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]

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

The Lucas polynomials use the same recurrence with different starting values: ^[2]

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

They can be defined for negative indices by ^[3]

${\displaystyle F_{-n}(x)=(-1)^{n-1}F_{n}(x),}$

${\displaystyle L_{-n}(x)=(-1)^{n}L_{n}(x).}$

The Fibonacci polynomials form a sequence of orthogonal polynomials with ${\displaystyle A_{n}=C_{n}=1}$ and ${\displaystyle B_{n}=0}$.

Examples

The first few Fibonacci polynomials are:

${\displaystyle F_{0}(x)=0\,}$

${\displaystyle F_{1}(x)=1\,}$

${\displaystyle F_{2}(x)=x\,}$

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

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

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

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

The first few Lucas polynomials are:

${\displaystyle L_{0}(x)=2\,}$

${\displaystyle L_{1}(x)=x\,}$

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

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

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

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

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

Properties

• The degree of F_n is n − 1 and the degree of L_n is n.

• 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 ordinary generating functions for the sequences are: ^[4]

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

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

• The polynomials can be expressed in terms of Lucas sequences as

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

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

• They can also be expressed in terms of Chebyshev polynomials ${\displaystyle {\mathcal {T}}_{n}(x)}$ and ${\displaystyle {\mathcal {U}}_{n}(x)}$ as

  ${\displaystyle F_{n}(x)=i^{n-1}\cdot {\mathcal {U}}_{n-1}({\tfrac {-ix}{2}}),\,}$

  ${\displaystyle L_{n}(x)=2\cdot i^{n}\cdot {\mathcal {T}}_{n}({\tfrac {-ix}{2}}),\,}$

where ${\displaystyle i}$ is the imaginary unit.

Identities

Main article: Lucas sequence

As particular cases of Lucas sequences, Fibonacci polynomials satisfy a number of identities, such as ^[3]

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

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

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

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

Closed form expressions, similar to Binet's formula are: ^[3]

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

where

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

are the solutions (in t) of

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

For Lucas Polynomials n > 0, we have

${\displaystyle L_{n}(x)=\sum _{k=0}^{\lfloor n/2\rfloor }{\frac {n}{n-k}}{\binom {n-k}{k}}x^{n-2k}.}$

A relationship between the Fibonacci polynomials and the standard basis polynomials is given by ^[5]

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

For example,

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

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

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

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

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), namely

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

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 ${\displaystyle F(n,k)={\begin{cases}\displaystyle {\binom {{\frac {1}{2}}(n+k-1)}{k}}&{\text{if }}n\not \equiv k{\pmod {2}},\\[12pt]0&{\text{else}}.\end{cases}}}$

This gives a way of reading the coefficients from Pascal's triangle as shown on the right.

References

1. Benjamin & Quinn p. 141
2. Benjamin & Quinn p. 142
3. Springer
4. Weisstein, Eric W. "Fibonacci Polynomial". MathWorld .
5. A proof starts from page 5 in Algebra Solutions Packet (no author).

• Benjamin, Arthur T.; Quinn, Jennifer J. (2003). "Fibonacci and Lucas Polynomial". Proofs that Really Count: The Art of Combinatorial Proof. Dolciani Mathematical Expositions. Vol. 27. Mathematical Association of America. p.  141. ISBN   978-0-88385-333-7.
• Philippou, Andreas N. (2001) [1994], "Fibonacci polynomials", Encyclopedia of Mathematics , EMS Press
• Philippou, Andreas N. (2001) [1994], "Lucas polynomials", Encyclopedia of Mathematics , EMS Press
• Weisstein, Eric W. "Lucas Polynomial". MathWorld .
• Jin, Z. On the Lucas polynomials and some of their new identities. Advances in Differential Equations 2018, 126 (2018). https://doi.org/10.1186/s13662-018-1527-9

Further reading

• Hoggatt, V. E.; Bicknell, Marjorie (1973). "Roots of Fibonacci polynomials". Fibonacci Quarterly . 11: 271–274. ISSN   0015-0517. MR   0332645.
• Hoggatt, V. E.; Long, Calvin T. (1974). "Divisibility properties of generalized Fibonacci Polynomials". Fibonacci Quarterly . 12: 113. MR   0352034.
• Ricci, Paolo Emilio (1995). "Generalized Lucas polynomials and Fibonacci polynomials". Rivista di Matematica della Università di Parma. V. Ser. 4: 137–146. MR   1395332.
• Yuan, Yi; Zhang, Wenpeng (2002). "Some identities involving the Fibonacci Polynomials". Fibonacci Quarterly. 40 (4): 314. MR   1920571.
• Cigler, Johann (2003). "q-Fibonacci polynomials". Fibonacci Quarterly (41): 31–40. MR   1962279.

External links

• OEIS sequenceA162515(Triangle of coefficients of polynomials defined by Binet form)
• OEIS sequenceA011973(Triangle of coefficients of Fibonacci polynomials)