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

7,1

Links

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

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

Crossrefs

Column k=2 of A230797.

Sequence in context: A205345 A302899 A266106 * A305683 A317063 A316930

Adjacent sequences: A264074 A264075 A264076 * A264078 A264079 A264080

Keyword

nonn

Author

Alois P. Heinz, Nov 02 2015

Status

approved