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 A078260 #23 Jun 25 2025 19:07:58
%S A078260 5,25,500,2500,100000,25000000,5000000000,125000000000,50000000000000,
%T A078260 500000000000000,500000000000000000,6250000000000000000,
%U A078260 5000000000000000000000,250000000000000000000000,10000000000000000000000000,1250000000000000000000000000,500000000000000000000000000000
%N A078260 a(n) = denominator(N), where N = 0.246...(2n) is the concatenation of the first n even numbers after decimal point.
%H A078260 Charlie Neder, <a href="/A078260/b078260.txt">Table of n, a(n) for n = 1..99</a>
%p A078260 a:= n-> (t-> denom(t/10^length(t)))(parse(cat(2*i$i=1..n))):
%p A078260 seq(a(n), n=1..17); # _Alois P. Heinz_, Jun 25 2025
%o A078260 (PARI) a(n) = {my(s = ""); for (k=1, n, s = concat(s, Str(2*k))); denominator(eval(s)/10^(#s));} \\ _Michel Marcus_, Jan 15 2019
%o A078260 (PARI) a(n) = c = concat(vector(n, i, Str(2*i))); denominator(eval(c) / 10^#Str(c)) \\ _David A. Corneth_, Jan 15 2019
%Y A078260 Cf. A078257 (similar, with concatenation of 1 to n), A078261 (numerators).
%K A078260 base,frac,nonn,easy
%O A078260 1,1
%A A078260 _Amarnath Murthy_, Nov 24 2002
%E A078260 More terms from _Sascha Kurz_, Jan 04 2003
%E A078260 a(5) and a(10) corrected, a(15)-a(17) from _Charlie Neder_, Jan 14 2019