OFFSET
1,2
LINKS
Colin Barker, Table of n, a(n) for n = 1..1000
David Lonoff and Jonah Ostroff, Symmetric Permutations Avoiding Two Patterns, Annals of Combinatorics 14 (1) pp.143-158 Springer, 2010; . - N. J. A. Sloane, Dec 27 2012
Lara Pudwell, Systematic Studies in Pattern Avoidance, 2005.
Index entries for linear recurrences with constant coefficients, signature (2,0,1,1,-1).
FORMULA
G.f.: x*(1 + x)*(1 - x + 2*x^2 - x^3) / ((1 + x^2)*(1 - 2*x - x^2 + x^3)).
a(n) = 2*a(n-1) + a(n-3) + a(n-4) - a(n-5) for n>5. - Colin Barker, Oct 20 2017
PROG
(PARI) Vec(x*(1 + x)*(1 - x + 2*x^2 - x^3) / ((1 + x^2)*(1 - 2*x - x^2 + x^3)) + O(x^40)) \\ Colin Barker, Oct 20 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Lara Pudwell, Feb 26 2006
STATUS
approved