%I #16 Jun 05 2013 00:51:19
%S 1,4,49,340,4235,39706,397068,3970918,39523168,395436300,3953296865
%N Number of numbers k such that all terms of the Collatz (3x+1) iteration of k are <= 10^n.
%e For n = 1, the four k are 1, 2, 4, and 8.
%t Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; Table[Length[Select[Range[10^n], Max[Collatz[#]] <= 10^n &]], {n, 0, 5}]
%Y Cf. A006884, A006885, A095384, A222291, A224540.
%K nonn,more
%O 0,2
%A _T. D. Noe_, Apr 24 2013
%E a(10) from _Donovan Johnson_, Jun 05 2013