The union of the terms in S000783.
1, 12, 28, 98, 386, 943, 1494, 1680, 2987, 7083, 57346, 252548, 331778, 524289, 596310, 2886352, 3247146, 3264428, 4585418, 5158596, 5772712, 13019668, 18341744, 24455681, 98041684, 136696632, 271114753, 361486064, 406672385, 481981441, 711611184, 722067240
1
T. D. Noe, Plot of 32 terms
Wikipedia, Collatz conjecture
(Mma) Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; t2 = {}; mx = 0; last = 1; cnt = 1; Do[c = Length[Collatz[n]]; If[c == last, cnt++, If[cnt > mx, mx = cnt; AppendTo[t2, {n - cnt, cnt}]]; last = c; cnt = 1], {n, 2, 100000}]; Transpose[t2][[1]]
Cf. S000783.
nonn
T. D. Noe, Dec 02 2015