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Start with the sequence a(1 to 4) = "1,3,2,3". Then in step s, append "1", "1,2", or "1,2,3", whichever ends with a(s+2).
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%I #7 Oct 19 2017 03:14:13

%S 1,3,2,3,1,2,1,2,3,1,1,2,1,1,2,1,2,3,1,1,1,2,1,1,1,2,1,1,2,1,2,3,1,1,

%T 1,1,2,1,1,1,1,2,1,1,1,2,1,1,2,1,2,3,1,1,1,1,1,2,1,1,1,1,1,2,1,1,1,1,

%U 2,1,1,1,2,1,1,2,1,2,3,1,1,1,1,1,1,2,1,1,1,1,1,1,2,1,1,1,1,1,2,1,1,1

%N Start with the sequence a(1 to 4) = "1,3,2,3". Then in step s, append "1", "1,2", or "1,2,3", whichever ends with a(s+2).

%C a(n)=3 just if n=(k^3+3*k^2-4*k+12)/6.

%K nonn

%O 1,2

%A _Benoit Cloitre_, Apr 14 2003

%E Edited by _Don Reble_, Nov 02 2005