OFFSET
0,4
EXAMPLE
a(5) = 5 since there are only five numbers 12, 13, 20, 21, 32 such that difference between number of halving and tripling steps is 5.
MATHEMATICA
Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; nn = 15; t = Table[0, {nn}]; Do[c = Collatz[n]; e = Select[c, EvenQ]; diff = 2*Length[e] - Length[c]; If[diff < nn - 1, t[[diff + 2]]++], {n, 2^(nn - 1)}]; t (* T. D. Noe, Mar 04 2013 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Jayanta Basu, Mar 04 2013
EXTENSIONS
Corrected and extended by T. D. Noe, Mar 06 2013
STATUS
approved