%I #19 May 08 2024 04:34:15
%S 2,3,6,8,12,15,21,25,34,38,48,54,66,72,84,92,108,117,135,143,161,171,
%T 193,205,225,237,264,276,304,316,346,362,392,408,432,450,486,504,540,
%U 556,596,614,656,676,712,734,780,804,846
%N A number of sum-free sets related to fractional parts of multiples of a rational number in the range 1/3 to 2/3.
%C Deserves a better title which mentions n in the sense that this is a sum-free set from a difference set with {1,....,n}.
%H Peter J. Cameron and Paul Erdős, <a href="http://dx.doi.org/10.1017/S0963548398003435">Notes on sum-free and related sets</a>, Combinat. Probabl. Comput. 8 (1&2) (1999), 95-107, Theorem 4.
%F a(n) = 1 + Sum_{j=1..n} A000010(3*j)/2.
%F a(n) ~ (27/(8*Pi^2)) * n^2. - _Amiram Eldar_, May 08 2024
%p A194881 := proc(n) 1+add(numtheory[phi](3*q),q=1..n)/2 ; end proc:
%p seq(A194881(n),n=1..80) ;
%t Accumulate[Table[EulerPhi[3*n], {n, 1, 60}]]/2 + 1 (* _Amiram Eldar_, May 08 2024 *)
%Y Cf. A000010, A195459, A372621.
%K nonn
%O 1,1
%A _R. J. Mathar_, Sep 04 2011