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A264451
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Number of permutations of [n] with exactly two (possibly overlapping) occurrences of the generalized pattern 12-3.
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2
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1, 13, 112, 843, 6089, 43887, 321357, 2411686, 18631631, 148490575, 1221894598, 10382648734, 91073271181, 824221683639, 7690752545310, 73932347642395, 731636916804531, 7447376286108705, 77913394499688645, 837113596053971008, 9229808681562243113
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OFFSET
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4,2
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LINKS
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EXAMPLE
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a(4) = 1: 1243.
a(5) = 13: 13254, 13524, 13542, 14235, 21354, 23154, 23514, 23541, 24135, 31254, 34125, 41253, 51243.
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MAPLE
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b:= proc(u, o) option remember; `if`(u+o=0, 1, add(
b(u-j, o+j-1), j=1..u)+add(convert(series(
b(u+j-1, o-j)*x^(o-j), x, 3), polynom), j=1..o))
end:
a:= n-> coeff(b(n, 0), x, 2):
seq(a(n), n=4..25);
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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