A213181 - OEIS

%I #11 Mar 01 2013 15:05:23

%S 0,0,1,1,1,1,3,1,4,1,3,2,2,3,2,1,3,4,4,2,1,3,2,2,5,2,17,4,4,2,16,1,6,

%T 3,2,5,4,4,6,2,17,1,6,4,4,2,16,2,5,5,5,3,2,17,17,4,7,4,6,3,3,16,15,1,

%U 6,6,5,4,3,2,16,5,18,4,2,5,5,6,6,2,4,17,17

%N Number of chains of even numbers of length 2 or more in the Collatz (3x+1) trajectory of n.

%C A pair of even numbers that appear side by side in Collatz trajectory of n is considered a chain of length 2 and likewise for chains of greater length.

%H T. D. Noe, <a href="/A213181/b213181.txt">Table of n, a(n) for n = 1..10000</a>

%e For n=3, Collatz trajectory of 3 is 3,10,5,16,8,4,2,1, hence the only chain is 16,8,4,2 and so a(3)=1; again for 12: 12,6,3,10,5,16,8,4,2,1 and as such there are two chains 12,6 and 16,8,4,2 so a(12)=2

%t Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; Table[c = Collatz[n]; cnt = 0; evenCnt = 0; Do[If[OddQ[i], evenCnt = 0, evenCnt++; If[evenCnt == 2, cnt++]], {i, c}]; cnt, {n, 100}] (* _T. D. Noe_, Feb 28 2013 *)

%Y Cf. A070165.

%K nonn

%O 1,7

%A _Jayanta Basu_, Feb 28 2013