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A006235
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Complexity of doubled cycle (regarding case n = 2 as a multigraph).
(Formerly M4849)
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7
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1, 12, 75, 384, 1805, 8100, 35287, 150528, 632025, 2620860, 10759331, 43804800, 177105253, 711809364, 2846259375, 11330543616, 44929049777, 177540878700, 699402223099, 2747583822720, 10766828545725, 42095796462852, 164244726238343, 639620518118400
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OFFSET
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1,2
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COMMENTS
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In plain English, a(n) is the number of spanning trees of the n-prism graph Y_n. - Eric W. Weisstein, Jul 15 2011
Also the number of spanning trees of the n-web graph. - Eric W. Weisstein, Jul 15 2011
Also the number of spanning trees of the n-dipyramidal graph. - Eric W. Weisstein, Jun 14 2018
Determinants of the spiral knots S(4,k,(1,-1,1)). a(k) = det(S(4,k,(1,-1,1))). These knots are also the weaving knots W(k,4) and the Turk's Head Links THK(4,k). - Ryan Stees, Dec 14 2014
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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N. Brothers, S. Evans, L. Taalman, L. Van Wyk, D. Witczak, and C. Yarnall, Spiral knots, Missouri J. of Math. Sci., 22 (2010).
M. DeLong, M. Russell, and J. Schrock, Colorability and determinants of T(m,n,r,s) twisted torus knots for n equiv. +/-1(mod m), Involve, Vol. 8 (2015), No. 3, 361-384.
Eric Weisstein's World of Mathematics, Web Graph
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FORMULA
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a(n) = (1/2)*n*(-2 + (2 - sqrt(3))^n + (2 + Sqrt(3))^n) (Kreweras). - Eric W. Weisstein, Jul 15 2011
G.f.: x(1+2x-10x^2+2x^3+x^4)/((1-x)*(1-4x+x^2))^2.
a(n) = 10a(n-1)-35a(n-2)+52a(n-3)-35a(n-4)+10a(n-5)-a(n-6), n>5.
a(n) = (n/2)*A129743(n). - Woong Kook and Seung Kyoon Shin (andrewk(AT)math.uri.edu), Jan 13 2009
a(k) = det(S(4,k,(1,-1,1))) = k*b(k)^2, where b(1)=1, b(2)=sqrt(6), b(k)=sqrt(6)*b(k-1) - b(k-2) = b(2)*b(k-1) - b(k-2). - Ryan Stees, Dec 14 2014
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EXAMPLE
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For k=3, b(3)=sqrt(6)b(2)-b(1)=6-1=5, so det(S(4,3,(1,-1,1)))=3*5^2=75.
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MAPLE
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A006235:=(1+2*z-10*z**2+2*z**3+z**4)/(z-1)**2/(z**2-4*z+1)**2; # conjectured (correctly) by Simon Plouffe in his 1992 dissertation
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MATHEMATICA
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LinearRecurrence[{10, -35, 52, -35, 10, -1}, {0, 1, 12, 75, 384, 1805}, 20]
Table[1/2 (-2 + (2 - Sqrt[3])^n + (2 + Sqrt[3])^n) n, {n, 0, 20}] // Expand
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PROG
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(PARI) a(n)=if(n<0, 0, polcoeff(x*(1+2*x-10*x^2+2*x^3+x^4)/((1-x)*(1-4*x+x^2))^2+x*O(x^n), n))
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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