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

2,1

Links

Chai Wah Wu, Table of n, a(n) for n = 2..322

P. De Geest, Repunits and Their Factors

Example

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

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 *)

Prog

(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

Crossrefs

Cf. A002275.

Sequence in context: A197817 A152144 A282344 * A202003 A109838 A014734

Adjacent sequences: A249624 A249625 A249626 * A249628 A249629 A249630

Keyword

nonn,base

Author

Jacob Vecht, Nov 02 2014

Extensions

More terms from Derek Orr, Nov 02 2014

Status

approved