A095889 Number of permutations of [n] with exactly 3 descents which avoid the pattern 4321.
10, 148, 1260, 8160, 44790, 220180, 1001000, 4295168, 17633122, 69921460, 269652100, 1016524352, 3760334958, 13690636212, 49172875520, 174559419680, 613383654170, 2136135472020, 7380351046300, 25319087913760, 86308819112550, 292526411823828, 986294675689560
Offset: 5
Links
- Colin Barker, Table of n, a(n) for n = 5..1000
- Index entries for linear recurrences with constant coefficients, signature (22,-216,1248,-4710,12180,-22004,27752,-23937,13446,-4428,648).
Programs
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PARI
Vec(2*x^5*(5 - 36*x + 82*x^2 - 36*x^3 - 87*x^4 + 80*x^5) / ((1 - x)^4*(1 - 2*x)^3*(1 - 3*x)^4) + O(x^30)) \\ Colin Barker, Nov 03 2017
Formula
G.f.: 2*x^5*(5 - 36*x + 82*x^2 - 36*x^3 - 87*x^4 + 80*x^5) / ((1 - x)^4*(1 - 2*x)^3*(1 - 3*x)^4).
a(n) = (n*(27 + 81*2^n - 115*3^n + 3*(27*2^n+2*3^n)*n + (-27+3^n)*n^2)) / 162. - Colin Barker, May 03 2019