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 A108966 #3 Dec 15 2017 17:36:51 %S A108966 1,5,16,36,61,149,725,1104,1276,1289,8575 %N A108966 Numbers n such that repunit(n) concatenated with its 10's complement is prime; or numbers n such that (1+7*10^n+100^n)/9 is prime. %C A108966 Larger values are only Fermat and Lucas PRPs. %e A108966 a(2)=5 because 10^5-11111 = 88889 and 1111188889 is prime. %Y A108966 Cf. A002275. %K A108966 base,hard,more,nonn %O A108966 1,2 %A A108966 _Jason Earls_, Jul 26 2005