A176973 Second smallest distinct prime factor of repunit(n) = (10^n-1)/9 (A002275), zero if repunit is prime.

0, 37, 101, 271, 7, 4649, 73, 37, 41, 513239, 7, 79, 239, 31, 17, 5363222357, 7, 0, 41, 37, 23, 0, 7, 271, 53, 37, 29, 16763, 7, 6943319, 17, 37, 103, 71, 7, 247629013, 909090909090909091, 37, 41, 1231, 7, 1527791, 23, 31, 47

Offset: 2

Author

Vladimir Joseph Stephan Orlovsky, Apr 29 2010

Keywords

• nonn

Comments

111=3*37, 1111=11*101, 11111=41*271, 111111=3*7*11*13*37,..

Links

• Ray Chandler, Table of n, a(n) for n = 2..382 (from Kamada link)
• Makoto Kamada, Factorizations of 11...11 (Repunit).

Crossrefs

Cf. A002275, A067063.

Programs

Mathematica

lst={};Do[If[Length[FactorInteger[(10^n-1)/9]]==1,lst=Append[lst,0],lst=Append[lst,FactorInteger[(10^n-1)/9][[2,1]]]],{n,2,60}];lst
(* Second program: *)
Table[If[Length@ # < 2, 0, #[[2, 1]]] &@ FactorInteger@ FromDigits@ ConstantArray[1, n], {n, 2, 46}] (* Michael De Vlieger, May 10 2017 *)

Extensions

Offset corrected to 2, description clarified by Ray Chandler, May 10 2017

b-file truncated at uncertain a(383) at the suggestion of Eric Chen by Max Alekseyev, May 13 2022