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A238803
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Number of ballot sequences of length 2n with exactly n fixed points.
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2
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1, 1, 3, 9, 29, 99, 357, 1351, 5343, 21993, 93923, 414969, 1892277, 8887291, 42912261, 212676951, 1080355463, 5617772049, 29868493827, 162204146857, 898874710797, 5078665886931, 29232738375653, 171294038649639, 1021117638212079, 6188701520663929
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OFFSET
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0,3
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COMMENTS
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The fixed points are in the positions 1,2,...,n.
Also the number of standard Young tableaux with 2n cells where n is the length of the maximal consecutive sequence 1,2,...,k in the first column. An alternate definition uses the first row.
All terms are odd because the counted structures come in pairs with exactly one exception.
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LINKS
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FORMULA
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a(n) = ((2*n-1)*a(n-1)+n*(n-2)*a(n-2))/(n-1) for n>1, a(0) = a(1) = 1.
a(n) = Sum_{k=0..n} C(n-1,k) * A000085(n-k).
a(n) ~ exp(2*sqrt(n)-n/2-1) * n^(n/2) / sqrt(2) * (1 - 1/(6*sqrt(n))). - Vaclav Kotesovec, Mar 07 2014
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EXAMPLE
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For n=3 we have the following a(3) = 9 ballot sequences: [1,2,3,1,1,1], [1,2,3,1,2,3], [1,2,3,1,1,2], [1,2,3,1,2,1], [1,2,3,1,4,1], [1,2,3,1,4,2], [1,2,3,1,1,4], [1,2,3,1,2,4], [1,2,3,1,4,5].
Their corresponding tableaux are:
: 1456 14 : 145 146 : 146 14 : 145 14 : 14 :
: 2 25 : 26 25 : 2 26 : 2 25 : 2 :
: 3 36 : 3 3 : 3 3 : 3 3 : 3 :
: : : 5 5 : 6 6 : 5 :
: : : : : 6 :
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MAPLE
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a:= proc(n) option remember; `if`(n<2, 1,
((2*n-1) *a(n-1) +n*(n-2) *a(n-2)) / (n-1))
end:
seq(a(n), n=0..35);
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MATHEMATICA
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RecurrenceTable[{a[0]==a[1]==1, a[n]==((2n-1)a[n-1]+n(n-2)a[n-2])/(n-1)}, a, {n, 30}] (* Harvey P. Dale, Jun 25 2014 *)
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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