This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A209048 #7 Mar 30 2012 17:31:31
%S A209048 1,2,4,3,12,11,32,27,75,192,427,171,1323,2187,6912,8192,4779,10923,
%T A209048 49152,109227,60075,170667,786432,699051,1092267,4893355,5810859,
%U A209048 11184811,25864875,95070891,101362347,44739243
%N A209048 Indices of the pentagonal numbers listed in A209049.
%C A209048 Pentagonal analog of A076550.
%t A209048 k = 1; t = Table[0, {50}]; While[k < 500000001, a = PrimeOmega[k] + PrimeOmega[3 k - 1] - 1; If[ t[[a + 1]] == 0, t[[a + 1]] = k; Print[{k, a}]]; k++]; t
%Y A209048 A209049, A076550.
%K A209048 nonn
%O A209048 0,2
%A A209048 _Robert G. Wilson v_, Mar 04 2012