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 A126473 Number of strings over a 5 symbol alphabet with adjacent symbols differing by three or less. 58
 1, 5, 23, 107, 497, 2309, 10727, 49835, 231521, 1075589, 4996919, 23214443, 107848529, 501037445, 2327695367, 10813893803, 50238661313, 233396326661, 1084301290583, 5037394142315, 23402480441009, 108722104190981 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS [Empirical] a(base,n)=a(base-1,n)+7^(n-1) for base>=3n-2; a(base,n)=a(base-1,n)+7^(n-1)-2 when base=3n-3 From Johannes W. Meijer, Aug 01 2010: (Start) The a(n) represent the number of n-move routes of a fairy chess piece starting in a given side square (m = 2, 4, 6 or 8) on a 3 X 3 chessboard. This fairy chess piece behaves like a king on the eight side and corner squares but on the central square the king goes crazy and turns into a red king, see A179596. For the side squares the 512 red kings lead to 47 different red king sequences, see the cross-references for some examples. The sequence above corresponds to four A[5] vectors with the decimal [binary] values 367 [1,0,1,1,0,1,1,1,1], 463 [1,1,1,0,0,1,1,1,1], 487 [1,1,1,1,0,0,1,1,1] and 493 [1,1,1,1,0,1,1,0,1]. These vectors lead for the corner squares to A179596 and for the central square to A179597. This sequence belongs to a family of sequences with g.f. (1+x)/(1-4*x-k*x^2). Red king sequences that are members of this family are A003947 (k=0), A015448 (k=1), A123347 (k=2), A126473 (k=3; this sequence) and A086347 (k=4). Other members of this family are A000351 (k=5), A001834 (k=-1), A111567 (k=-2), A048473 (k=-3) and A053220 (k=-4) Inverse binomial transform of A154244. (End) Equals the INVERT transform of A055099: (1, 4, 14, 50, 178,...). - Gary W. Adamson, Aug 14 2010 Number of one-sided n-step walks taking steps from {E, W, N, NE, NW}. - Shanzhen Gao, May 10 2011 For n>=1, a(n) equals the numbers of words of length n-1 on alphabet {0,1,2,3,4} containing no subwords 00 and 11. - Milan Janjic, Jan 31 2015 REFERENCES S. Gao, H. Niederhausen, Sequences Arising From Prudent Self-Avoiding Walks (submitted to INTEGERS: The Electronic Journal of Combinatorial Number Theory). LINKS Shanzhen Gao, Keh-Hsun Chen, Tackling Sequences From Prudent Self-Avoiding Walks, FCS'14, The 2014 International Conference on Foundations of Computer Science. Index entries for linear recurrences with constant coefficients, signature (4,3) FORMULA From Johannes W. Meijer, Aug 01 2010: (Start) G.f.: (1+x)/(1-4*x-3*x^2). a(n) = 4*a(n-1) + 3*a(n-2) with a(0) = 1 and a(1) = 5. a(n) = ((1+3/sqrt(7))/2)*(A)^(-n) + ((1-3/sqrt(7))/2)*(B)^(-n) with A = (-2 + sqrt(7))/3 and B = (-2-sqrt(7))/3. Limit(a(n+k)/a(k),k=infinity) = (-1)^(n+1)*A000244(n)/(A015530(n)*sqrt(7)-A108851(n)) (End) MAPLE with(LinearAlgebra): nmax:=19; m:=2; A[5]:= [1, 0, 1, 1, 0, 1, 1, 1, 1]: A:=Matrix([[0, 1, 0, 1, 1, 0, 0, 0, 0], [1, 0, 1, 1, 1, 1, 0, 0, 0], [0, 1, 0, 0, 1, 1, 0, 0, 0], [1, 1, 0, 0, 1, 0, 1, 1, 0], A[5], [0, 1, 1, 0, 1, 0, 0, 1, 1], [0, 0, 0, 1, 1, 0, 0, 1, 0], [0, 0, 0, 1, 1, 1, 1, 0, 1], [0, 0, 0, 0, 1, 1, 0, 1, 0]]): for n from 0 to nmax do B(n):=A^n: a(n):= add(B(n)[m, k], k=1..9): od: seq(a(n), n=0..nmax); # Johannes W. Meijer, Aug 01 2010 PROG (S/R) stvar \$[N]:(0..M-1) init \$[]:=0 asgn \$[]->{*} kill +[i in 0..N-2]((\$[i]`-\$[i+1]`>3)+(\$[i+1]`-\$[i]`>3)) (PARI) a(n)=([0, 1; 3, 4]^n*[1; 5])[1, 1] \\ Charles R Greathouse IV, May 10 2016 CROSSREFS Cf. 5 symbol differing by two or less A126392, one or less A057960. Cf. Red king sequences side squares [numerical value A[5]]: A086347 [495], A179598 [239], A126473 [367], A123347 [335], A179602 [95], A154964 [31], A015448 [327], A152187 [27], A003947 [325], A108981 [11], A007483 [2]. - Johannes W. Meijer, Aug 01 2010 Cf. A055099. Sequence in context: A270530 A128732 A026894 * A238112 A109877 A179598 Adjacent sequences:  A126470 A126471 A126472 * A126474 A126475 A126476 KEYWORD nonn,easy AUTHOR R. H. Hardin, Dec 27 2006 EXTENSIONS Edited by Johannes W. Meijer, Aug 10 2010 STATUS approved

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Last modified August 9 11:00 EDT 2020. Contains 336323 sequences. (Running on oeis4.)