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 A115541 #6 Jul 15 2021 09:16:38 %S A115541 1020408163265306122449,4081632653061224489796,9183673469387755102041, %T A115541 106678449690695619239994950132559020325716449943188991289, %U A115541 123721752304002019946976391869713420022724403484408534276 %N A115541 Numbers k such that the concatenation of 5*k with k gives a square. %e A115541 5102040816326530612245_1020408163265306122449 = 7142857142857142857143^2. %Y A115541 Cf. A102567, A106497, A115527 - A115556. %K A115541 nonn,base %O A115541 1,1 %A A115541 _Giovanni Resta_, Jan 25 2006