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 A070993 #16 Sep 22 2013 15:44:02 %S A070993 3,7,9,15,19,25,33,39,51,91,121,159,166,183,243,250,333,376,411,432, %T A070993 487,501,649,667,865,889,975,1153,1185,1299,1335,1731,1779,2307,3643, %U A070993 4857,7287 %N A070993 Numbers n such that the trajectory of n under the "3x+1" map reaches n+1. %C A070993 From Collatz conjecture, the trajectory of n never reaches n again. Is this sequence finite? (it seems there are no further terms below 10^6). %C A070993 There are no more terms < 10^9. - _Donovan Johnson_, Sep 22 2013 %e A070993 Trajectory of 39 is (118, 59, 178, 89, 268, 134, 67, 202, 101, 304, 152, 76, 38, 19, 58, 29, 88, 44, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1) which contains 39+1=40, so 39 is in the sequence. %t A070993 Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; Select[Range[100000], MemberQ[Collatz[#], # + 1] &] (* _T. D. Noe_, Feb 22 2013 *) %o A070993 (PARI) for(n=1,10000,s=n; t=0; while(s!=1,t++; if(s%2==0,s=s/2,s=3*s+1); if(s==n-1,print1(n,","); ); )) %Y A070993 Cf. A070165 (Collatz trajectories), A221213, A222293, A070991. %K A070993 nonn %O A070993 1,1 %A A070993 _Benoit Cloitre_ and Boris Gourevitch (boris(AT)pi314.net), May 18 2002 %E A070993 Corrected by _T. D. Noe_, Oct 25 2006