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 A244638 #13 Oct 20 2019 21:56:53
%S A244638 2,3,7,19,31,41,73,97,193,257,313,487,859,937,1249,2539,3331,3947,
%T A244638 5351,5839,7963,9257,12343,21943,31687,45127,60169,78791,115547,
%U A244638 180463,213881,234239,270271,376603,875681,1023871,1252663,1564063,2585279,4063723,5649499,9973919,11200681,39824647,41464303,73583071,95592191,226588897,1359533387,2263333321,3349304527
%N A244638 In the '3x+1' problem, primes which as starting values set new records for number of steps to reach 1, where a step means either 'divide by two' or 'triple plus one and then divide by two'.
%H A244638 Wikipedia, <a href="http://en.wikipedia.org/wiki/Collatz_conjecture"> Collatz Conjecture</a>
%H A244638 <a href="/index/3#3x1">Index entries for sequences related to 3x+1 (or Collatz) problem</a>
%t A244638 f[n_] := Length@ NestWhileList[ If[ OddQ@ #, (3 # + 1)/2, #/2] &, n, # > 1 &] mx = 0; p = 2; lst = {}; While[p < 10^10/2, a = f@ p; If[a > mx, mx = a; Print[{PrimePi@p, p, a - 1}]; AppendTo[ lst, p]]; p = NextPrime@ p]; lst
%Y A244638 Cf. A006877, A006577, A006884.
%K A244638 nonn
%O A244638 1,1
%A A244638 _Zak Seidov_ and _Robert G. Wilson v_, Jul 03 2014