cp's OEIS Frontend

A292771 If sigma(n)-phi(n) is even then (sigma(n)-phi(n))/2 otherwise -1.

%I A292771 #13 Oct 01 2017 12:57:41
%S A292771 0,1,1,-1,1,5,1,-1,-1,7,1,12,1,9,8,-1,1,-1,1,17,10,13,1,26,-1,15,11,
%T A292771 22,1,32,1,-1,14,19,12,-1,1,21,16,37,1,42,1,32,27,25,1,54,-1,-1,20,37,
%U A292771 1,51,16,48,22,31,1,76,1,33,34,-1,18,62,1,47,26,60,1,-1,1,39,42,52,18,72,1,77,-1,43,1
%N A292771 If sigma(n)-phi(n) is even then (sigma(n)-phi(n))/2 otherwise -1.
%C A292771 a(n) = 1 if and only if n is prime. - _Robert Israel_, Sep 28 2017
%H A292771 Robert Israel, <a href="/A292771/b292771.txt">Table of n, a(n) for n = 1..10000</a>
%p A292771 f:= proc(n) local v;
%p A292771    v:= numtheory:-sigma(n)-numtheory:-phi(n);
%p A292771    if v::even then v/2 else -1 fi
%p A292771 end proc:
%p A292771 map(f, [$1..100]); # _Robert Israel_, Sep 28 2017
%Y A292771 Cf. A051612, A000010, A000203, A292770.
%K A292771 sign
%O A292771 1,6
%A A292771 _N. J. A. Sloane_, Sep 28 2017