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 A109743 #7 Aug 03 2021 19:01:31 %S A109743 1,3,9,41,231,1551,12037,105861,1039539,11268059,133585089,1718984673, %T A109743 23857531519,355190381031,5646040979901,95432906354621, %U A109743 1709068974897867,32325104169244851,643869687410800249,13471540400560532889,295384662114697463319 %N A109743 a(2)=1; for n>2, a(n) = A109742(n)/3. %Y A109743 Cf. A000166, A109742. %K A109743 nonn %O A109743 2,2 %A A109743 _N. J. A. Sloane_, Aug 13 2005