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 A020806 Decimal expansion of 1/7. 31
 1, 4, 2, 8, 5, 7, 1, 4, 2, 8, 5, 7, 1, 4, 2, 8, 5, 7, 1, 4, 2, 8, 5, 7, 1, 4, 2, 8, 5, 7, 1, 4, 2, 8, 5, 7, 1, 4, 2, 8, 5, 7, 1, 4, 2, 8, 5, 7, 1, 4, 2, 8, 5, 7, 1, 4, 2, 8, 5, 7, 1, 4, 2, 8, 5, 7, 1, 4, 2, 8, 5, 7, 1, 4, 2, 8, 5, 7, 1, 4, 2, 8, 5, 7, 1, 4, 2, 8, 5, 7, 1, 4, 2, 8, 5, 7, 1, 4, 2 (list; constant; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS 142857 and 999999 = 7*142857 are first and last Kaprekar numbers with six digits. Note a(n) + a(n+3) = 9. (142857^2 = 20408122449; 20408 + 122449 = 142857.) a(n)^2 = 1, 16, 4, 64, 25, 49, ... - Paul Curtz, Aug 24 2009 The constant 19 + 1/7 = 19.142857... is the Kirchhoff index of the MÃ¶bius ladder graph on v=8 vertices. The laplacian matrix has the eigenvalues 4 (one time), 4-sqrt(2) (2 times), 4+sqrt(2) (2 times), 2 (2 times) and 0 (one time). Then the Kirchhoff index is v times the sum over the inverse, nonzero eigenvalues. - R. J. Mathar, Feb 13 2011 Decimal expansion of -99*(zeta(-5) + zeta(-9)) - 1. - Arkadiusz Wesolowski, Sep 15 2013 Also, decimal expansion of Sum_{i>0} 1/8^i. - Bruno Berselli, Jan 03 2014 The points whose coordinates are overlapping pairs of digits of this sequence, (1, 4), (4, 2), (2, 8), (8, 5), (5, 7) and (7, 1), all lie on one ellipse, with equation 19*x^2 + 36*x*y + 41*y^2 - 333*x - 531*y = -1638. Overlapping pairs of pairs of digits, (14, 28), (42, 85), (28, 57), (85, 71), (57, 14), (71, 42), also yield 6 points on one ellipse, with equation -165104*x^2 + 160804*x*y + 8385498*x - 41651*y^2 - 3836349*y = 7999600. (See book by Wells and MathWorld link.) - M. F. Hasler, Oct 25 2017 REFERENCES H. Rademacher and O. Toeplitz, Von Zahlen und Figuren (Springer 1930, reprinted 1968), ch. 19, 'Die periodischen DezimalbrÃ¼che'. D. Wells, The Penguin Dictionary of Curious and Interesting Numbers. Middlesex, England: Penguin Books, 1986. LINKS J. Hall, One-Seventh Ellipse, on MathWorld, by E. W. Weisstein. Index entries for linear recurrences with constant coefficients, signature (1,0,-1,1). FORMULA A028416(1)=7; A002371(A049084(7)) = A002371(4) = 6. a(n+6) = a(n), a(n+6/2) = 9 - a(n). - Reinhard Zumkeller, Oct 06 2008 a(n) = (1/30)*(39*(n mod 6)-((n+1) mod 6)+24*((n+2) mod 6)-21*((n+3) mod 6)+19*((n+4) mod 6)-6*((n+5) mod 6)). - Paolo P. Lava, Jan 21 2009 From Colin Barker, Aug 14 2012: (Start) a(n) = a(n-1) - a(n-3) + a(n-4) for n>3. G.f.: (1+3*x-2*x^2+7*x^3) / ((1-x)*(1+x)*(1-x+x^2)). (End) a(n) = A068028(n+2). - Zak Seidov, Mar 26 2015 a(n) = (27 - 11*cos(n*Pi) - 10*cos(n*Pi/3) - 6*sqrt(3)*sin(n*Pi/3))/6. - Wesley Ivan Hurt, Jun 28 2016 EXAMPLE 0.142857142857142857... MAPLE Digits:=100: evalf(1/7); # Wesley Ivan Hurt, Jun 28 2016 MATHEMATICA CoefficientList[Series[(1 + 3 x - 2 x^2 + 7 x^3) / ((1 - x) (1 + x) (1 - x + x^2)), {x, 0, 100}], x] (* Vincenzo Librandi, Mar 27 2015 *) PROG (MAGMA) I:=[1, 4, 2, 8]; [n le 4 select I[n] else Self(n-1)-Self(n-3)+Self(n-4): n in [1..100]]; // Vincenzo Librandi, Mar 27 2015 (PARI) 1/7. \\ Charles R Greathouse IV, Sep 24 2015 (PARI) digits(10^99\7) \\ M. F. Hasler, Oct 25 2017 CROSSREFS Cf. A002371, A028416, A049084, A068028. Sequence in context: A000727 A030181 A021879 * A030210 A098798 A131783 Adjacent sequences:  A020803 A020804 A020805 * A020807 A020808 A020809 KEYWORD nonn,cons,easy AUTHOR STATUS approved

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Last modified May 26 09:37 EDT 2020. Contains 334620 sequences. (Running on oeis4.)