Linearized polynomials over finite fields revisited∗ Baofeng Wu,† Zhuojun Liu‡ Abstract We give new characterizations of the algebra Ln(Fqn ) formed by all linearized polynomials over the finite field Fqn after briefly sur- veying some known ones. One isomorphism we construct is between ∨ L F n F F n n( q ) and the composition algebra qn ⊗Fq q . The other iso- morphism we construct is between Ln(Fqn ) and the so-called Dickson matrix algebra Dn(Fqn ). We also further study the relations between a linearized polynomial and its associated Dickson matrix, generalizing a well-known criterion of Dickson on linearized permutation polyno- mials. Adjugate polynomial of a linearized polynomial is then intro- duced, and connections between them are discussed. Both of the new characterizations can bring us more simple approaches to establish a special form of representations of linearized polynomials proposed re- cently by several authors. Structure of the subalgebra Ln(Fqm ) which are formed by all linearized polynomials over a subfield Fqm of Fqn where m|n are also described. Keywords Linearized polynomial; Composition algebra; Dickson arXiv:1211.5475v2 [math.RA] 1 Jan 2013 matrix algebra; Representation. ∗Partially supported by National Basic Research Program of China (2011CB302400). †Key Laboratory of Mathematics Mechanization, AMSS, Chinese Academy of Sciences, Beijing 100190, China. Email:
[email protected] ‡Key Laboratory of Mathematics Mechanization, AMSS, Chinese Academy of Sciences, Beijing 100190, China. Email:
[email protected] 1 1 Introduction n Let Fq and Fqn be the finite fields with q and q elements respectively, where q is a prime or a prime power.