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 A013991 Number of edge-disjoint paths between opposite corners of a 2 X n grid. 6
 1, 1, 4, 16, 72, 335, 1562, 7273, 33850, 157534, 733148, 3412005, 15879172, 73900265, 343925312, 1600598044, 7449042040, 34667184251, 161338016046, 750852888177, 3494403076902, 16262643529850, 75684907767980, 352230881365025, 1639251436594792, 7628931517771089 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 REFERENCES rec.puzzles Dec 10 1995. LINKS Matthew House, Table of n, a(n) for n = 0..1491 Index entries for linear recurrences with constant coefficients, signature (6,-6,-4,16,-16,7). FORMULA G.f.: (-7*x^6+15*x^5-12*x^4+2*x^3+4*x^2-5*x+1)/[(1-x+x^2)*(1-5*x+9*x^3-7*x^4)]. MATHEMATICA Join[{1}, LinearRecurrence[{6, -6, -4, 16, -16, 7}, {1, 4, 16, 72, 335, 1562}, 25]] (* Vincenzo Librandi, Sep 18 2018 *) PROG (PARI) Vec((-7*x^6+15*x^5-12*x^4+2*x^3+4*x^2-5*x+1)/((1-x+x^2)*(1-5*x+9*x^3-7*x^4)) + O(x^30)) \\ Michel Marcus, Sep 18 2018 (MAGMA) I:=[1, 1, 4, 16, 72, 335, 1562]; [n le 7 select I[n] else 6*Self(n-1) - 6*Self(n-2) - 4*Self(n-3) + 16*Self(n-4) - 16*Self(n-5) + 7*Self(n-6): n in [1..30]]; // Vincenzo Librandi, Sep 18 2018 CROSSREFS Cf. A013990, A013992, A013993, A013994, A013995, A013996, A013997. Sequence in context: A158784 A180141 A226282 * A151245 A151246 A152807 Adjacent sequences:  A013988 A013989 A013990 * A013992 A013993 A013994 KEYWORD nonn,easy AUTHOR EXTENSIONS More terms from Matthew House, Dec 24 2016 STATUS approved

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Last modified April 13 10:24 EDT 2021. Contains 342935 sequences. (Running on oeis4.)