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A094806 Number of (s(0), s(1), ..., s(2n)) such that 0 < s(i) < 8 and |s(i) - s(i-1)| = 1 for i = 1,2,...,2n, s(0) = 1, s(2n) = 5. 3
1, 5, 20, 74, 264, 924, 3200, 11016, 37792, 129392, 442496, 1512224, 5165952, 17643456, 60250112, 205729920, 702452224, 2398414592, 8188884992, 27958972928, 95458646016, 325917686784, 1112755552256, 3799191029760, 12971261403136, 44286680330240 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,2

COMMENTS

In general a(n)= 2/m*Sum_{r=1..m-1} sin(r*j*Pi/m)sin(r*k*Pi/m)(2*cos(r*Pi/m))^(2n)) counts (s(0), s(1), ..., s(2n)) such that 0 < s(i) < m and |s(i) - s(i-1)| = 1 for i = 1,2,....,2n, s(0) = j, s(2n) = k.

LINKS

Harvey P. Dale, Table of n, a(n) for n = 2..1000

Index entries for linear recurrences with constant coefficients, signature (6,-10,4).

FORMULA

a(n) = (1/4)*Sum_{k=1..7} sin(Pi*k/8)sin(5*Pi*k/8)(2*cos(Pi*k/8))^(2n).

a(n) = 6*a(n-1) - 10*a(n-2) + 4*a(n-3).

G.f.: x^2*(x-1) / ( (2*x-1)*(2*x^2-4*x+1) )

a(n) = (-2^n+(-(2-sqrt(2))^n+(2+sqrt(2))^n)/sqrt(2))/4. - Colin Barker, Apr 27 2016

4*a(n) = 2*A007070(n-1)-2^n.- R. J. Mathar, Nov 14 2019

MATHEMATICA

f[n_] := FullSimplify[ TrigToExp[(1/4)Sum[ Sin[Pi*k/8]Sin[5Pi*k/8](2Cos[Pi*k/8])^(2n), {k, 1, 7}]]]; Table[ f[n], {n, 2, 25}] (* Robert G. Wilson v, Jun 18 2004 *)

LinearRecurrence[{6, -10, 4}, {1, 5, 20}, 30] (* Harvey P. Dale, Mar 04 2015 *)

CROSSREFS

Sequence in context: A034535 A316222 A273718 * A289596 A026639 A248326

Adjacent sequences:  A094803 A094804 A094805 * A094807 A094808 A094809

KEYWORD

nonn,easy

AUTHOR

Herbert Kociemba, Jun 11 2004

STATUS

approved

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Last modified April 19 05:26 EDT 2021. Contains 343105 sequences. (Running on oeis4.)