%I #10 Aug 12 2023 11:07:34
%S 1,21,99,369,1607,7051,31743,144683,666527,3094947,14463569,67946603,
%T 320593259,1518250599,7212838277,34360479199,164079145839,
%U 785173661987,3764380920489,18077996769707,86948519336037,418760656546517
%N Number of base 21 circular n-digit numbers with adjacent digits differing by 2 or less.
%C [Empirical] a(base,n)=a(base-1,n)+A005191(n+1) for base>=2.int(n/2)+1.
%C See A285280 for confirmation of linear recurrence and code to produce sequence. - _Ray Chandler_, Aug 12 2023.
%H Ray Chandler, <a href="/A124949/b124949.txt">Table of n, a(n) for n = 0..99</a>
%H <a href="/index/Rec#order_21">Index entries for linear recurrences with constant coefficients</a>, signature (21, -171, 627, -595, -2635, 6891, 1703, -19279, 6565, 26454, -13848, -20145, 11191, 8590, -4346, -1925, 775, 190, -50, -5, 1).
%o (S/R) stvar $[N]:(0..M-1) init $[]:=0 asgn $[]->{*} kill +[i in 0..N-1](($[i]`-$[(i+1)mod N]`>2)+($[(i+1)mod N]`-$[i]`>2))
%Y Cf. Row 21 of A285280.
%K nonn,base
%O 0,2
%A _R. H. Hardin_, Dec 28 2006