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 A020762 Decimal expansion of 1/sqrt(5). 8
 4, 4, 7, 2, 1, 3, 5, 9, 5, 4, 9, 9, 9, 5, 7, 9, 3, 9, 2, 8, 1, 8, 3, 4, 7, 3, 3, 7, 4, 6, 2, 5, 5, 2, 4, 7, 0, 8, 8, 1, 2, 3, 6, 7, 1, 9, 2, 2, 3, 0, 5, 1, 4, 4, 8, 5, 4, 1, 7, 9, 4, 4, 9, 0, 8, 2, 1, 0, 4, 1, 8, 5, 1, 2, 7, 5, 6, 0, 9, 7, 9, 8, 8, 2, 8, 8, 2, 8, 8, 1, 6, 7, 5, 7, 5, 6, 4, 5, 4, 9, 9, 3, 9, 0, 1 (list; constant; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS This number is the cosine of the central angle of a regular icosahedron; see A105199 for the angle itself. - Clark Kimberling, Feb 10 2009 Largest radius of ten circles tangent to a circle of radius 1. - Charles R Greathouse IV, Jan 14 2013 LINKS Ivan Panchenko, Table of n, a(n) for n = 0..1000 FORMULA 1/sqrt(5) = cos(arctan(2)). - Clark Kimberling, Feb 10 2009 Equals lim_{n -> infinity} A000045(n)/A000032(n). - Bruno Berselli, Jan 22 2018 From Christian Katzmann, Mar 19 2018: (Start) Equals Sum_{n>=0} (2*n)!/(n!^2*3^(2*n+1)). Equals Sum_{n>=0} 5*(2*n+1)!/(n!^2*3^(2*n+3)). (End) EXAMPLE 0.447213595499957939281834733746255247088123671922305144854179449082104... MATHEMATICA RealDigits[5^(-1/2), 10, 150] (* Stefan Steinerberger, Apr 08 2006 *) Circs[n_] := With[{r = Sin[Pi/n]/(1 - Sin[Pi/n])}, Graphics[Append[   Table[Circle[(r + 1) {Sin[2 Pi k/n], Cos[2 Pi k/n]}, r], {k, n}],     {Blue, Circle[{0, 0}, 1]}]]] Circs[10] (* Charles R Greathouse IV, Jan 14 2013 *) PROG (PARI) 1/sqrt(5) \\ Charles R Greathouse IV, Jan 14 2013 CROSSREFS Cf. A000032, A000045. Sequence in context: A021695 A131844 A010476 * A204158 A053510 A197009 Adjacent sequences:  A020759 A020760 A020761 * A020763 A020764 A020765 KEYWORD nonn,cons AUTHOR EXTENSIONS More terms from Stefan Steinerberger, Apr 08 2006 STATUS approved

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Last modified August 14 19:45 EDT 2020. Contains 336483 sequences. (Running on oeis4.)