A176973 - OEIS

%I #22 Mar 22 2024 18:45:03

%S 0,37,101,271,7,4649,73,37,41,513239,7,79,239,31,17,5363222357,7,0,41,

%T 37,23,0,7,271,53,37,29,16763,7,6943319,17,37,103,71,7,247629013,

%U 909090909090909091,37,41,1231,7,1527791,23,31,47

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

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

%H Ray Chandler, <a href="/A176973/b176973.txt">Table of n, a(n) for n = 2..382</a> (from Kamada link)

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/repunit">Factorizations of 11...11 (Repunit)</a>.

%t 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

%t (* Second program: *)

%t Table[If[Length@ # < 2, 0, #[[2, 1]]] &@ FactorInteger@ FromDigits@ ConstantArray[1, n], {n, 2, 46}] (* _Michael De Vlieger_, May 10 2017 *)

%Y Cf. A002275, A067063.

%K nonn

%O 2,2

%A _Vladimir Joseph Stephan Orlovsky_, Apr 29 2010

%E Offset corrected to 2, description clarified by _Ray Chandler_, May 10 2017

%E b-file truncated at uncertain a(383) at the suggestion of _Eric Chen_ by _Max Alekseyev_, May 13 2022