3+4*x^4))/((1-x)^4*(1-2*x)).
a(n) = 6*a(n-1) - 14*a(n-2) + 16*a(n-3) - 9*a(n-4) + 2*a(n-5).
a(n) = 2^(n+2) - (n^3-n^2+10*n+4)/2.
MATHEMATICA
LinearRecurrence[{6, -14, 16, -9, 2}, {1, 2, 6, 18, 51}, 50] (* Harvey P. Dale, Oct 16 2011 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Gill Barequet, Oct 04 2011
STATUS
approved