# 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]

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

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] $L_n(x) = \begin{cases} 2, & \mbox{if } n = 0 \\ x, & \mbox{if } n = 1 \\ x L_{n - 1}(x) + L_{n - 2}(x), & \mbox{if } n \geq 2. \end{cases}$

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 Fn at x = 2. The degrees of Fn is n − 1 and the degree of Ln 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]

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

Other identities include:[4]

$F_{m+n}(x)=F_{m+1}(x)F_n(x)+F_m(x)F_{n-1}(x)\,$
$L_{m+n}(x)=L_m(x)L_n(x)-(-1)^nL_{m-n}(x)\,$
$F_{n+1}(x)F_{n-1}(x)- F_n(x)^2=(-1)^n\,$
$F_{2n}(x)=F_n(x)L_n(x).\,$

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

$t^2-xt-1=0.\,$

## 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 xk in Fn(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. ^ a b Benjamin & Quinn p. 141
2. ^ Benjamin & Quinn p. 142
3. ^
4. ^ a b c Springer