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 A115055 Lower level digraph derived from a voltage graph. 3
 0, 1, 0, 0, 1, 3, 3, 2, 6, 15, 21, 24, 42, 86, 138, 192, 305, 546, 906, 1381, 2175, 3651, 6042, 9582, 15225, 24901, 40836, 65748, 105364, 170796, 278184, 450017, 724968, 1172412, 1902321, 3080367, 4975551, 8044478, 13029534, 21096027, 34114553 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,6 COMMENTS Lower level digraph derived from a voltage graph (Gross's covering graph construction) that is a generalized Fibonacci Markov. In matrix terms gives a 6 X 6 Markov with characteristic Polynomial (-1 - x + x^2)*(1 + 2*x + 2*x^2 + x^3 + x^4). This digraph construction gives a complex substructure to the Fibonacci Pisot that is not Pisot. Gross's covering graph constructions called voltage graphs are abstractions from lower level graphs. limit_{n to Infinity} (a(n+1)/a(n)) = Golden Mean. REFERENCES J. L. Gross and T. W. Tucker, Topological Graph Theory, Wiley, 1987; see Figure 2.5 p. 62 LINKS G. C. Greubel, Table of n, a(n) for n = 1..1000 Index entries for linear recurrences with constant coefficients, signature (0,0,1,3,3,1). FORMULA Let M be the 6x6 matrix given by: M = {{0, 0, 0, 0, 1, 1}, {1, 0, 0, 0, 0, 0}, {1, 1, 0, 0, 0, 0}, {0, 0, 1, 0, 0, 0}, {0, 0, 1, 1, 0, 0}, {0, 0, 0, 0, 1, 0}}, then v(n) = M.v(n-1), where a(n) = v(n)(1). From Vladimir Kruchinin, Oct 12 2011: (Start) G.f.: x/(1-(x+x^2)^3). a(n) = Sum_{k=0..n} binomial(3*k,n-3*k). (End) a(n) = a(n-3) + 3*a(n-4) + 3*a(n-5) + a(n-6). - G. C. Greubel, Mar 22 2019 MATHEMATICA (* Gross page 62 voltage group L3 : weights set to one *) M = {{0, 0, 0, 0, 1, 1}, {1, 0, 0, 0, 0, 0}, {1, 1, 0, 0, 0, 0}, {0, 0, 1, 0, 0, 0}, {0, 0, 1, 1, 0, 0}, {0, 0, 0, 0, 1, 0}} v[1] = {0, 0, 0, 0, 0, 1} v[n_] := v[n] = M.v[n - 1] a = Table[Floor[v[n][[1]]], {n, 1, 50}] (* alternate program *) LinearRecurrence[{0, 0, 1, 3, 3, 1}, {0, 1, 0, 0, 1, 3}, 50] (* G. C. Greubel, Mar 22 2019 *) PROG (PARI) my(x='x+O('x^50)); concat([0], Vec(x/(1-(x+x^2)^3))) \\ G. C. Greubel, Mar 22 2019 (Magma) R:=PowerSeriesRing(Integers(), 50); [0] cat Coefficients(R!( x/(1-(x+x^2)^3) )); // G. C. Greubel, Mar 22 2019 (Sage) (x/(1-(x+x^2)^3)).series(x, 50).coefficients(x, sparse=False) # G. C. Greubel, Mar 22 2019 (GAP) a:=[0, 1, 0, 0, 1, 3];; for n in [7..50] do a[n]:=a[n-3]+3*a[n-4]+ 3*a[n-5]+a[n-6]; od; a; # G. C. Greubel, Mar 22 2019 CROSSREFS Sequence in context: A200174 A266153 A086636 * A158468 A238278 A200770 Adjacent sequences: A115052 A115053 A115054 * A115056 A115057 A115058 KEYWORD nonn AUTHOR Roger L. Bagula, Dec 09 2006 EXTENSIONS Edited by G. C. Greubel, Mar 22 2019 STATUS approved

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