%I #5 Mar 31 2012 12:35:05
%S 1,21,99,475,2293,11111,53973,262649,1279855,6243155,30479985,
%T 148910385,727918557,3559976043,17417455619,85244931269,417326666085,
%U 2043565752795,10008988327829,49030629716903,240219997723397
%N Number of base 21 n-digit numbers with adjacent digits differing by two or less.
%C [Empirical] a(base,n)=a(base-1,n)+5^(n-1) for base>=2n-1; a(base,n)=a(base-1,n)+5^(n-1)-2 when base=2n-2
%o (S/R) stvar $[N]:(0..M-1) init $[]:=0 asgn $[]->{*} kill +[i in 0..N-2](($[i]`-$[i+1]`>2)+($[i+1]`-$[i]`>2))
%Y Cf. Base 21 differing by one or less A126375.
%K nonn,base
%O 0,2
%A _R. H. Hardin_, Dec 28 2006