A250263 Number of permutations p of [n] such that p(i) > p(i+1) iff i == 1 (mod 7).

1, 1, 1, 2, 3, 4, 5, 6, 7, 55, 306, 1145, 3454, 8997, 21008, 45031, 630421, 5954062, 36439007, 174011100, 697278853, 2442137774, 7677528495, 154507257827, 2060124800270, 17511127577941, 114431145225190, 619024078342189, 2890746049056048, 11979440405949527

Offset: 0

Author

Alois P. Heinz, Nov 15 2014

Keywords

• nonn

Links

• Alois P. Heinz, Table of n, a(n) for n = 0..500

Crossrefs

Column k=7 of A250261.

Programs

Maple

b:= proc(u, o, t) option remember; `if`(u+o=0, 1,
     `if`(t=1, add(b(u-j, o+j-1, irem(t+1, 7)), j=1..u),
               add(b(u+j-1, o-j, irem(t+1, 7)), j=1..o)))
    end:
a:= n-> b(0, n, 0):
seq(a(n), n=0..35);

Mathematica

b[u_, o_, t_] := b[u, o, t] = If[u + o == 0, 1, If[t == 1, Sum[b[u - j, o + j - 1, Mod[t + 1, 7]], {j, 1, u}], Sum[b[u + j - 1, o - j, Mod[t + 1, 7]], {j, 1, o}]]];
a[n_] := b[0, n, 0];
Table[a[n], {n, 0, 35}] (* Jean-François Alcover, Jul 22 2019, after Alois P. Heinz *)