OFFSET
-1,2
COMMENTS
a(n) is the number of permutations pi of [n+4] such that s(pi) is one of 132456...(n+4), 231456...(n+4), 312456...(n+4), or 321456...(n+4) and s denotes West's stack-sorting map. - Colin Defant, Jan 14 2019
LINKS
Vincenzo Librandi, Table of n, a(n) for n = -1..1000
FORMULA
G.f.: (2 - 9*x + 6*x^2 + 2*x^3 + (5*x-2)*sqrt(1-4*x))/(x^4) = 6*x + 14*x^2*G(0); G(k) = 1 + 1/(k + 1 - 2*x*(k+1)*(k+2)*(2*k+9)/(2*x*(k+2)*(2*k+9) + (k+7)/G(k+1))); (continued fraction, 3-step ). - Sergei N. Gladkovskii, Jan 08 2012
-(n+4)*(n-1)*a(n) + 2*n*(2*n+3)*a(n-1) = 0, n > 0. - R. J. Mathar, Dec 15 2015
G.f.: x*C(x)^6 + 3*(1+C(x))C(x)^4, where C(x) = (1-sqrt(1-4x))/(2x) is the Catalan function. - Colin Defant, Jan 14 2019
MATHEMATICA
Table[6 (n + 1) (2 n + 6)! / ((n + 3)! (n + 5)!), {n, -1, 25}] (* Vincenzo Librandi, Jan 15 2019 *)
PROG
(Magma) [6*(n+1)*Factorial(2*n+6)/(Factorial(n+3)*Factorial(n+5)): n in [-1..40]]; // Vincenzo Librandi, Jan 15 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Grisha Berkolaiko (grisha(AT)ORY.ph.biu.ac.il)
STATUS
approved