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Search: id:A010693
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| 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3, 2, 3
(list; graph; listen)
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OFFSET
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0,1
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COMMENT
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a(n) = smallest prime divisor of n!! for n >= 2. For biggest prime divisor of n!! see A139421. - Artur Jasinski (grafix(AT)csl.pl), Apr 21 2008
a(n) = A020639(A016767(n)) for n>0. [From Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Jan 29 2009]
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LINKS
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INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 466
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FORMULA
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a(n) = 5/2 - ((-1)^n)/2.
a(n) = 2 + n mod 2 = A007395(n) + A000035(n). - Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Mar 23 2005
Contribution from Jaume Oliver Lafont (joliverlafont(AT)gmail.com), Mar 20 2009: (Start)
G.f.:(2+3*x)/(1-x^2)
Linear recurrence: a(0)=2, a(1)=3, a(n)=a(n-2) for n>=2 (End)
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MATHEMATICA
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Table[5/2 - (-1)^n/2, {n, 0, 100}] or a = {}; Do[b = First[First[FactorInteger[n!! ]]]; AppendTo[a, b], {n, 2, 1000}]; a - Artur Jasinski (grafix(AT)csl.pl), Apr 21 2008
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CROSSREFS
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Cf. A139421.
Sequence in context: A145384 A117666 A165587 this_sequence A158478 A139713 A023524
Adjacent sequences: A010690 A010691 A010692 this_sequence A010694 A010695 A010696
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KEYWORD
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nonn,easy
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AUTHOR
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N. J. A. Sloane (njas(AT)research.att.com).
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