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A275539
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a(n) = n! + n*(n-1)!!.
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1
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1, 2, 4, 12, 36, 160, 810, 5376, 41160, 366336, 3638250, 39959040, 479126340, 6227619840, 87180183090, 1307684044800, 20922822320400, 355687603568640, 6402374325997650, 121645103938928640, 2432902021271221500, 51090942249743155200, 1124000728080092512650
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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) = n! + n*(n-1)!! = n*((n-1)! + (n-1)!!).
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MAPLE
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a:= proc(n) option remember; `if`(n<5, [1, 2, 4, 12, 36]
[n+1], ((n-3)^2*n*a(n-1) +(n-4)*n*(n-1)*a(n-2)
-(n-3)*n*(n-1)*(n-2)*a(n-3))/((n-4)*(n-2)))
end:
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MATHEMATICA
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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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