University of Crete School of Sciences and Engineering Department of Mathematics and Applied Mathematics Doctoral Thesis Polynomials with Special Properties over Finite Fields Giorgos N. Kapetanakis Advisor: Theodoulos Garefalakis HERAKLION 2014 To my family Nikos, Maria and Eri and to the memory of my uncle Dimitris Committee PhD advisory committee • Theodoulos Garefalakis, Associate Professor, Department of Mathematics and Applied Mathematics, University of Crete, Greece (Advisor). • Aristides Kontogeorgis, Associate Professor, Department of Mathematics, Na- tional and Kapodistrian University of Athens, Greece. • Nikos Tzanakis, Professor, Department of Mathematics and Applied Mathemat- ics, University of Crete, Greece. Dissertation committee • Jannis Antoniadis, Professor, Department of Mathematics and Applied Mathe- matics, University of Crete, Greece. • Maria Chlouveraki, Assistant Professor, Laboratoire de Mathématiques, Uni- versité de Versailles - St. Quentin, France. • Dimitrios Dais, Associate Professor, Department of Mathematics and Applied Mathematics, University of Crete, Greece. • Dimitrios Poulakis, Professor, Department of Mathematics, Aristotle University of Thessaloniki, Greece. v vi Committee Acknowledgments First of all, I wish to thank my supervisor Prof. Theodoulos Garefalakis, not only for his support, patience, encouragement and the fruitful conversations we had, but also for constantly motivating me and introducing me to the world of finite fields and their wonderful international academic community, to which I am also grateful for many reasons. I also note that the work present in Chapter 3 of this thesis is joint with him. I also wish to thank Profs. Alexis Kouvidakis and Nikos Tzanakis from the Uni- versity of Crete and Profs. Evangelos Raptis and Dimitris Varsos from the University of Athens for doing their best in teaching me algebra. Additionally, I would like to thank the academic staff of the (former) Department of Mathematics of the University of Crete for sustaining a great productive academic environment, even in those diffi- cult times. Last but not least, I wish to thank Athanasios Triantis; it was my honor to learn from a so committed mathematician and teacher in my high-school years. I also wish to thank my parents Nikos and Maria and my sister Eri for their sup- port, encouragement and trust in my abilities and my beloved uncle Dimitris, who passed away in 2012, for making it impossible to count the reasons that I am grate- ful to him and for being one of the most wonderful persons I ever met. This thesis is naturally dedicated to my family and my uncle Dimitris. Additionally, I wish to thank my girlfriend, Popi, for making this journey far more interesting, challenging and pleasant. I also wish to thank all my friends, but especially Thanos Tsouanas, who mentioned me in both his MSc and PhD theses. Finally, I wish to thank Maria Michael Manassaki Foundation and the University of Crete’s Special Research Account (research grand No. 3744) for their support. Exclusively free (as in freedom) and open-source software was used, running on Linux machines. Non-trivial (and some trivial :-) ) calculations were performed with Sage . Typeset was done with XƎLATEX, using the Linux Libertine font . vii viii Acknowledgments Abstract In this work, we are interested in the existence of polynomials with special properties over finite fields. In Chapter 2 some background material is presented. We present some basic concepts of characters of finite abelian groups and we prove some basic results. Next, we focus on Dirichlet characters and on the characters of the additive and the multiplicative groups of a finite field. We conclude this chapter with anex- pression of the characteristic function of generators of cyclic R-modules, where R is a Euclidean domain, known as Vinogradov’s formula. In Chapter 3, we consider a special case of the Hansen-Mullen conjecture. In particular, we consider the existence of self-reciprocal monic irreducible polynomials of degree 2n over Fq, where q is odd, with some coefficient prescribed. First, we use Carlitz’s characterization of self-reciprocal polynomials over odd finite fields and, with the help of Dirichlet characters, we prove asymptotic conditions for the existence of polynomials with the desired properties. As a conclusion, we restrict ourselves to the first n=2 (hence also to the last n=2) coefficients, where our results are more efficient, and completely solve the resulting problem. In Chapter 4 we extend the primitive normal basis theorem and its strong version. Namely, we consider the existence of polynomials whose roots are simultaneously primitive, produce a normal basis and some given Möbius transformation of those roots also produce a normal basis. First, we characterize elements with the desired properties and with the help of characters, we end up with some sufficient condi- tions, which we furtherly relax using sieving techniques. In the end, we prove our desired results, with roughly the same exceptions as the ones appearing in the strong primitive normal basis theorem. In Chapter 5, we work in the same pattern as in Chapter 4, only here we demand that the Möbius transformation of the roots of the polynomial is also primitive. We roughly follow the same steps and prove that there exists a polynomial over a finite field such that its roots are simultaneously primitive and produce a normal basisand some given Möbius tranformation of its roots also possess both properties, given that the cardinality of the field and the degree of the polynomial are large enough. Keywords: finite field, primitive element, normal basis, free element, self-reciprocal polynomials, character sums, Hansen-Mullen conjecture 2010 MSC: 11T30, 12E05, 11T06, 11T24, 12E20, 12E10 ix x Abstract Contents Committee v Acknowledgments vii Abstract ix Contents 1 1 Introduction 3 1.1 The Hansen-Mullen conjecture ...................... 4 1.2 Extending the (strong) primitive normal basis theorem . 4 2 Background material 7 2.1 Characters and character sums ...................... 7 2.1.1 Dirichlet characters ........................ 8 2.1.2 Additive and multiplicative characters . 11 2.2 Vinogradov’s formula ........................... 13 3 The H-M conjecture for self-reciprocal irreducible polynomials 17 3.1 Preliminaries ................................ 17 3.2 Weighted sum ............................... 19 3.3 The restriction k ≤ n=2 .......................... 22 4 Extending the (strong) primitive normal basis theorem I 25 4.1 Some estimates .............................. 25 4.2 The sieve .................................. 29 4.3 The case m = 2 .............................. 31 4.4 Evaluations ................................. 32 4.5 Completion of the proof .......................... 39 5 Extending the (strong) primitive normal basis theorem II 45 5.1 Some estimates .............................. 45 5.1.1 Matrices that are neither upper triangular nor anti-diagonal . 46 1 2 Contents 5.1.2 Upper triangular matrices that are not diagonal . 48 5.1.3 Anti-diagonal matrices ...................... 50 5.1.4 Diagonal matrices ........................ 51 5.2 The sieve .................................. 52 5.3 Evaluations ................................. 55 A Computer input and output 61 A.1 Computations of Chapter 3 ........................ 61 A.2 Computations of Chapter 4 ........................ 63 A.3 Computations of Chapter 5 ........................ 75 Bibliography 85 Index 89 CHAPTER 1 Introduction In this thesis, some existence results for irreducible polynomials over finite fields, with special properties are shown. These properties include combinations of primitiveness, freeness (i.e. a root of the polynomial form a normal basis) and having some coeffi- cients prescribed. Also, since an irreducible polynomial over a finite field is fully characterized by its roots, we consider the roots of the polynomials, instead of the polynomials themselves, if such a replacement is convenient or natural. In particular, we study the Hansen-Mullen conjecture for self-reciprocal irreducible polynomials and we extend the primitive normal basis theorem and its strong version. Although the origins of the study of finite fields are found in antiquity, the formal study of finite fields has its roots in beginning of the 19th century and Gauss’ book Disquisitiones Arithmeticae [29]. The first though to extensively work on finite fields was, a few years later, Galois in Sur la théorie des nombres [22]. This work is a landmark to the subject and, consequently, many authors use the term Galois field to denote a finite field. The interested reader is referred to[48, Chapter 1] and the references therein for more detailed coverage of the history of the theory of finite fields. Throughout this thesis, Fq will stand for the finite field of q elements, Fqm for its extension of degree m, where m ≥ 1 and Fp as its prime field. It is well-known that p should be a prime number, also known as the characteristic of all the men- tioned finite fields, and q should power of p. The polynomial ring over a finite field, besides its great theoretical interest, also has numerous important applications, in- cluding efficient computation in finite fields, fast Fourier transform, coding theory and cryptography. The main idea behind our techniques dates back to the 50’s and the work ofCarlitz [2, 3], yet remains popular among authors in this line of research. Namely, first, we ex- press the characteristic or a characteristic-like