The generalized Collatz iteration, n*X+1, is finite for these n, starting from X=3.
3, 5, 9, 21, 85, 341, 1365, 5461, 21845, 87381
1
As pointed out by Crandall, for n > 3, there are few generalized iterations that are finite. Note that s(n+1) = 4*s(n)+1 for n > 3.
T. D. Noe, Plot of 10 terms
R. E. Crandall, On the “3x+1” problem, Math. Comp., Vol. 32, Num. 144, October 1978, pp. 1281-1292.
(Mma) mxLen = 1000; CollatzX[m_, n_] := NestWhileList[If[EvenQ[#], #/2, (m*# + 1)/2] &, n, # > 1 &, 1, mxLen]; Select[Range[3, 100000, 2], Length[CollatzX[#, 3]] <= mxLen &]
Cf. A006370.
nonn,more
T. D. Noe, Apr 03 2015