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%I #7 Mar 31 2012 12:35:06
%S 1,21,169,1401,11729,98733,833745,7053777,59744071,506355371,
%T 4293250829,36409762185,308822547173,2619602438143,22221971988277,
%U 188513351680519,1599222568187231,13566880665362081,115094253790715069
%N Number of base 21 n-digit numbers with adjacent digits differing by four or less.
%C [Empirical] a(base,n)=a(base-1,n)+9^(n-1) for base>=4n-3; a(base,n)=a(base-1,n)+9^(n-1)-2 when base=4n-4
%o (S/R) stvar $[N]:(0..M-1) init $[]:=0 asgn $[]->{*} kill +[i in 0..N-2](($[i]`-$[i+1]`>4)+($[i+1]`-$[i]`>4))
%Y Cf. Base 21 differing by three or less A126489, two or less A126408, one or less A126375.
%K nonn,base
%O 0,2
%A _R. H. Hardin_, Dec 28 2006