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 A115539 #6 Jul 15 2021 09:16:54 %S A115539 2040816326530612245,8163265306122448980,18367346938775510205, %T A115539 265008112493239588967009194159005,346133044889129259058950784207680, %U A115539 438074634937804218496484586262845,540832882639264467279610600324500 %N A115539 Numbers k such that the concatenation of k with 5*k gives a square. %e A115539 2040816326530612245_10204081632653061225 = 14285714285714285715^2. %Y A115539 Cf. A102567, A106497, A115527 - A115556. %K A115539 nonn,base %O A115539 1,1 %A A115539 _Giovanni Resta_, Jan 25 2006