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A051450 Number of positive rational knots with 2n+1 crossings. 7
1, 2, 5, 12, 30, 76, 195, 504, 1309, 3410, 8900, 23256, 60813, 159094, 416325, 1089648, 2852242, 7466468, 19546175, 51170460, 133962621, 350713222, 918170280, 2403786672, 6293172025, 16475700746, 43133883845, 112925875764 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The number of positive rational knots with even crossing number is zero.

a(n) = (1/2)(coefficient of x in the reduction by x^2->x+1 of the polynomial p(n,x) = 1+x^n+x^(2n)); see A192464.  Reductions of polynomials by substitutions such as x^2->x+1 are introduced at A192232. - Clark Kimberling, Jul 01 2011

LINKS

Colin Barker, Table of n, a(n) for n = 1..1000

Index entries for linear recurrences with constant coefficients, signature (4,-3,-2,1).

FORMULA

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

G.f. (version 2): x*(1-2*x)/((1-x-x^2)*(1-3*x+x^2)). - N. J. A. Sloane, Jan 21 2001

Binomial transform of Fib(n)(1-(-1)^n)/2. Binomial transform of (Fib(n)+Fib(-n))/2. - Paul Barry, Apr 23 2004

Let phi be the golden ratio (1+Sqrt[5])/2. Then a(n)= (phi^n - (-phi)^(-n) + (1+phi)^n - (1+phi)^(-n))/(2Sqrt[5]) or a(n) = Floor[(1 + phi^n + (1+phi)^n)/(2Sqrt[5])]. - Herbert Kociemba, May 12 2004

Also, number of (s(0), s(1), ..., s(n)) such that 0 < s(i) < 5 and |s(i) - s(i-1)| <= 1 for i = 1, 2, ...., n, s(0) = 1, s(n) = 2. a(n)=(2/5)*Sum(k, 1, 4, sin(Pi*k/5)sin(2Pi*k/5)(1+2cos(Pi*k/5))^n). - Herbert Kociemba, Jun 07 2004

a(n) = (Fibonacci(2*n)+Fibonacci(n))/2. - Vladeta Jovovic, Jul 17 2004

Convolution of F(n) and F(2n-1). a(n)=sum{k=0..n, F(2k-1)F(n-k)}. - Paul Barry, Jul 26 2004

a(n) = 4*a(n-1)-3*a(n-2)-2*a(n-3)+a(n-4). - Colin Barker, Nov 01 2014

EXAMPLE

a(4) = 12 because we have 12 positive rational knots with 9 crossings: 9_1 to 9_7, 9_9, 9_10, 9_13, 9_18 and 9_23 (in Alexander-Briggs notation).

PROG

(PARI) Vec(x*(2*x-1)/((x^2-3*x+1)*(x^2+x-1)) + O(x^100)) \\ Colin Barker, Nov 01 2014

(MAGMA) [(Fibonacci(2*n)+Fibonacci(n))/2: n in [1..30]]; // Vincenzo Librandi, Nov 01 2014

CROSSREFS

Cf. A000045.

Sequence in context: A092247 A108360 A051163 * A038508 A105695 A244884

Adjacent sequences:  A051447 A051448 A051449 * A051451 A051452 A051453

KEYWORD

easy,nonn

AUTHOR

Alexander Stoimenow (stoimeno(AT)math.toronto.edu)

EXTENSIONS

More terms from James A. Sellers, Dec 09 1999

STATUS

approved

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Last modified September 26 06:54 EDT 2017. Contains 292502 sequences.