This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A089017 #10 Oct 23 2013 15:10:08
%S A089017 8,9,10,11,12,13,14,15,16,18,19,20,21,22,23,24,25,26,27,28,29,30,31,
%T A089017 32,33,34,35,36,37,38,40,41,42,43,44,45,46,47,48,50,51,52,53,54,55,56,
%U A089017 57,58,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,78,79,80,81,82
%N A089017 n for which the number consisting of a string of n 3's and a terminal 1 is not prime.
%C A089017 Complement of A055520(n)=A055557(n) - 1. The first n for which {10^(n+1) - 7}/3 is composite is thus n=8,corresponding to 333333331=17*19607843.
%t A089017 Select[Range[2,90],!PrimeQ[FromDigits[PadLeft[{1},#,3]]]&]-1 (* _Harvey P. Dale_, Jun 19 2012 *)
%o A089017 (PARI) is(n)=ispseudoprime((10^(n+1)-7)/3) \\ _Charles R Greathouse IV_, Oct 23 2013
%Y A089017 Cf. A055520, A089018.
%K A089017 base,nonn
%O A089017 1,1
%A A089017 _Lekraj Beedassy_, Nov 04 2003