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 A220867 Number of perfect matchings in the twisted graph (C_4 X C_n)_(1234). 1
 2, 14, 80, 194, 888, 2702, 11040, 37634, 146024, 524174, 1988720, 7300802, 27436248, 101687054, 380604480, 1416317954, 5292200648, 19726764302, 73658763920, 274758382274 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Andrew Howroyd, Table of n, a(n) for n = 1..30 S. Butler and S. Osborne, Counting tilings by taking walks, 2012. FORMULA Empirical g.f.: -2 * x * ( 3*x^6 - 11*x^5 - 19*x^4 + 74*x^3 - 7*x^2 - 3*x - 1 ) / ( (x^2 - 4*x + 1) * (x^2 - 2*x - 1) * (x^2 + 2*x - 1) ). - Joerg Arndt, Nov 17 2015 CROSSREFS Sequence in context: A121200 A112408 A026291 * A216985 A361813 A102401 Adjacent sequences: A220864 A220865 A220866 * A220868 A220869 A220870 KEYWORD nonn AUTHOR N. J. A. Sloane, Dec 27 2012 EXTENSIONS a(1)-a(2), a(11)-a(20) from Andrew Howroyd, Nov 15 2015 STATUS approved

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Last modified August 15 02:04 EDT 2024. Contains 375171 sequences. (Running on oeis4.)