A136023 - OEIS

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A136023

The largest prime factor among all composites <= 10^n.

5, 47, 499, 4999, 49999, 499979, 4999999, 49999991, 499999993, 4999999937, 49999999967, 499999999979, 4999999999937, 49999999999981, 499999999999999, 4999999999999997, 49999999999999993, 499999999999999931, 4999999999999999963, 49999999999999999951

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OFFSET

1,1

COMMENTS

This is the largest single divisor contributing to A136021(n).

If 4 concatenated with n-1 nines is prime, it will be the largest factor. This candidate does not work for n=6 because 499999 is not prime.

LINKS

Robert Israel, Table of n, a(n) for n = 1..501

EXAMPLE

a(3)=499 because it is the largest prime factor of all k<=10^3; the largest k in that interval with this factor is k=2*499 = 998.

MAPLE

5, seq(prevprime(5*10^n), n=1..100); # Robert Israel, Jul 21 2014