A249627 Product of the lowest and highest prime factors of n-th repunit A002275(n).

121, 111, 1111, 11111, 111, 1111111, 1507, 1001001, 100001, 11111111111, 29703, 14064697609, 10000001, 8718483, 64705883, 11111111111111111, 1001001, 1234567901234567900987654320987654321, 307571, 32516067, 5645629, 123456790123456790123454320987654320987654321

Offset: 2

Author

Jacob Vecht, Nov 02 2014

Keywords

• nonn
• base

Examples

			Highest and lowest prime factors of 111111 are 3 and 37, so the sixth term is 111.
		

Links

• Chai Wah Wu, Table of n, a(n) for n = 2..322
• P. De Geest, Repunits and Their Factors

Crossrefs

Cf. A002275.

Programs

Mathematica

plh[n_]:=Module[{fi=FactorInteger[FromDigits[PadRight[{},n,1]]]},fi[[1,1]]fi[[-1,1]]]; Array[plh,30,2] (* Harvey P. Dale, May 04 2021 *)

PARI

for(n=2,20,p=(10^n-1)/9;print1(factor(p)[1,1]*factor(p)[#factor(p)[,1],1],", ")) \\ Derek Orr, Nov 02 2014

Python

from sympy import factorint
A249627_list = [min(fs)*max(fs) for fs in (factorint((10**n-1)//9) for n in range(2,31))] # Chai Wah Wu, Nov 04 2019

Extensions

More terms from Derek Orr, Nov 02 2014