A105432 - OEIS

%I #21 Feb 05 2025 18:41:59

%S 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 32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,

%U 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 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 Complement of A097683.

%C Numbers k such that (10^n + 17)/9 is composite (see A098406).

%H Harvey P. Dale, <a href="/A105432/b105432.txt">Table of n, a(n) for n = 1..1000</a>

%t IntegerLength/@Select[Table[FromDigits[PadLeft[{3},n,1]],{n,80}],CompositeQ] (* _Harvey P. Dale_, Feb 05 2025 *)

%o (PARI) isok(n) = ! isprime((10^n + 17)/9); \\ _Michel Marcus_, Jul 27 2017

%Y Cf. A098406, A105433.

%K nonn,base

%O 1,1

%A _Lekraj Beedassy_, Apr 08 2005