A217731 Numbers n such that in Collatz (3x+1) trajectory of n, the number of terms < n equals number of terms > n.

1, 19, 39, 75, 201, 428, 462, 550, 583, 593, 638, 702, 755, 1486, 2395, 3023, 3583, 3867, 5342, 6998, 7419, 8283, 10367, 10447, 10524, 10567, 10879, 11219, 12379, 13647, 13650, 14252, 14561, 14783, 15230, 15871, 16871, 16875, 17121, 17385, 18046, 19279, 19691

Offset: 1

Author

Jayanta Basu, Mar 26 2013

Keywords

• nonn

Examples

			19 is in the list because Collatz trajectory of 19 is {19, 58, 29, 88, 44, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1} and number of terms < 19 = 10 = number of terms > 19.
		

Links

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

Crossrefs

Cf. A076228, A222597.

Programs

Mathematica

Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3*# + 1] &, n, # > 1 &]; t = {};Do[If[Length[Select[Collatz[n], # < n &]] == Length[Select[Collatz[n], # > n &]], AppendTo[t, n]], {n, 50000}]; t