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 A116294 #10 Jul 09 2018 03:17:42
%S A116294 3,7,78,80919,91809,326510,475025,524975,673490,4323777,4767132,
%T A116294 5232868,5676223,4083911141,4975000250,5024999750,5916088859,
%U A116294 9503960496,9604950396,4186904462792,4313465946775,5686534053225,5813095537208,7504715871407,7631277355390
%N A116294 Numbers k such that k*(k+1) gives the concatenation of two numbers m and m+1.
%H A116294 Giovanni Resta, <a href="/A116294/b116294.txt">Table of n, a(n) for n = 1..2000</a>
%e A116294 80919 * (80919 + 1) = 6547965480, the concatenation of 65479 and 65479 + 1.
%t A116294 Union @@ ((x /. List@ ToRules@ Reduce[x (x+1) == 10^# y +y+1 && x>0 && 10^(#-1) <= y+1 < 10^#, {x,y}, Integers]) & /@ Range[13] /. x->{}) (* _Giovanni Resta_, Jul 08 2018 *)
%Y A116294 Cf. A116163, A116285, A116295, A116301.
%K A116294 nonn,base
%O A116294 1,1
%A A116294 _Giovanni Resta_, Feb 06 2006
%E A116294 More terms from _Giovanni Resta_, Jul 08 2018