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.
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, 0, 37, 116, 0, 0, 128, 132, 45, 0, 144, 49, 152, 0, 15, 0, 168, 0, 176, 180, 61, 0, 192, 43, 200, 0, 69, 212, 0, 0, 224, 228, 51, 0, 240, 81, 0, 0, 85, 260, 264, 0, 272, 276, 93, 0, 288
Offset: 1
Keywords
Examples
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.
Programs
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Mathematica
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}]