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 A073010 Decimal expansion of Pi/sqrt(27). 26
 6, 0, 4, 5, 9, 9, 7, 8, 8, 0, 7, 8, 0, 7, 2, 6, 1, 6, 8, 6, 4, 6, 9, 2, 7, 5, 2, 5, 4, 7, 3, 8, 5, 2, 4, 4, 0, 9, 4, 6, 8, 8, 7, 4, 9, 3, 6, 4, 2, 4, 6, 8, 5, 8, 5, 2, 3, 2, 9, 4, 9, 7, 8, 4, 6, 2, 7, 0, 7, 7, 2, 7, 0, 4, 2, 1, 1, 7, 9, 6, 1, 2, 2, 8, 0, 4, 1, 6, 6, 2, 7, 3, 7, 3, 5, 3, 3, 8, 9, 6, 1, 8, 7, 4, 0 (list; constant; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS Original name: Decimal expansion of sum(1/(n*binomial(2*n,n)), n=1..infinity). This appears to be Pi/sqrt(27). See A111510. - Marco Matosic, Feb 27 2008 This is Pi*sqrt(3)/9 = A019676*A002194, see eq (12) in Lehmer link. - R. J. Mathar, Mar 04 2009 Value of the Dirichlet L-series of the non-principal character modulo m=3 (A102283) at s=1. - R. J. Mathar, Oct 03 2011 Construct the largest possible circle inside a given equilateral triangle. This constant is the ratio of the area of the circle to the area of the triangle (A245670 is analogous square in triangle). - Rick L. Shepherd, Jul 29 2014 REFERENCES Jolley, Summation of Series, Dover (1961) eq (81) page 16. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 J. M. Borwein, R. Girgensohn, Evaluations of binomial series, Aequat. Math. 70 (2005) 25-36. Etienne Fouvry, Claude Levesque, Michel Waldschmidt, Representation of integers by cyclotomic binary forms, arXiv:1712.09019 [math.NT], 2017. D. H. Lehmer, Interesting Series Involving the Central Binomial Coefficient, Am. Math. Monthly 92 (1985) 449-457. See eq (12). Courtney Moen, Infinite series with binomial coefficients, Math. Mag. 64 (1) (1991) 53-55. Simon Plouffe, Sum(1/(n*binomial(2*n,n)), n=1..infinity), see p. 87. Eric Weisstein's World of Mathematics, Central Binomial Coefficient FORMULA -Pi/(3*sqrt(3)) = Sum_{n=0..infinity} (1/(6*n+1) - 2/(6*n+2) - 3/(6*n+3) - 1/(6*n+4) + 2/(6*n+5) + 3/(6*n+6)). - Mats Granvik, Sep 08 2013 Also equals integral_{0..infinity} 2*x/((x^2+1)*(x^4+x^2+1)) dx. [Jean-François Alcover, Sep 10 2013] From Peter Bala, Feb 16 2015: (Start) Pi/sqrt(27) = Sum {n >= 0} 1/((3*n + 1)*(3*n + 2)) = 1 - 1/2 + 1/4 - 1/5 + 1/7 - 1/8 + .... Continued fraction: 1/(1 + 1^2/(1 + 2^2/(2 + 4^2/(1 + 5^2/(2 + ... + (3*n + 1)^2/(1 + (3*n + 2)^2/(2 + ... ))))))). Pi/sqrt(27) = int {t = 0 .. 1/2} 1/(t^2 - t + 1) dt = int {t = 0 .. 1/2} (1 + t - t^3 - t^4)/(1 - t^6) dt. Pi/sqrt(27) = 1/4*Sum {n >= 0} (-1)^n*(9*n + 5)/( (3*n + 1)*(3*n + 2)*8^n ). BBP-type formulas: Pi/sqrt(27) = Sum {n >= 0} (1/64)^(n+1)*( 32/(6*n + 1) + 16/(6*n + 2) - 4/(6*n + 4) - 2/(6*n + 5) ) follows from the above integral representation. Pi/sqrt(27) = Sum {n >= 0} (-1)^n*(1/27)^(n+1)*( 9/(6*n + 1) + 9/(6*n + 2) + 6/(6*n + 3) + 3/(6*n + 4) + 1/(6*n + 5) ) follows from the result: Pi/3 = int {t = 0 .. 1/sqrt(3)} 1/(1 - sqrt(3)*t + t^2) dt. (End) Equals the integral_{x=0..infinity} x*I_0(x)*K_0(x)^2 dx over a triple product of modified Bessel functions. - R. J. Mathar, Oct 14 2015 EXAMPLE 0.60459978807807261686469275254738524409468... MATHEMATICA RealDigits[ N [Sum[1/(n*Binomial[2n, n]), {n, 1, Infinity}], 110]] [[1]] RealDigits[Pi/(3*Sqrt[3]), 10, 105][[1]] (* T. D. Noe, Sep 11 2013 *) PROG (PARI) Pi/sqrt(27) \\ Charles R Greathouse IV, Sep 11 2013 (MAGMA) R:=RealField(106); SetDefaultRealField(R); n:=Pi(R)/Sqrt(27); Reverse(Intseq(Floor(10^105*n))); // Bruno Berselli, Mar 12 2018 CROSSREFS Sequence in context: A209835 A298528 A021947 * A100120 A132709 A197148 Adjacent sequences:  A073007 A073008 A073009 * A073011 A073012 A073013 KEYWORD nonn,cons,changed AUTHOR Robert G. Wilson v, Aug 03 2002 STATUS approved

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Last modified March 17 14:13 EDT 2018. Contains 300565 sequences. (Running on oeis4.)