OFFSET
0,3
COMMENTS
When C(n) is prime (or 1), then a(n) = 1; that is, n is in A005849.
On the penultimate page of their paper, Grau and Luca ask for "a good (large) lower bound on this quantity which is valid for all n and which tends to infinity with n."
LINKS
Amiram Eldar, Table of n, a(n) for n = 0..848
José María Grau Ribas and Florian Luca, Cullen numbers with the Lehmer property, Proceedings of the American Mathematical Society, Vol. 140, No. 1 (2012), pp. 129-134, preprint, arXiv:1103.3578 [math.NT], Mar 18, 2011.
EXAMPLE
a(2) = 3 because the second Cullen number is 9; phi(9) = 6, therefore 6/gcd(8, 6) = 6/2 = 3.
MATHEMATICA
cullen[n_] := n(2^n) + 1; Table[EulerPhi[cullen[n]]/GCD[cullen[n] - 1, EulerPhi[cullen[n]]], {n, 0, 39}]
PROG
(PARI) a(n)=my(C=n<<n, p=eulerphi(C+1)); p/gcd(C, p) \\ Charles R Greathouse IV, Feb 05 2013
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Alonso del Arte, Sep 20 2011
STATUS
approved