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A176973 Second smallest distinct prime factor of repunit(n) = (10^n-1)/9 (A002275), zero if repunit is prime. 1
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 (list; graph; refs; listen; history; text; internal format)
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..438 (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 A090496 A005107

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

STATUS

approved

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Last modified May 24 18:34 EDT 2019. Contains 323534 sequences. (Running on oeis4.)