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%I #29 Aug 20 2019 19:51:21
%S 1,15,27,35,39,51,63,69,75,77,87,91,95,99,115,119,123,125,135,143,147,
%T 153,155,159,171,175,183,187,189,195,203,207,215,219,231,235,237,243,
%U 245,247,249,255,259,267,275,279,285,287,291,299,303,305,309,315,319,323
%N Numbers k such that phi(r) + phi(s) = k has no solution, where phi is Euler totient function.
%C Problem C1 of Unsolved Problems in Number Theory asks if there are even numbers in this sequence.
%D Richard K. Guy, Unsolved Problems in Number Theory, Springer, 2004, section C1, p. 160.
%F a(n) = A005277(n-1) + 1 if there are no even numbers in this sequence.
%t v = Union@ EulerPhi@ Range@ 1680; Complement[Range@ 400, Join[2 v, Plus @@@ Subsets[v, {2}]]] (* _Giovanni Resta_, Jul 18 2019 *)
%o (PARI) is(n)=sum(i=1,n-1,istotient(i)&&istotient(n-i))==0
%Y Cf. A000010, A002202, A005277, A307853.
%K nonn
%O 1,2
%A _Jack Zhang_, May 01 2019
%E More terms from _Jinyuan Wang_, May 03 2019
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