%I #8 Mar 31 2012 12:35:02
%S 1,18,252,3168,44472,634368,9124092,131601852,1900243080,27449395092,
%T 396572715092,5729777752772,82786984432932,1196161590550776,
%U 17282993079833922,249717254801438778,3608098064502340024
%N Number of base 18 circular n-digit numbers with adjacent digits differing by 9 or less.
%C [Empirical] a(base,n)=a(base-1,n)+F(9) for base>=9.int(n/2)+1 and F(d) is the largest coefficient in (1+x+...+x^(2d))^n
%o (S/R) stvar $[N]:(0..M-1) init $[]:=0 asgn $[]->{*} kill +[i in 0..N-1](($[i]`-$[(i+1)mod N]`>9)+($[(i+1)mod N]`-$[i]`>9))
%K nonn,base
%O 0,2
%A _R. H. Hardin_, Dec 28 2006