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 A130904 #9 Mar 31 2012 10:22:30 %S A130904 3,4,6,9,10,15,16,18,25,27,28,30,33,34,48,49,51,52,54,55,57,58,63,64, %T A130904 66,91,93,94,96,99,100,102,105,106,108,111,112,114,119,121,123,124, %U A130904 126,129,130,169,180,183,184,186,187,189,190,195,196,198 %N A130904 Numbers n such that the trajectory of the map n -> (n + lpf(n)) / 2 reaches 3, where lpf(n) is the least prime factor of n (A020639). %C A130904 The only prime term is 3 because all primes map to themselves. %e A130904 (15 + 3) / 2 = 9, (9 + 3) / 2 = 6, (6 + 2) / 2 = 4, and (4 + 2) / 2 = 3, thus 15 is in the sequence. %e A130904 (21 + 3) / 2 = 12, (12 + 2) / 2 = 7, and (7 + 7) / 2 = 7, so 21 never reaches 3 and therefore is not in the sequence. %Y A130904 Cf. A177980, A020639, A061228. %K A130904 nonn %O A130904 1,1 %A A130904 _Grant Garcia_, Jan 06 2011