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 A078261 #18 Jun 25 2025 19:15:09
%S A078261 1,6,123,617,24681,6170253,1234050607,30851265177,12340506070809,
%T A078261 123405060708091,123405060708091011,1542563258851137639,
%U A078261 1234050607080910111213,61702530354045505560657,2468101214161820222426283,308512651770227527803285379,123405060708091011121314151617
%N A078261 a(n) = numerator(N) where N = 0.246...(2n) is the concatenation of the first n even numbers after the decimal point.
%H A078261 Alois P. Heinz, <a href="/A078261/b078261.txt">Table of n, a(n) for n = 1..351</a>
%p A078261 a:= n-> (t-> numer(t/10^length(t)))(parse(cat(2*i$i=1..n))):
%p A078261 seq(a(n), n=1..17); # _Alois P. Heinz_, Jun 25 2025
%o A078261 (PARI) a(n) = {my(s = ""); for (k=1, n, s = concat(s, Str(2*k))); numerator(eval(s)/10^(#s));} \\ _Michel Marcus_, Jan 15 2019
%Y A078261 Cf. A078258 (similar, with concatenation of 1 to n), A078260 (denominators).
%K A078261 base,nonn
%O A078261 1,2
%A A078261 _Amarnath Murthy_, Nov 24 2002
%E A078261 More terms from Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Apr 19 2003
%E A078261 More terms from _Michel Marcus_, Jan 15 2019