OFFSET
1,4
COMMENTS
For n>1: a(n)>=0 and a(n)=0 iff (n,n+2) is a twin prime pair.
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..1000
MATHEMATICA
Table[DivisorSigma[1, n]-EulerPhi[n+2], {n, 80}] (* Harvey P. Dale, Dec 25 2012 *)
PROG
(PARI) a(n)=sigma(n)-eulerphi(n+2) \\ Charles R Greathouse IV, Feb 19 2013
CROSSREFS
KEYWORD
sign
AUTHOR
Reinhard Zumkeller, Dec 06 2002
STATUS
approved