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A070960
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a(1) = 1; a(n) = n!*(3/2) for n>=2.
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12
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1, 3, 9, 36, 180, 1080, 7560, 60480, 544320, 5443200, 59875200, 718502400, 9340531200, 130767436800, 1961511552000, 31384184832000, 533531142144000, 9603560558592000, 182467650613248000, 3649353012264960000, 76636413257564160000, 1686001091666411520000, 38778025108327464960000
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OFFSET
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1,2
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COMMENTS
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Let g be a permutation of [1..n] having, say, j_i cycles of length i, with Sum_i i*j_i = n; sequence gives Sum_{g} Sum_{i} (j_1 + j_2). - N. J. A. Sloane, Jul 22 2009
a(n) is the greatest integer that can be obtained from the integers {1, 2, 3, ..., n} using each number at most once and the operators +, -, *, /.
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LINKS
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FORMULA
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Sum_{n>=1} 1/a(n) = (2*e-1)/3.
Sum_{n>=1} (-1)^(n+1)/a(n) = 1 - 2/(3*e). (End)
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EXAMPLE
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a(5) = 180 because the greatest number we can obtain using 1, 2, 3, 4, 5 is 180 which is (1+2)*3*4*5.
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MATHEMATICA
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PROG
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(Haskell)
a070960 n = if n == 1 then 1 else 3 * a000142 n `div` 2
a070960_list = map (flip div 2) fs where fs = 3 : zipWith (*) [2..] fs
(PARI) a(n) = if (n==1, 1, n!*3/2); \\ Michel Marcus, Dec 03 2022
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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Koksal Karakus (karakusk(AT)hotmail.com), May 24 2002
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EXTENSIONS
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STATUS
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approved
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