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A226012
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Number of unimodal functions f:[n]->[2n].
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2
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1, 2, 16, 161, 1716, 18832, 210574, 2385644, 27290916, 314537894, 3646709616, 42483615330, 496908084660, 5831654186256, 68636514069496, 809835178438996, 9575879777488676, 113445872396014898, 1346272950075766624, 16000494256911975827, 190424554847852203816
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OFFSET
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0,2
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LINKS
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FORMULA
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a(n) ~ 5^(5*n-1/2) / (9*2^(8*n-5/2)*sqrt(Pi*n)). - Vaclav Kotesovec, Jul 16 2014
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EXAMPLE
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a(2) = 16: [1,1], [1,2], [1,3], [1,4], [2,1], [2,2], [2,3], [2,4], [3,1], [3,2], [3,3], [3,4], [4,1], [4,2], [4,3], [4,4].
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MAPLE
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a:= proc(n) option remember; `if`(n<3, 2^(n^2),
((2166498*n^7 -16827434*n^6 +54145990*n^5 -93141070*n^4
+92008232*n^3 -51863736*n^2 +15330240*n -1814400) *a(n-1)
-5*(5*n-9)*(5*n-8)*(5*n-7)*(5*n-6)
*(333*n^3-595*n^2+338*n-60) *a(n-2)) / (16*(4*n-3)*
(2*n-1)*(4*n-5)*(333*n^3-1594*n^2+2527*n-1326)*n))
end:
seq(a(n), n=0..30);
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MATHEMATICA
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A[n_, k_] := If[n==0, 1, Sum[Binomial[n + 2j - 1, 2j], {j, 0, k n - 1}]];
a[n_] := A[n, 2];
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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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