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 A110780 #5 Nov 14 2021 12:37:39 %S A110780 11,99999,111111111,9999999999,11111111111111111111111111111111111111, %T A110780 99999999999999999999999999999999999999999999999 %N A110780 Copies of 1 and 9 alternately such that every partial concatenation is a prime. %e A110780 11, 1199999, 1199999111111111, ... are all prime. %Y A110780 Cf. A110774, A110775, A110776, A110777, A110778, A110781. %K A110780 base,nonn %O A110780 1,1 %A A110780 _Amarnath Murthy_, Aug 12 2005 %E A110780 More terms from _Joshua Zucker_, Jan 11 2006 %E A110780 The next term has more than 800 1's and is too large to include.