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 A179005 #15 Jul 06 2015 23:24:58 %S A179005 0,1,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,20,21,22,24,25,26,27,28, %T A179005 29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51, %U A179005 52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69 %N A179005 Indices of nonprime repunits: nonnegative numbers n such that 11...111 = (10^n - 1)/9 is composite. %C A179005 The complement of A004023 in the set of nonnegative integers. %e A179005 3 appears because the 3-digit repunit 111 is composite (37*3). %e A179005 4 appears because the 4-digit repunit 1111 is composite (101*11). %o A179005 (PARI) isok(n) = !isprime((10^n - 1)/9); \\ _Michel Marcus_, Sep 05 2013 %Y A179005 Complement of A004023. Cf. A002275, A004022, A031974, A259102. %K A179005 nonn %O A179005 1,3 %A A179005 _Andrew McFarland_, Jan 03 2011