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%I #6 Nov 09 2019 18:54:21
%S 1,1221,12212121221,1221212122121221212212121221,
%T 122121212212122121221212122121221212122121221212122121221212212121221
%N a(n) = concatenation of a(n-1),2,a(n-2),2,a(n-1), with a(1)=1, a(2)=1221.
%C In the Allombert et al. (2018) paper the sequence starts with a(0) = the empty string.
%D 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 nonn
%O 1,2
%A _N. J. A. Sloane_, Nov 09 2019