OFFSET
1,2
LINKS
Colin Barker, Table of n, a(n) for n = 1..1000
D. Callan, T. Mansour, Enumeration of small Wilf classes avoiding 1324 and two other 4-letter patterns, arXiv:1705.00933 [math.CO] (2017), Table 2 No 85.
Lara Pudwell, Systematic Studies in Pattern Avoidance, 2005.
Index entries for linear recurrences with constant coefficients, signature (7,-18,21,-11,2).
FORMULA
G.f.: x*(1 - 5*x + 10*x^2 - 6*x^3 + 3*x^4) / ((1 - x)^2*(1 - 2*x)*(1 - 3*x + x^2)).
a(n) = 2^(-1-n)*(-7*4^n+5*(3+sqrt(5))^n - sqrt(5)*(3+sqrt(5))^n + (3-sqrt(5))^n*(5+sqrt(5)) + 3*2^(1+n)*n). - Colin Barker, Nov 02 2017
a(n) = 3*n + 5*Fibonacci(2*n - 1) - 7*2^(n - 1). - Ehren Metcalfe, Nov 08 2017
MATHEMATICA
LinearRecurrence[{7, -18, 21, -11, 2}, {1, 2, 6, 21, 73}, 40] (* Harvey P. Dale, Jan 16 2019 *)
PROG
(PARI) Vec(x*(1 - 5*x + 10*x^2 - 6*x^3 + 3*x^4) / ((1 - x)^2*(1 - 2*x)*(1 - 3*x + x^2)) + O(x^30)) \\ Colin Barker, Nov 02 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Lara Pudwell, Feb 26 2006
STATUS
approved