cp's OEIS Frontend

A087264 a(n) is the least initial value of a 3x+1 trajectory in which 4n is the largest(peak) term or a(n)=0 if 4n cannot be a peak.

%I A087264 #6 Oct 15 2013 22:32:17
%S A087264 4,8,0,3,20,24,0,32,0,13,0,48,7,56,0,21,68,72,0,80,84,19,0,96,33,104,
%T A087264 0,37,116,0,0,128,132,45,0,144,49,152,0,15,0,168,0,176,180,61,0,192,
%U A087264 43,200,0,69,212,0,0,224,228,51,0,240,81,0,0,85,260,264,0,272,276,93,0,288
%N A087264 a(n) is the least initial value of a 3x+1 trajectory in which 4n is the largest(peak) term or a(n)=0 if 4n cannot be a peak.
%e A087264 n=10: 4n=40 arises as a peak-term in 3x+1 trajectory started at 13, so a[10]=13 and 13 is the least initial value including 40 as a max-term.
%t A087264 c[x_] := (1-Mod[x, 2])*(x/2)+Mod[x, 2]*(3*x+1); c[1]=1; fpl[x_] := Max[Delete[FixedPointList[c, x], -1]] t=Table[fpl[w], {w, 1, 15000}]; Table[Min[Flatten[Position[t, 4*j]]], {j, 1, 256}]
%Y A087264 Cf. A025586, A087263.
%K A087264 nonn
%O A087264 1,1
%A A087264 _Labos Elemer_, Sep 11 2003