OFFSET
3,5
COMMENTS
Conjecture : a(n)=0 if and only if n is a Fermat prime (A019434).
The conjecture appears to be false, a(83623937)=0 and 83623937 is not a Fermat number (A000215). See A232720 for sequence of n such that a(n)=0. - Michel Marcus, Nov 28 2013
FORMULA
a(n)=-1 if n is of the form 2^x+2;
a(n)=-3 if n is in a subsequence b(k) = 8, 12, 16, 24, 28, 36, 40, 48, 60, 64, 76, 84, 88, 96..( b(k) seems to be asymptotic to c*n*log(n) with c=2.28..).
MATHEMATICA
Table[2*EulerPhi[n-1]-n, {n, 2, 90}] (* Harvey P. Dale, May 22 2020 *)
PROG
(PARI) a(n) = 2*eulerphi(n-2) - (n-1); \\ Michel Marcus, Nov 28 2013
CROSSREFS
KEYWORD
easy,sign
AUTHOR
Benoit Cloitre, Sep 15 2002
STATUS
approved