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 A173643 #5 Mar 30 2012 18:41:21 %S A173643 1,2,3,4,5,6,8,10,12,13,16,20,21,24,26,32,40,42,48,52,53,64,80,84,85, %T A173643 96,104,106,113,128,160,168,170,192,208,212,213,226,227,256,320,336, %U A173643 340,341,384,416,424,426,452,453,454,512,640,672,680 %N A173643 Positive numbers of form 2^m - 2^l - 3*2^k (A172233) divisible by 9, divided by 9. %C A173643 Böhm and Sontacchi show a(n) needs the 3x+1 operator at most twice to reach 1 in the Collatz 3x+1 problem. %C A173643 Conjecture: Odd part of a(n) is of form [(1/6)*(8^m-(-1)^m-3)*4^k-1]/3, k,m>0. %D A173643 C. Böhm and G. Sontacchi: On the Existence of Cycles of given Length in Integer Sequences like x_(n+1) = x_n/2 if x_n even, and x_(n+1) = 3x_n + 1 otherwise. Atti della Accademia Nazionale dei Lincei. Rendiconti. Classe di Scienze Fisiche, Matematiche e Naturali. Serie VIII 64 (1978), 260-264. %Y A173643 Cf. A172143. %K A173643 nonn %O A173643 1,2 %A A173643 _Ralf Stephan_, Nov 24 2010