The OEIS is supported by the many generous donors to the OEIS Foundation.
%I #12 May 03 2021 02:09:28
%S 9,13,14,15,19,20,21,44,45,74,75,109,110,111,200,201,284,285,620,621,
%T 830,831,1304,1305,1490,1491,1879,1880,1881,2090,2091,2144,2145,2690,
%U 2691,2804,2805,3469,3470,3471,5420,5421,5659,5660,5661,9440,9441,11834,11835
%N Numbers k such that A335660(k) = 2.
%H Altug Alkan, Andrew R. Booker, and Florian Luca, <a href="https://arxiv.org/abs/2006.08013">On a recursively defined sequence involving the prime counting function</a>, arXiv:2006.08013 [math.NT], 2020.
%t f[1] = 1; f[n_] := f[n] = PrimePi[n] - PrimePi[Sum[f[k], {k, 1, n - 1}]]; m = 12000 ; Position[Range[m] - Accumulate @ Array[f, m], 2] // Flatten (* _Amiram Eldar_, May 03 2021 *)
%Y Cf. A000720, A334714, A335294, A335660.
%K nonn
%O 1,1
%A _Seiichi Manyama_, Jun 17 2020