%I #10 Jul 18 2017 08:03:05
%S 1,15,115,915,7355,59369,479887,3880945,31391483,253929457,2054110259,
%T 16616430593,134416594859,1087347726489,8795978441155,71154097145873,
%U 575593242083915,4656198285106057,37665804590772499,304693389569808737
%N Number of base 15 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.
%F G.f.: (1+6*x-18*x^2-48*x^3-6*x^4+24*x^5+8*x^6) / (1-9*x+2*x^2+42*x^3+14*x^4-20*x^5-8*x^6) (conjectured). - _Colin Barker_, Jul 18 2017
%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 15 differing by three or less A126483, two or less A126402, one or less A126369.
%K nonn,base
%O 0,2
%A _R. H. Hardin_, Dec 28 2006