A390053 - OEIS

A390053

a(1) = 1. For n > 1, a(n) is the largest prime factor of A390052(n).

1, 2, 3, 2, 5, 3, 7, 2, 3, 5, 11, 7, 31, 41, 17, 61, 71, 3, 19, 5, 7, 11, 23, 7, 13, 31, 3, 41, 29, 5, 31, 23, 11, 43, 53, 7, 73, 83, 31, 5, 41, 7, 43, 11, 5, 23, 47, 7, 47, 5, 17, 13, 53, 5, 11, 13, 19, 29, 59, 5, 61, 31, 7, 23, 13, 11, 67, 43, 23, 7, 71, 3, 73

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OFFSET

1,2

LINKS

Michael S. Branicky, Table of n, a(n) for n = 1..10000

EXAMPLE

a(153) = 59 since the digits of 153 can be rearranged to 135,315,351,513 and 531; and 531 has the largest prime factor of the set (59).

MATHEMATICA

A390053[n_] := Max[FactorInteger[Map[FromDigits, Permutations[IntegerDigits[n]]]][[All, -1, 1]]];

Array[A390053, 100] (* Paolo Xausa, Oct 31 2025 *)

PROG

(Python)

from sympy import factorint

from itertools import permutations

def a(n):

if n == 1: return 1

m, argm = 0, None

for p in permutations(str(n)):

t = int("".join(p))

f = 0 if t == 1 else max(factorint(t))

if f > m: m, argm = f, t

return m

print([a(n) for n in range(1, 74)])

CROSSREFS

Cf. A390052.

Sequence in context: A378856 A163457 A285708 * A072593 A394544 A348907

Adjacent sequences: A390050 A390051 A390052 * A390054 A390055 A390056

KEYWORD

nonn,look,base

AUTHOR

Michael S. Branicky and Ali Sada, Oct 22 2025

STATUS

approved