In mathematics, a permutation polynomial (for a given ring) is a polynomial that acts as a permutation of the elements of the ring, i.e. the map is a bijection. In case the ring is a finite field, the Dickson polynomials, which are closely related to the Chebyshev polynomials, provide examples.
Over a finite field, every function, so in particular every permutation of the elements of that field, can be written as a polynomial function.
In the case of finite rings Z/nZ, such polynomials have also been studied and applied in the interleaver component of error detection and correction algorithms.
Let be the finite field of characteristic , that is, the field having elements where for some prime . A polynomial with coefficients in (symbolically written as ) is a permutation polynomial of if the function from to itself defined by is a permutation of .
Due to the finiteness of , this definition can be expressed in several equivalent ways:
A characterization of which polynomials are permutation polynomials is given by
(Hermite's Criterion) is a permutation polynomial of if and only if the following two conditions hold:
If is a permutation polynomial defined over the finite field , then so is for all and in . The permutation polynomial is in normalized form if and are chosen so that is monic, and (provided the characteristic does not divide the degree of the polynomial) the coefficient of is 0.
There are many open questions concerning permutation polynomials defined over finite fields.
Hermite's criterion is computationally intensive and can be difficult to use in making theoretical conclusions. However, Dickson was able to use it to find all permutation polynomials of degree at most five over all finite fields. These results are:
A list of all monic permutation polynomials of degree six in normalized form can be found in .
Beyond the above examples, the following list, while not exhaustive, contains almost all of the known major classes of permutation polynomials over finite fields.
These can also be obtained from the recursion
with the initial conditions and . The first few Dickson polynomials are:
If and then permutes GF(q) if and only if . If then and the previous result holds.
The linearized polynomials that are permutation polynomials over form a group under the operation of composition modulo , which is known as the Betti-Mathieu group, isomorphic to the general linear group .
An exceptional polynomial over is a polynomial in which is a permutation polynomial on for infinitely many .
A permutation polynomial over of degree at most is exceptional over .
Every permutation of is induced by an exceptional polynomial.
If a polynomial with integer coefficients (i.e., in ) is a permutation polynomial over for infinitely many primes , then it is the composition of linear and Dickson polynomials. (See Schur's conjecture below).
In finite geometry coordinate descriptions of certain point sets can provide examples of permutation polynomials of higher degree. In particular, the points forming an oval in a finite projective plane, with a power of 2, can be coordinatized in such a way that the relationship between the coordinates is given by an o-polynomial, which is a special type of permutation polynomial over the finite field .
The problem of testing whether a given polynomial over a finite field is a permutation polynomial can be solved in polynomial time.
A polynomial is a permutation polynomial in variables over if the equation has exactly solutions in for each .
For the finite ring Z/nZ one can construct quadratic permutation polynomials. Actually it is possible if and only if n is divisible by p<sup>2</sup> for some prime number p. The construction is surprisingly simple, nevertheless it can produce permutations with certain good properties. That is why it has been used in the interleaver component of turbo codes in 3GPP Long Term Evolution mobile telecommunication standard (see 3GPP technical specification 36.212 e.g. page 14 in version 8.8.0).
Consider for the ring Z/4Z. One sees: so the polynomial defines the permutation
Consider the same polynomial for the other ring Z/8Z. One sees: so the polynomial defines the permutation
Consider for the ring Z/p<sup>k</sup>Z.
Lemma: for k=1 (i.e. Z/pZ) such polynomial defines a permutation only in the case a=0 and b not equal to zero. So the polynomial is not quadratic, but linear.
Lemma: for k>1, p>2 (Z/p<sup>k</sup>Z) such polynomial defines a permutation if and only if and .
Consider , where p<sub>t</sub> are prime numbers.
Lemma: any polynomial defines a permutation for the ring Z/nZ if and only if all the polynomials defines the permutations for all rings , where are remainders of modulo .
As a corollary one can construct plenty quadratic permutation polynomials using the following simple construction. Consider , assume that k<sub>1</sub> >1.
Consider , such that , but ; assume that , i > 1. And assume that for all . (For example, one can take and ). Then such polynomial defines a permutation.
To see this we observe that for all primes p<sub>i</sub>, i > 1, the reduction of this quadratic polynomial modulo p<sub>i</sub> is actually linear polynomial and hence is permutation by trivial reason. For the first prime number we should use the lemma discussed previously to see that it defines the permutation.
For example, consider and polynomial . It defines a permutation
A polynomial g(x) for the ring Z/p<sup>k</sup>Z is a permutation polynomial if and only if it permutes the finite field Z/pZ and for all x in Z/p<sup>k</sup>Z, where g′(x) is the formal derivative of g(x).
Let K be an algebraic number field with R the ring of integers. The term "Schur's conjecture" refers to the assertion that, if a polynomial f defined over K is a permutation polynomial on R/P for infinitely many prime ideals P, then f is the composition of Dickson polynomials, degree-one polynomials, and polynomials of the form x<sup>k</sup>. In fact, Schur did not make any conjecture in this direction. The notion that he did is due to Fried, who gave a flawed proof of a false version of the result. Correct proofs have been given by Turnwald and Müller.