A292770 - OEIS

%I #12 Jun 19 2022 18:42:52

%S 1,2,3,-1,5,7,7,-1,-1,11,11,16,13,15,16,-1,17,-1,19,25,22,23,23,34,-1,

%T 27,29,34,29,40,31,-1,34,35,36,-1,37,39,40,53,41,54,43,52,51,47,47,70,

%U -1,-1,52,61,53,69,56,72,58,59,59,92,61,63,70,-1,66,82,67,79,70,84,71,-1,73,75,82,88,78,96

%N If sigma(n)+phi(n) is even then (sigma(n)+phi(n))/2 otherwise -1.

%H Robert Israel, <a href="/A292770/b292770.txt">Table of n, a(n) for n = 1..10000</a>

%p f:= proc(n) local v;

%p    v:= numtheory:-sigma(n)+numtheory:-phi(n);

%p    if v::even then v/2 else -1 fi

%p end proc:

%p map(f, [$1..100]); # _Robert Israel_, Sep 28 2017

%t Table[If[EvenQ[DivisorSigma[1,n]+EulerPhi[n]],(DivisorSigma[1,n]+EulerPhi[n])/2,-1],{n,120}] (* _Harvey P. Dale_, Jun 19 2022 *)

%Y Cf. A065387, A291790, A292108, A292766, A000010, A000203, A292771.

%K sign

%O 1,2

%A _N. J. A. Sloane_, Sep 28 2017