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%I #7 Mar 31 2012 12:35:01
%S 1,21,135,693,4159,25291,157635,995659,6357119,40921479,265145505,
%T 1727138841,11300204035,74208972755,488874737353,3229363648333,
%U 21382430152111,141869013922279,942972599847861,6277675948529895
%N Number of base 21 circular n-digit numbers with adjacent digits differing by 3 or less.
%C [Empirical] a(base,n)=a(base-1,n)+A025012(n+1) for base>=3.int(n/2)+1
%o (S/R) stvar $[N]:(0..M-1) init $[]:=0 asgn $[]->{*} kill +[i in 0..N-1](($[i]`-$[(i+1)mod N]`>3)+($[(i+1)mod N]`-$[i]`>3))
%K nonn,base
%O 0,2
%A _R. H. Hardin_, Dec 28 2006