Fibonacci polynomials

In mathematics, the Fibonacci polynomials are a polynomial sequence which can be considered as a generalization of the Fibonacci numbers. The polynomials generated in a similar way from the Lucas numbers are called Lucas polynomials.

Definition

These Fibonacci polynomials are defined by a recurrence relation:^[1]

F_{n}(x)={\begin{cases}0,&{\mbox{if }}n=0\\1,&{\mbox{if }}n=1\\xF_{{n-1}}(x)+F_{{n-2}}(x),&{\mbox{if }}n\geq 2\end{cases}}

The first few Fibonacci polynomials are:

F_{0}(x)=0\,

F_{1}(x)=1\,

F_{2}(x)=x\,

F_{3}(x)=x^{2}+1\,

F_{4}(x)=x^{3}+2x\,

F_{5}(x)=x^{4}+3x^{2}+1\,

F_{6}(x)=x^{5}+4x^{3}+3x\,

The Lucas polynomials use the same recurrence with different starting values:^[2] $L_{n}(x)={\begin{cases}2,&{\mbox{if }}n=0\\x,&{\mbox{if }}n=1\\xL_{{n-1}}(x)+L_{{n-2}}(x),&{\mbox{if }}n\geq 2.\end{cases}}$

The first few Lucas polynomials are:

L_{0}(x)=2\,

L_{1}(x)=x\,

L_{2}(x)=x^{2}+2\,

L_{3}(x)=x^{3}+3x\,

L_{4}(x)=x^{4}+4x^{2}+2\,

L_{5}(x)=x^{5}+5x^{3}+5x\,

L_{6}(x)=x^{6}+6x^{4}+9x^{2}+2.\,

The Fibonacci and Lucas numbers are recovered by evaluating the polynomials at x = 1; Pell numbers are recovered by evaluating F_n at x = 2. The degrees of F_n is n − 1 and the degree of L_n is n. The ordinary generating function for the sequences are:^[3]

\sum _{{n=0}}^{\infty }F_{n}(x)t^{n}={\frac {t}{1-xt-t^{2}}}

\sum _{{n=0}}^{\infty }L_{n}(x)t^{n}={\frac {2-xt}{1-xt-t^{2}}}.

The polynomials can be expressed in terms of Lucas sequences as

F_{n}(x)=U_{n}(x,-1),\,

L_{n}(x)=V_{n}(x,-1).\,

Identities

Main article: Lucas sequence

As particular cases of Lucas sequences, Fibonacci polynomials satisfy a number of identities.

First, they can be defined for negative indices by^[4]

F_{{-n}}(x)=(-1)^{{n-1}}F_{{n}}(x),\,L_{{-n}}(x)=(-1)^{n}L_{{n}}(x).

Other identities include:^[4]

F_{{m+n}}(x)=F_{{m+1}}(x)F_{n}(x)+F_{m}(x)F_{{n-1}}(x)\,

L_{{m+n}}(x)=L_{m}(x)L_{n}(x)-(-1)^{n}L_{{m-n}}(x)\,

F_{{n+1}}(x)F_{{n-1}}(x)-F_{n}(x)^{2}=(-1)^{n}\,

F_{{2n}}(x)=F_{n}(x)L_{n}(x).\,

Closed form expressions, similar to Binet's formula are:^[4]

F_{n}(x)={\frac {\alpha (x)^{n}-\beta (x)^{n}}{\alpha (x)-\beta (x)}},\,L_{n}(x)=\alpha (x)^{n}+\beta (x)^{n},

where

\alpha (x)={\frac {x+{\sqrt {x^{2}+4}}}{2}},\,\beta (x)={\frac {x-{\sqrt {x^{2}+4}}}{2}}

are the solutions (in t) of

t^{2}-xt-1=0.\,

A relationship between the Fibonacci polynomials and the standard basis polynomials is given by

x^{n}=F_{n+1}(x)+\sum _{k=1}^{\lfloor n/2\rfloor }(-1)^{k}\left[{\binom {n}{k}}-{\binom {n}{k-1}}\right]F_{n+1-2k}(x).

For example,

x^{4}=F_{5}(x)-3F_{3}(x)+2F_{1}(x)\,

x^{5}=F_{6}(x)-4F_{4}(x)+4F_{2}(x)\,

x^{6}=F_{7}(x)-5F_{5}(x)+9F_{3}(x)-5F_{1}(x)\,

x^{7}=F_{8}(x)-6F_{6}(x)+14F_{4}(x)-14F_{2}(x)\,

A proof of this fact is given starting from page 5 here.

Combinatorial interpretation

The coefficients of the Fibonacci polynomials can be read off from Pascal's triangle following the "shallow" diagonals (shown in red). The sums of the coefficients are the Fibonacci numbers.

If F(n,k) is the coefficient of x^k in F_n(x), so

F_{n}(x)=\sum _{{k=0}}^{n}F(n,k)x^{k},\,

then F(n,k) is the number of ways an n−1 by 1 rectangle can be tiled with 2 by 1 dominoes and 1 by 1 squares so that exactly k squares are used.^[1] Equivalently, F(n,k) is the number of ways of writing n−1 as an ordered sum involving only 1 and 2, so that 1 is used exactly k times. For example F(6,3)=4 and 5 can be written in 4 ways, 1+1+1+2, 1+1+2+1, 1+2+1+1, 2+1+1+1, as a sum involving only 1 and 2 with 1 used 3 times. By counting the number of times 1 and 2 are both used in such a sum, it is evident that F(n,k) is equal to the binomial coefficient

F(n,k)={\binom {{\tfrac {n+k-1}{2}}}{k}}

when n and k have opposite parity. This gives a way of reading the coefficients from Pascal's triangle as shown on the right.

References

1 2 Benjamin & Quinn p. 141
↑ Benjamin & Quinn p. 142
↑ Weisstein, Eric W. "Fibonacci Polynomial". MathWorld.
1 2 3 Springer

Benjamin, Arthur T.; Quinn, Jennifer J. (2003). "§9.4 Fibonacci and Lucas Polynomial". Proofs that Really Count. MAA. p. 141. ISBN 0-88385-333-7.
Philippou, Andreas N. (2001), "Fibonacci polynomials", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
Philippou, Andreas N. (2001), "Lucas polynomials", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
Weisstein, Eric W. "Lucas Polynomial". MathWorld.

External links

"Sloane's A162515 : Triangle of coefficients of polynomials defined by Binet form...". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
"Sloane's A011973 : Triangle of coefficients of Fibonacci polynomials.". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.

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