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A292172
Number of permutations p of [n] such that 0p has a nonincreasing jump sequence beginning with six.
2
52, 166, 425, 1116, 3392, 10872, 38795, 102634, 260334, 651704, 1707641, 4477199, 12277412, 30970764, 75239813, 181934538, 440594545, 1063081418, 2625678546, 6286974074, 14803016600, 34534616815, 80129926919, 185059517397, 431845849360, 997735570874
OFFSET
6,1
COMMENTS
An up-jump j occurs at position i in p if p_{i} > p_{i-1} and j is the index of p_i in the increasingly sorted list of those elements in {p_{i}, ..., p_{n}} that are larger than p_{i-1}. A down-jump j occurs at position i in p if p_{i} < p_{i-1} and j is the index of p_i in the decreasingly sorted list of those elements in {p_{i}, ..., p_{n}} that are smaller than p_{i-1}. First index in the lists is 1 here.
LINKS
EXAMPLE
a(6) = 52: 612345, 613245, 613425, 613452, 613524, 613542, 614235, 614253, 614325, 614352, 614532, 615234, 615243, 615324, 615342, 615432, 621345, 623145, 623415, 623451, 624135, 624153, 624315, 624351, 624531, 625134, 625143, 625314, 625341, 625431, 631245, 631425, 631452, 632145, 632415, 632451, 634215, 634251, 634521, 635214, 635241, 635421, 642135, 642315, 642351, 642513, 642531, 643215, 643251, 643521, 645321, 654321.
MAPLE
b:= proc(u, o, t) option remember; `if`(u+o=0, 1,
add(b(u-j, o+j-1, j), j=1..min(t, u))+
add(b(u+j-1, o-j, j), j=1..min(t, o)))
end:
a:= n-> b(0, n, 6)-b(0, n, 5):
seq(a(n), n=6..50);
CROSSREFS
Column k=6 of A291684.
Sequence in context: A297627 A049059 A345240 * A166390 A161478 A288919
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Sep 10 2017
STATUS
approved