A087272 a(n) is the largest prime number in 3x+1 trajectory initiated at n.

2, 5, 2, 5, 5, 17, 2, 17, 5, 17, 5, 13, 17, 53, 2, 17, 17, 29, 5, 2, 17, 53, 5, 29, 13, 1619, 17, 29, 53, 1619, 2, 29, 17, 53, 17, 37, 29, 101, 5, 1619, 2, 43, 17, 17, 53, 1619, 5, 37, 29, 29, 13, 53, 1619, 1619, 17, 43, 29, 101, 53, 61, 1619, 1619, 2, 37, 29, 101, 17, 13, 53

Offset

2,1

Links

Rémy Sigrist, Table of n, a(n) for n = 2..10000

Mathematica

c[x_] := (1-Mod[x, 2])*(x/2)+Mod[x, 2]*(3*x+1); c[1]=1; fpl[x_] := Delete[FixedPointList[c, x], -1] ofp[x_] := Part[fpl[x], Flatten[Position[PrimeQ[fpl[x]], True]]] Table[Max[ofp[w]], {w, 1, 256}]
Table[Max[Select[NestWhileList[If[EvenQ[#], #/2, 3#+1]&, n, #>1&], PrimeQ]], {n, 2, 70}] (* Harvey P. Dale, Feb 27 2023 *)

Prog

(PARI) a(n) = my (mx=2); while (n>1, if (isprime(n), mx=max(mx, n)); n=if (n%2, 3*n+1, n/2)); mx \\ Rémy Sigrist, Oct 08 2018

Crossrefs

Cf. A025586, A055510, A087232.

Sequence in context: A104289 A309697 A177435 * A299963 A107060 A016589

Adjacent sequences: A087269 A087270 A087271 * A087273 A087274 A087275

Keyword

nonn

Author

Labos Elemer, Sep 18 2003

Extensions

Offset corrected by Rémy Sigrist, Oct 08 2018

Status

approved