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A165998
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Denominators of Taylor series expansion of 1/(3*x)*log((1+x)/(1-x)^2)
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26
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1, 6, 3, 12, 5, 18, 7, 24, 9, 30, 11, 36, 13, 42, 15, 48, 17, 54, 19, 60, 21, 66, 23, 72, 25, 78, 27, 84, 29, 90, 31, 96, 33, 102, 35, 108, 37, 114, 39, 120, 41, 126, 43, 132, 45, 138, 47, 144, 49, 150, 51, 156, 53, 162, 55, 168, 57, 174, 59, 180, 61, 186, 63, 192, 65, 198
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OFFSET
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0,2
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COMMENTS
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Numerators are all 1.
Setting x=1/3 into
1/(3*x)*log((1+x)/(1-x)^2) = Sum_{k>=0} x^k/((2-(-1)^k)*(k+1)),
log(3) = Sum_{k>=0} 1/((2-(-1)^k)*(k+1)*3^k)
= Sum_{k>=0} (9/(2k+1)+1/(2k+2))/9^(k+1)
is obtained.
It appears that this is also the first differences of the generalized decagonal numbers A074377. - Omar E. Pol, Sep 10 2011
It appears that this is also A005408 and positive terms of A008588 interleaved. - Omar E. Pol, May 28 2012
With offset 1 this sequence is multiplicative (in fact, a generalized totient function): a(p^e) = p^e for any odd prime p and a(2^e) = 3*2^e for e >= 1. - Charles R Greathouse IV, Mar 09 2015
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LINKS
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Vincenzo Librandi, Table of n, a(n) for n = 0..1004
Tian-Xiao He, Peter J.-S. Shiue, Zihan Nie, Minghao Chen, Recursive sequences and Girard-Waring identities with applications in sequence transformation, Electronic Research Archive (2020) Vol. 28, No. 2, 1049-1062.
Index entries for linear recurrences with constant coefficients, signature (0,2,0,-1)
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FORMULA
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G.f.: (1+6*x+x^2)/(1-x^2)^2.
a(n) = (2-(-1)^n)*(n+1) (see PARI's code by Jaume Oliver Lafont).
a(2n)= 2n+1. a(2n+1) = 6*(n+1). - R. J. Mathar, Apr 02 2011
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MATHEMATICA
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LinearRecurrence[{0, 2, 0, -1}, {1, 6, 3, 12}, 50] (* Vincenzo Librandi, Feb 22 2012 *)
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PROG
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(PARI) a(n)=(2-(-1)^n)*(n+1)
(MAGMA) [(2-(-1)^n)*(n+1): n in [0..350]]; // Vincenzo Librandi, Apr 04 2011
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CROSSREFS
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Cf. A154920.
Sequence in context: A131894 A335393 A040033 * A322091 A329583 A050132
Adjacent sequences: A165995 A165996 A165997 * A165999 A166000 A166001
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KEYWORD
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frac,nonn,easy
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AUTHOR
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Jaume Oliver Lafont, Oct 03 2009
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STATUS
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approved
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