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A020762 Decimal expansion of 1/sqrt(5). 7
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

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

Sequence in context: A021695 A131844 A010476 * A204158 A053510 A197009

Adjacent sequences:  A020759 A020760 A020761 * A020763 A020764 A020765

KEYWORD

nonn,cons

AUTHOR

N. J. A. Sloane

EXTENSIONS

More terms from Stefan Steinerberger, Apr 08 2006

STATUS

approved

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Last modified November 18 10:38 EST 2017. Contains 294887 sequences.