Definitions

# Dickson polynomial

In mathematics, the Dickson polynomials, denoted Dn(x,α), form a polynomial sequence studied by .

Over the complex numbers, Dickson polynomials are essentially equivalent to Chebyshev polynomials with a change of variable, and in fact Dickson polynomials are sometimes called Chebyshev polynomials. Dickson polynomials are mainly studied over finite fields, when they are not equivalent to Chebyshev polynomials. One of the main reasons for interest in them is that for fixed α, they give many examples of permutation polynomials: polynomials acting as permutations of finite fields.

## Definition

D0(x,α) = 2, and for n > 0 Dickson polynomials (of the first kind) are given by

$D_n\left(x,alpha\right)=sum_\left\{p=0\right\}^\left\{lfloor n/2rfloor\right\}frac\left\{n\right\}\left\{n-p\right\} binom\left\{n-p\right\}\left\{p\right\} \left(-alpha\right)^p x^\left\{n-2p\right\}.$

The first few Dickson polynomials are

$D_0\left(x,alpha\right) = 2 ,$

$D_1\left(x,alpha\right) = x ,$

$D_2\left(x,alpha\right) = x^2 - 2alpha ,$

$D_3\left(x,alpha\right) = x^3 - 3xalpha ,$

$D_4\left(x,alpha\right) = x^4 - 4x^2alpha + 2alpha^2 ,$

The Dickson polynomials of the second kind En are defined by

$E_n\left(x,alpha\right)=sum_\left\{p=0\right\}^\left\{lfloor n/2rfloor\right\}binom\left\{n-p\right\}\left\{p\right\} \left(-alpha\right)^p x^\left\{n-2p\right\}.$
They have not been studied much, and have properties similar to those of Dickson polynomials of the first kind. The first few Dickson polynomials of the second kind are

$E_0\left(x,alpha\right) = 1 ,$

$E_1\left(x,alpha\right) = x ,$

$E_2\left(x,alpha\right) = x^2 - alpha ,$

$E_3\left(x,alpha\right) = x^3 - 2xalpha ,$

$E_4\left(x,alpha\right) = x^4 - 3x^2alpha + alpha^2 ,$

## Properties

The Dn satisfy the identities

$D_n\left(u + alpha/u,alpha\right) = u^n + \left(alpha/u\right)^n , ;$
$D_\left\{mn\right\}\left(x,alpha\right) = D_m\left(D_n\left(x,alpha\right),alpha^n\right) , .$

For n≥2 the Dickson polynomials satisfy the recurrence relation

$D_n\left(x,alpha\right) = xD_\left\{n-1\right\}\left(x,alpha\right)-alpha D_\left\{n-2\right\}\left(x,alpha\right) ,$
$E_n\left(x,alpha\right) = xE_\left\{n-1\right\}\left(x,alpha\right)-alpha E_\left\{n-2\right\}\left(x,alpha\right) ,$

The Dickson polynomial Dn = y is a solution of the ordinary differential equation

$\left(x^2-4alpha\right)y + xy\text{'} - n^2y=0 ,$
and the Dickson polynomial En = y is a solution of the differential equation
$\left(x^2-4alpha\right)y + 3xy\text{'} - n\left(n+2\right)y=0 ,$
Their ordinary generating functions are
$sum_nD_n\left(x,alpha\right)z^n = frac\left\{2-xz\right\}\left\{1-xz+alpha z^2\right\} ,$
$sum_nE_n\left(x,alpha\right)z^n = frac\left\{1-xz\right\}\left\{1-xz+alpha z^2\right\} ,$

$D_n\left(2xa,a^2\right)= 2a^\left\{n\right\}T_n\left(x\right) ,$
$E_n\left(2xa,a^2\right)= a^\left\{n\right\}U_n\left(x\right) ,$

## Permutation polynomials and Dickson polynomials

A permutation polynomial (for a given finite field) is one that acts as a permutation of the elements of the finite field.

The Dickson polynomial Dn(x,α) (considered as a function of x with α fixed) is a permutation polynomial for the field with q elements whenever n is coprime to q2−1.

M. proved Schur's conjecture that any integral polynomial that is a permutation polynomial for infinitely many prime fields is a composition of monomials, (rational) linear polynomials, and Dickson polynomials. (The article uses Chebyshev polynomials rather than Dickson polynomials, but Chebyshev polynomials and Dickson polynomials over the rationals can be converted into each other by composition with rational linear maps.)

## References

|last=Lidl|first= R.|last2=Mullen|first2= G. L.|last3= Turnwald|first3= G. |title=Dickson polynomials |series=Pitman Monographs and Surveys in Pure and Applied Mathematics|volume= 65|publisher= Longman Scientific & Technical, Harlow; copublished in the United States with John Wiley & Sons, Inc., New York |year=1993| ISBN= 0-582-09119-5}}

• Thermistocles M. Rassias (1991). Topics in Polynomials of One and Several Variables and Their Applications: A Legacy of P.L.Chebyshev. World Scientific.
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