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A123637
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a(n) = 1 + 1*n + 1*n*2 + 1*n*2*(n-1) + 1*n*2*(n-1)*3 + 1*n*2*(n-1)*3*(n-2) + ... + n!*(n+1)!.
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2
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1, 4, 23, 238, 4285, 120056, 4807699, 259889218, 18207958073, 1603405689580, 173263178533711, 22534190356771094, 3471514311529290613, 625057269686305463008, 130043797443156653379275
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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) = Sum_{k=0..2*n} (floor((k + 2)/2)! * n!)/((n - floor((k + 1)/2))!). - G. C. Greubel, Oct 26 2017
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EXAMPLE
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a(n) equals the sum of (2n+1) terms:
a(0) = 1;
a(1) = 1 + 1*1 + 1*1*2 = 4;
a(2) = 1 + 1*2 + 1*2*2 + 1*2*2*1 + 1*2*2*1*3 = 23;
a(3) = 1 + 1*3 + 1*3*2 + 1*3*2*2 + 1*3*2*2*3 + 1*3*2*2*3*1 + 1*3*2*2*3*1*4 = 238.
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MATHEMATICA
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Join[{1}, Table[Sum[(Floor[(k + 2)/2]! * n!)/((n - Floor[(k + 1)/2])!), {k, 0, 2*n}], {n, 1, 50}]] (* G. C. Greubel, Oct 26 2017 *)
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PROG
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(PARI) {a(n)=sum(k=1, 2*n+1, prod(j=1, k, ((j+1)\2)*(j%2)+(n+1-(j\2))*((j-1)%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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