%I #19 Mar 26 2013 16:38:25
%S 1,19,39,75,201,428,462,550,583,593,638,702,755,1486,2395,3023,3583,
%T 3867,5342,6998,7419,8283,10367,10447,10524,10567,10879,11219,12379,
%U 13647,13650,14252,14561,14783,15230,15871,16871,16875,17121,17385,18046,19279,19691
%N Numbers n such that in Collatz (3x+1) trajectory of n, the number of terms < n equals number of terms > n.
%H T. D. Noe, <a href="/A217731/b217731.txt">Table of n, a(n) for n = 1..10000</a>
%e 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.
%t 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
%Y Cf. A076228, A222597.
%K nonn
%O 1,2
%A _Jayanta Basu_, Mar 26 2013