cp's OEIS Frontend

A105432 Numbers n such that the near-repdigit number consisting of n-1 1's followed by a terminal 3 (i.e., of the form 111...1113) is composite.

%I A105432 #21 Feb 05 2025 18:41:59
%S A105432 4,6,7,8,10,12,13,14,15,16,17,18,19,20,21,22,23,25,26,27,28,29,30,31,
%T A105432 32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,
%U A105432 55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78
%N A105432 Numbers n such that the near-repdigit number consisting of n-1 1's followed by a terminal 3 (i.e., of the form 111...1113) is composite.
%C A105432 Complement of A097683.
%C A105432 Numbers k such that (10^n + 17)/9 is composite (see A098406).
%H A105432 Harvey P. Dale, <a href="/A105432/b105432.txt">Table of n, a(n) for n = 1..1000</a>
%t A105432 IntegerLength/@Select[Table[FromDigits[PadLeft[{3},n,1]],{n,80}],CompositeQ] (* _Harvey P. Dale_, Feb 05 2025 *)
%o A105432 (PARI) isok(n) = ! isprime((10^n + 17)/9); \\ _Michel Marcus_, Jul 27 2017
%Y A105432 Cf. A098406, A105433.
%K A105432 nonn,base
%O A105432 1,1
%A A105432 _Lekraj Beedassy_, Apr 08 2005