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

2,2

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).

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 *)

Crossrefs

Cf. A002275, A067063.

Sequence in context: A044605 A130229 A142941 * A105019 A351141 A090496

Adjacent sequences: A176970 A176971 A176972 * A176974 A176975 A176976

Keyword

nonn

Author

Vladimir Joseph Stephan Orlovsky, Apr 29 2010

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

Status

approved