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 A328991 #6 Nov 09 2019 18:54:21 %S A328991 1,1221,12212121221,1221212122121221212212121221, %T A328991 122121212212122121221212122121221212122121221212122121221212212121221 %N A328991 a(n) = concatenation of a(n-1),2,a(n-2),2,a(n-1), with a(1)=1, a(2)=1221. %C A328991 In the Allombert et al. (2018) paper the sequence starts with a(0) = the empty string. %D A328991 Allombert, Bill, Nicolas Brisebarre, and Alain Lasjaunias. "On a two-valued sequence and related continued fractions in power series fields." The Ramanujan Journal 45.3 (2018): 859-871. See W_n in Theorem 2. %K A328991 nonn %O A328991 1,2 %A A328991 _N. J. A. Sloane_, Nov 09 2019