A269042 - OEIS

%I #18 Apr 01 2017 19:13:26

%S 0,0,1,132,15767,2190688,370531683,77182248916,19835792076675,

%T 6266271456118776,2413632612087046844,1120958514818713738544,

%U 619918692943471064695593,403190647991638511052901232,304867528413299672718870216538,265248225675908889875489731636920

%N Number of permutations of [2n] avoiding the pattern 12...n.

%H Alois P. Heinz, <a href="/A269042/b269042.txt">Table of n, a(n) for n = 0..30</a>

%F a(n) = (2n)! - A269021(n).

%F a(n) = A214015(2n,n-1) for n>0.

%F a(n) ~ (2*n)!. - _Vaclav Kotesovec_, Mar 26 2016

%e a(2) = 1: 4321.

%e a(3) = 132: 165432, 216543, 261543, 265143, 265413, 265431, 316542, ..., 653412, 653421, 654132, 654213, 654231, 654312, 654321.

%p h:= proc(l) (n-> add(i, i=l)!/mul(mul(1+l[i]-j+add(`if`(

%p       l[k]>=j, 1, 0), k=i+1..n), j=1..l[i]), i=1..n))(nops(l))

%p     end:

%p g:= (n, i, l)-> `if`(n=0 or i=1, h([l[], 1$n])^2, `if`(i<1, 0,

%p                  add(g(n-i*j, i-1, [l[], i$j]), j=0..n/i))):

%p a:= n-> `if`(n=0, 0, g(2*n, n-1, [])):

%p seq(a(n), n=0..15);

%t h[l_] := Function[n, Total[l]!/Product[Product[1+l[[i]]-j+Sum[If[l[[k]] >= j, 1, 0], { k, i+1, n}], {j, 1, l[[i]]}], {i, 1, n}]][Length[l]];

%t g[n_, i_, l_] := If[n == 0 || i == 1, h[Join[l, Table[1, {n}]]]^2, If[i < 1, 0, Sum[g[n - i*j, i-1, Join[l, Table[i, {j}]]], {j, 0, n/i}]]];

%t a[n_] := If[n == 0, 0, g[2n, n-1, {}]];

%t Table[a[n], {n, 0, 15}] (* _Jean-François Alcover_, Apr 01 2017, translated from Maple *)

%Y Cf. A010050, A214015, A267532, A269021.

%K nonn

%O 0,4

%A _Alois P. Heinz_, Feb 18 2016