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%I #2 Feb 27 2009 03:00:00
%S 1,121,1213231,1213231424341,121323142434152535451,
%T 1213231424341525354516263646561,
%U 1213231424341525354516263646561727374757671
%N a(1)=1 a(2)=121 and a(n)=a(n-1) n 2 n 3 n ... n (n-1) n 1, using concatenation.
%C An example of transition complete sequences, when n <= 9. One needs to consider the base 10 complications when n >= 10. The number of digits is given by d(n)=d(n-1)+2n-2 which is A002061.
%e a(4)=1213231424341=1213231 + 4 2 4 3 4 1.
%Y Cf. A108713, A002061.
%K nonn,base
%O 1,2
%A Douglas Stones (dssto1(AT)student.monash.edu.au), Oct 03 2006