OFFSET
1,2
LINKS
Colin Barker, Table of n, a(n) for n = 1..1000
Darla Kremer and Wai Chee Shiu, Finite transition matrices for permutations avoiding pairs of length four patterns, Discrete Math. 268 (2003), 171-183. MR1983276 (2004b:05006). See Table 1.
Lara Pudwell, Systematic Studies in Pattern Avoidance, 2005.
Index entries for linear recurrences with constant coefficients, signature (7,-16,16,-4).
FORMULA
G.f.: -x*(x-1)*(2*x-1)^2 / (4*x^4-16*x^3+16*x^2-7*x+1).
a(n) = 7*a(n-1) - 16*a(n-2) + 16*a(n-3) - 4*a(n-4) for n>3. - Colin Barker, Oct 20 2017
MATHEMATICA
CoefficientList[Series[-(x - 1)*(2*x - 1)^2/(4*x^4 - 16*x^3 + 16*x^2 - 7*x + 1), {x, 0, 30}], x] (* Wesley Ivan Hurt, Sep 04 2022 *)
PROG
(PARI) Vec(x*(1 - x)*(1 - 2*x)^2 / (1 - 7*x + 16*x^2 - 16*x^3 + 4*x^4) + O(x^40)) \\ Colin Barker, Oct 20 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Lara Pudwell, Feb 26 2006
STATUS
approved