A136024 Largest prime factor of odd composites less than 10^n.

3, 31, 331, 3331, 33331, 333331, 3333331, 33333331, 333333313, 3333333323, 33333333329, 333333333323, 3333333333301, 33333333333323, 333333333333307, 3333333333333301, 33333333333333323

Offset: 1

Author

Enoch Haga, Dec 12 2007

Keywords

• easy
• nonn
• changed

Comments

Last instance of the largest prime factor of odd N <= 10^n-1 associated with A136021.

This sequence is not the same as A051200. E.g., A051200(9)=333333331 is not prime and is different from a(9)=333333313. However, if A051200(n) is prime, then a(n)=A051200(n).

Examples

			a(1)=31 because it is the largest factor of odd N <= 10^2-1. The value of odd N where this factor first occurs is 3*31 = 93.
		

Crossrefs

Cf. A051200, A136021.

Programs

Mathematica

a[n_]:=NextPrime[10^n/3,-1]; Array[a,17] (* Stefano Spezia, Aug 31 2025 *)

PARI

a(n)=precprime(10^n\3)

Formula

a(n) = precprime(10^n/3) = A007917((10^n-1)/3). - Max Alekseyev, Sep 29 2015

Extensions

Clarified and extended by Charles R Greathouse IV, Oct 11 2009