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 A154920 Denominators of a ternary BBP-type formula for log(3). 7
 1, 18, 27, 324, 405, 4374, 5103, 52488, 59049, 590490, 649539, 6377292, 6908733, 66961566, 71744535, 688747536, 731794257, 6973568802, 7360989291, 69735688020, 73222472421, 690383311398, 721764371007, 6778308875544 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS log(3) = Sum_{k>=0} (9/(2k+1)+1/(2k+2))/9^(k+1). log(3) = 1 + Sum_{k>=0} (1/(2k+2)+1/(2k+3))/9^(k+1). LINKS David H. Bailey, A Compendium of BBP-Type Formulas for Mathematical Constants, page 14. [From Jaume Oliver Lafont, Sep 25 2009] Index entries for linear recurrences with constant coefficients, signature (0,18,0,-81). FORMULA a(n) = (n+1)*9^[(n+1)/2] = 18*a(n-2) - 81*a(n-4). Sum_{n>=0} 1/a(n) = log(3). G.f.: (1+18*x+9*x^2)/(1-9*x^2)^2. - Jaume Oliver Lafont, Jan 29 2009 a(n) = (2-(-1)^n)*(n+1)*3^n. - Jaume Oliver Lafont, Sep 27 2009 MATHEMATICA LinearRecurrence[{0, 18, 0, -81}, {1, 18, 27, 324}, 30] (* Harvey P. Dale, Jan 10 2017 *) PROG (PARI) a(n)=(n+1)*9^((n+1)\2) \\ Jaume Oliver Lafont, Mar 25 2009 (MAGMA) [(2-(-1)^n)*(n+1)*3^n: n in [0..30]]; // Vincenzo Librandi, Jul 06 2015 CROSSREFS Cf. A002391, A058962. Cf. A164985, A165132. - Jaume Oliver Lafont, Sep 25 2009 Sequence in context: A038632 A138336 A166630 * A094224 A128858 A141782 Adjacent sequences:  A154917 A154918 A154919 * A154921 A154922 A154923 KEYWORD nonn AUTHOR Jaume Oliver Lafont, Jan 17 2009, Jan 18 2009 STATUS approved

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Last modified October 19 16:34 EDT 2018. Contains 316367 sequences. (Running on oeis4.)