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A010693 Period 2. +0
9
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)
OFFSET

0,1

COMMENT

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]

LINKS

INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 466

FORMULA

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)

MATHEMATICA

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

CROSSREFS

Cf. A139421.

Sequence in context: A145384 A117666 A165587 this_sequence A158478 A139713 A023524

Adjacent sequences: A010690 A010691 A010692 this_sequence A010694 A010695 A010696

KEYWORD

nonn,easy

AUTHOR

N. J. A. Sloane (njas(AT)research.att.com).

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Last modified November 25 08:46 EST 2009. Contains 167481 sequences.


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