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 A115540 #6 Jul 15 2021 09:16:46 %S A115540 14285714285714285715,28571428571428571430,42857142857142857145, %T A115540 1627906976744186046511627906976745,1860465116279069767441860465116280 %N A115540 Numbers k such that k^2 is the concatenation of two numbers m and 5*m. %e A115540 14285714285714285715^2 = 2040816326530612245_10204081632653061225. %Y A115540 Cf. A102567, A106497, A115527 - A115556. %K A115540 nonn,base %O A115540 1,1 %A A115540 _Giovanni Resta_, Jan 25 2006