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 A274243 #16 Jul 02 2016 01:39:19
%S A274243 11,13,22,26,44,52,53,67,88,104,105,106,113,121,131,134,165,176,187,
%T A274243 208,210,211,212,226,227,231,242,243,257,261,262,268,273,289,291,293,
%U A274243 325,329,330,352,373,374,416,419,420,422,424,431,447,452,454,461,462,473
%N A274243 Numbers n for which the sum of the odd numbers in the Collatz (3x+1) iteration of n is prime.
%C A274243 The corresponding primes are 47, 19, 47, 19, 47, 19, 59, 263, 47, 19, 947, 59, 199, 19777, 419, 263, 20359, 47, 1759, 19, 947, 1291, 59, 199, 569, 23813, 19777, 20173,...
%e A274243 11 is in the sequence because the Collatz trajectory of 11 is 11 -> 34 -> 17 -> 52 -> 26 -> 13 -> 40 -> 20 -> 10 -> 5 -> 16 -> 8 -> 4 -> 2 -> 1 and the sum of the odd terms is 11 + 17 + 13 + 5 + 1 = 47 is prime.
%t A274243 lst={};coll[n_]:=NestWhileList[If[EvenQ[#],#/2,3#+1]&,n,#>1&];a:=Select[coll[n],OddQ[#]&];Do[s=Sum[a[[i]],{i,1,Length[a]}];If[PrimeQ[s],AppendTo[lst,n]],{n,1,500}];lst
%Y A274243 Cf. A033493, A225748.
%K A274243 nonn
%O A274243 1,1
%A A274243 _Michel Lagneau_, Jul 01 2016