A264077 Number of permutations of [n] with exactly two (possibly overlapping) occurrences of the consecutive step pattern up, down, up, down.

272, 4352, 42880, 530432, 7662336, 101713920, 1402932224, 21604831232, 343423787008, 5608501018624, 97560785780736, 1783363198386176, 33762252617416704, 668148809474244608, 13830009274919288832, 297227159309157138432, 6632506903059366936576

Offset: 7

Author

Alois P. Heinz, Nov 02 2015

Keywords

• nonn

Links

• Alois P. Heinz, Table of n, a(n) for n = 7..460

Crossrefs

Column k=2 of A230797.

Programs

Maple

b:= proc(u, o, t, c) option remember; `if`(c>2, 0, `if`(u+o=0,
     `if`(c=2, 1, 0), add(b(u-j, o-1+j, [2,2,4,2][t], c), j=1..u)+
      add(b(u+j-1, o-j, [1,3,1,3][t], c+`if`(t=4,1,0)), j=1..o)))
    end:
a:= n-> b(0, n, 1, 0);
seq(a(n), n=7..25);  # Alois P. Heinz, Nov 02 2015