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A264077 Number of permutations of [n] with exactly two (possibly overlapping) occurrences of the consecutive step pattern up, down, up, down. 2
272, 4352, 42880, 530432, 7662336, 101713920, 1402932224, 21604831232, 343423787008, 5608501018624, 97560785780736, 1783363198386176, 33762252617416704, 668148809474244608, 13830009274919288832, 297227159309157138432, 6632506903059366936576 (list; graph; refs; listen; history; text; internal format)
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

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Last modified January 23 11:16 EST 2022. Contains 350512 sequences. (Running on oeis4.)