1 Introduction
Let be the finite field of order and be a mapping from to itself. is said to be a to mapping if for any . to mappings over finite fields in even characteristic have wide applications in symmetric cryptography, in particular in the construction of APN functions, bent functions, semibent functions and so on. For example, to mappings over finite fields with characteristic allow to construct bent Boolean functions in bivariate representation from the socalled class introduced by Carlet and Mesnager [3]. In addition, to mappings over can also determine semibent Boolean functions in bivariate representation from the MaioranaMaFarland class [13]. For more applications of to mappings over finite fields, we refer to [13, Section 6].
Very recently, Mesnager and Qu [13] provided a systematic study of to mappings over finite fields with arbitrary characteristic including characterizations by the Walsh transform, several constructions (an AGWlike criterion, those from permutation polynomials or linear translators), some classical classes of polynomials (linearized polynomials, monomials, low degree polynomials, etc) and many explicit applications of to mappings. In this paper^{1}^{1}1 Some parts of the present paper have been accepted in the proceedings of the conference IWSDA 2019, mainly the determination of to mappings with degree and two classes of to trinomials and four classes of to quadrinomials. However, due to the limit of length, only sketch of the proofs have been included in the proceedings paper. we push further the study initiated in [13] and we focus on even characteristic (binary case being the most interesting for applications). More specifically, in [13], the authors determined completely the to mappings of degree at most 4 over finite fields. In this paper, we firstly consider to polynomials of degree over . Our method is based on the HasseWeil bound (see e.g. [6, 14]), which has been used recently in the study of permutation polynomials (see e.g. [1, 5]). Next, we focus on to mappings with few terms. The characterization of to monomials is trivial. The authors [13] presented four classes of to binomials of the form thanks to the known results on hyperoval sets (see Proposition 4.2). In the present paper, we investigate by MAGMA all to binomials of the form on , where , and . It turns out that for odd, up to the oequivalence (see Definition 2.8), these experiment examples can be explained by the four classes of to binomials given in Proposition 4.2. For the case when is even, there is only one class of to binomials that is over . For to trinomials and quadrinomials, we derive, up to the QMequivalence (see Definition 2.9), two classes of to trinomials of the form , which explain all the experiment examples of to trinomials over with . We also present twelve classes of to quadrinomials of the form over . Our method in this part uses the multivariate method introduced by Dobbertin [4] and the key point is to determine the number of solutions of equations with a high degree. In addition, we shall use an important tool (which is the resultant of two polynomials) to treat the case of quadrinomials.
The remainder of this paper is organized as follows. Section 2 introduces some basic notions which will be used in the manuscript. Based on the HasseWeil bound, Section 3 determines completely the to mappings with degree over . In Section 4, we consider to polynomials with few terms, including mainly experiment examples obtained by MAGMA and two classes of to trinomials over . In Section 5, we present twelve classes of to quadrinomials over . Finally, Section 6 is a conclusion. Throughout this paper, for any , we assume denotes the trace function from to , i.e., for any . Particularly, when , we use to denote the absolute trace function over , i.e., for any . The algebraic closure of is denoted by . For any sets , denotes the cardinality of .
2 Preliminaries
In this section, we introduce some basic notions on the HasseWeil bound as well as some known results concerning the solutions of equations with low degree (quadratic, cubic and quartic). We also recall the resultant of two polynomials which will be useful in our subsequent proofs. Finally, we introduce and recall two equivalences to study to polynomials.
2a The HasseWeil bound
In this subsection, we recall some well known results on algebraic curves and algebraic function fields, mainly the HasseWeil bound. These classical results can be found in most of the textbooks on algebraic curves and algebraic function fields.
Lemma 2.1.
Let be a function field and be perfect. Let denote the genus of . Then we have the following upper bound on the genus.
Lemma 2.2.
Given two plane curves and and a point on the plane, the intersection number of and at the point is defined by seven axioms. We do not include its precise and long definitions here. For more details, we refer to [7].
Lemma 2.3.
[7, Bézout’s Theorem] Let and be two projective plane curves over an algebraically closed field , having no component in common. Let and be the polynomials associated with and respectively. Then
where the sum runs over all points in the projective plane .
2B Solutions of equations with low degree
In this subsection, we introduce some known lemmas about the solutions of some equations with low degree (quadratic, cubic, quartic), which will be used the proofs of our results.
Lemma 2.4.
[9] Let and . Then the quadratic equation has solutions in if and only if .
Lemma 2.5.
[2] Let , where . Then the cubic equation has a unique solution in if and only if .
If is a quartic polynomial over which factors as a product of two linear factors times an irreducible quadratic, we write ; if is a cubic irreducible polynomial over , we write . In [12], P. A. Leonard and K. S. Williams characterized the factorization of a quartic polynomial over as follows.
Lemma 2.6.
[12] Let with and . Let and denote roots of when they exist in . Set . Then the factorization of over is characterized as follows:

if and only if and ;

if and only if and , ;

if and only if ;

if and only if and ;

if and only if and .
2C Resultant of polynomials
In this subsection, we recall some basic facts about the resultant of two polynomials. Given two nonzero polynomials of degrees and respectively
with and coefficients in a field or in an integral domain , their resultant is the determinant of the following matrix:
For a field and two polynomials , we use to denote the resultant of and with respect to . It is the resultant of and when considered as polynomials in the single variable . In this case, belongs in the ideal generated by and , and thus any satisfying and is such that (see [9]).
2D Oequivalence and QMequivalence between two 2to1 mappings
A permutation polynomial over is called an ovalpolynomial (for short opolynomial) if , and for each ,
is a permutation polynomial. It is well known that there is a close relation between opolynomials and to mappings as follows.
Lemma 2.7.
A polynomial from to itself with is an opolynomial if and only if is to for every .
Each opolynomial defines an (hyper)oval. And each hyperoval defines opolynomials. Two opolynomials are called (projectively) oequivalent^{2}^{2}2Note that for a special type of bent functions, socalled Niho bent functions there is a general equivalence relation called oequivalence which is induced from the equivalence of opolynomials., if they define equivalent hyperovals. Hyperovals being called equivalent if they are mapped to each other by collineations (i.e. permutations mapping lines to lines). For example, for the following omonomials
are oequivalent to each other. We can naturally define an equivalent relation between to (polynomial) mappings as follows.
Definition 2.8.
Let and be to (polynomial) mappings where . Then and are said to be oequivalent if the corresponding opolynomials and are equivalent.
The oequivalence between two 2to1 mappings will play an important role in our classification of to binomials. Namely, the oequivalence plays a major role in explaining the experiment results of to binomials. In particular, under the oequivalence, Proposition 4.2 can explain all experiment results on to binomials over with and odd.
Next, we recall another equivalence between polynomials. Let with . As we all know, if permutes , then is a to polynomial over if and only if is to polynomial over .
It is trivial that a monomial is a permutation polynomial over if and only if Let and be two polynomials in satisfying that , where and is an integer such that . Then is to if and only if so is . Consequently, we recall the following notion of QMequivalence [15, 10].
Definition 2.9.
[15] Two polynomials and in are said to be quasimultiplicative (QM) equivalence if there exists an integer with and for some nonzero elements in .
Using the QMequivalence, we can simplify the experiment results on to trinomials (resp. quadrinomials). As indicated in Table II, there are for example only two to trinomials of the form over up to the QMequivalence. In addition, we can avoid getting equivalent to polynomials having the same terms.
3 to mappings with degree over
In this section, we completely determine the to mappings with degree over . Clearly, for any polynomials with degree , is to over if and only if so is , where and . Hence, it is suffisant to consider with normalized form, i.e., is monic, , and when , the coefficient of is . That is to say, in this part, we suffice to consider where since .
Theorem 3.1.
Let and where . Then is not to over .
Proof.
We assume that is to over . According to the definition, has exactly two solutions and for any . In addition, for any , we have
Thus for any ,
(1) 
has exactly one solution in .
In the following, we assume that and . Let
and Then
where and . We also assume that . Thus Eq. (1) has exactly one solution in if and only if
(2) 
does, where . Let . According to Lemmas 2.5 and 2.6, if has exactly one solution in , then and
(3) 
for any satisfying and . Indeed,
where and . It should be noted that is equivalent to .
Assume that and . Let
and
Then . Together with Eq. (3) and that there exist at most elements such that , as well as that at most elements such that , we have
(4) 
If is irreducible over , let be transcendentals over with . Then by Lemma 2.2, the functional fields has genus
Then by the HasseWeil bound, i.e., Lemma 2.1, we have
when , which is contradictory with (4).
Therefore, is not irreducible over and we assume that where , are irreducible and If , choose such that . Then . Assume that . Then or , say Then . Hence and we have
Thanks to Bézout’s Theorem, i.e., Lemma 2.3,
which is also contradictory with (4). Thus . Namely, there exists some such that
Hence,
(5) 
Obviously, the degree of is . Assume that After comparing the coefficients of Eq. (5), we have
(.1)  
(.2)  
(.3)  
(.4)  
(.5)  
(.6)  
(.7)  
(.8)  
(.9)  
(.10)  
(.11)  
(.12)  
(.13) 
In the above equation system, denotes that the equation in the same row is from comparing the coefficient of degree . From (.1)(.7), we obtain , and . Together with (.13), we have
Then it follows from (.6) that , and from (.10), we have . Thus in the following, we suffice to consider the case . When , from (.13), (.11) and , we have , and thus or .
When , and Eq. (2) becomes
(7) 
where . Let Then if is to, must be irreducible from Lemma 2.6. However, it is clear that there exist some such that has or solutions in , which means can not be irreducible. Thus is not to in the case.
When , Eq. (1) becomes
(8) 
Let . Then from Eq. (8), we get , having or solutions in , which is contrary with that Eq. (1) has only one solution in for any .
Therefore when , is not to over , which completes the proof. ∎
Remark 3.2.
As for the case , there exist some to mappings with the form of where . We obtain them by MAGMA and list them in Table I, where is a primitive element in .
No.  No.  No.  

,  , 
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