A213181 Number of chains of even numbers of length 2 or more in the Collatz (3x+1) trajectory of n.

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

Offset

1,7

Comments

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.

Links

T. D. Noe, Table of n, a(n) for n = 1..10000

Example

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

Mathematica

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 *)

Crossrefs

Cf. A070165.

Sequence in context: A090740 A094603 A165595 * A301848 A325610 A278536

Adjacent sequences: A213178 A213179 A213180 * A213182 A213183 A213184

Keyword

nonn

Author

Jayanta Basu, Feb 28 2013

Status

approved