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 A115438 #13 Jul 15 2021 08:52:38 %S A115438 2,310,453,548,691,856,4382,5619,72730,346533,653468,9090908,94117646, %T A115438 334665333,336032387,378253328,390977442,439928491,483516486, %U A115438 516483515,560071510,609022559,621746673,663967614,665334668 %N A115438 Numbers whose square is the concatenation of two numbers k and k+4. %C A115438 From _Farideh Firoozbakht_, Nov 26 2006: (Start) %C A115438 1. All numbers of the form f(n)=3(n).4.6(n).5.3(n+1) are in the sequence because if k(n)=1(n).2.0(n+1).8(n).5 then f(n)^2= k(n).(k(n)+4). For example f(3)=333466653333; k(3)=111200008885 and f(3)^2=333466653333^2=k(3).(k(3)+4)=111200008885.111200008889. %C A115438 2. All numbers of the form g(n)=6(n).5.3(n).4.6(n).8 are in the sequence because g(0)=548 is in the sequence(548^2=300.304) and for n>0 if h(n)=4(n).2.6(n-1).70.2(n).0 then g(n)^2=h(n).(h(n)+4). For example g(5)=666665333334666668; h(5)=444442666670222220 and g(5)^2=h(5).(h(5)+4)=444442666670222220.444442666670222224. (End) %e A115438 120085_120089 = 346533^2. %Y A115438 Cf. A030467, A106497, A115437, A115427, A115439, A115440, A115441, A115442, A115443, A115444, A115445, A115446, A115447. %K A115438 base,nonn %O A115438 1,1 %A A115438 _Giovanni Resta_, Jan 24 2006 %E A115438 The initial "2" (which is admittedly somewhat dubious) added by _N. J. A. Sloane_, Aug 13 2008