OFFSET
1,2
REFERENCES
Richard K. Guy, Unsolved Problems in Number Theory, Springer, 2004, p. 139.
LINKS
Paul Erdős and Ronald L. Graham, Old and new problems and results in combinatorial number theory, Monographies de L'Enseignement Mathematique, L'enseignement Mathématique, Université de Genève, 1980, p. 61.
Eric Weisstein's MathWorld, Totient Function.
Wikipedia, Euler's totient function.
FORMULA
Equals Sum_{k>=1} A007431(k)/(2^k - 1). - Amiram Eldar, Jun 23 2020
EXAMPLE
1.36763080198502235079050814621308813907489199896...
MATHEMATICA
digits = 99; m0 = 10; dd = 10; Clear[f]; f[m_] := f[m] = Sum[EulerPhi[n]/2^n, {n, 1, m}] // N[#, digits + 2*dd]&; f[m = m0] ; While[RealDigits[f[2*m], 10, digits + dd ] != RealDigits[f[m], 10, digits + dd ], m = 2*m; Print[m]]; RealDigits[f[m], 10, digits] // First
PROG
(PARI) suminf(n=1, eulerphi(n)/2^n) \\ Charles R Greathouse IV, Apr 20 2016
CROSSREFS
KEYWORD
nonn,cons
AUTHOR
Jean-François Alcover, Apr 13 2015
STATUS
approved