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 A221213 #29 Feb 22 2013 13:03:27
%S A221213 42,42,57,52,46,53,58,57,54,49,56,59,49,62,61,59,69,59,50,54,72,64,65,
%T A221213 55,57,54,55,66,61,60,62,61,64,73,62,59,71,63,62,58,68,72,63,59,57,65,
%U A221213 70,59,60,59,71,55,64,54,66,75,67,62,64,64,73,68,68,67,58,61
%N A221213 Conjectured total number of times that k-n appears in the Collatz (3x+1) sequence of k for k = 1, 2, 3,....
%C A221213 Values are tested for natural numbers up to 1000000.
%D A221213 R. K. Guy, Unsolved Problems in Number Theory, E16.
%H A221213 T. D. Noe, <a href="/A221213/b221213.txt">Table of n, a(n) for n = 1..10000</a>
%e A221213 a(1) = 42 = total number of k such that k-1 appears in the Collatz sequence of k, that is, the number of terms in A070991.
%t A221213 Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; nn = 75; t = Table[0, {nn}]; lastChange = 10; k = 0; While[k < 2*lastChange, k++; c = Collatz[k]; d = Intersection[Range[nn], k - c]; If[Length[d] > 0, lastChange = k; t[[d]]++]]; t (* _T. D. Noe_, Feb 21 2013 *)
%Y A221213 Cf. A070165, A070991.
%K A221213 nonn
%O A221213 1,1
%A A221213 _Jayanta Basu_, Feb 21 2013