Question

Let $\theta$ be a root of $x^4+x^3+1$ in $G F_{16}$. Construct the minimal polynomial of $\theta^3+\theta^2$.

   Let $\theta$ be a root of $x^4+x^3+1$ in $G F_{16}$. Construct the minimal polynomial of $\theta^3+\theta^2$.
Applied Algebra: Codes, Ciphers and Discrete Algorithms
Applied Algebra: Codes, Ciphers and Discrete Algorithms
Darel W. Hardy, Fred… 2nd Edition
Chapter 10, Problem 7 ↓

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Since $GF(16)$ has characteristic 2, the field contains 16 elements and can be represented as $\mathbb{F}_2[\alpha]$ where $\alpha$ is a root of an irreducible polynomial of degree 4 over $\mathbb{F}_2$.  Show more…

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Let $\theta$ be a root of $x^4+x^3+1$ in $G F_{16}$. Construct the minimal polynomial of $\theta^3+\theta^2$.
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Key Concepts

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Finite Fields
Finite fields, also known as Galois fields, are algebraic structures with a finite number of elements where both addition and multiplication (excluding zero) are well-defined and satisfy the field axioms. They are commonly denoted GF(q), where q is a prime power. In this context, GF(16) is a finite field with 16 elements, and understanding its structure is key to operations such as element representation and arithmetic computation.
Field Extensions
Field extensions occur when a larger field is constructed from a smaller one by adjoining new elements that are roots of polynomials that have no roots in the smaller field. GF(16) can be seen as an extension of GF(2), typically constructed using an irreducible polynomial of degree 4. Recognizing and working within these extensions is central to understanding the algebraic relationships between different elements in the field.
Minimal Polynomial
The minimal polynomial of an element in a field extension is the monic polynomial of lowest degree with coefficients in the base field for which the element is a root. Determining the minimal polynomial is crucial because it encapsulates the algebraic dependencies of that element over the base field, and it also plays an important role in computations involving field elements and solving equations within the field.
Irreducible Polynomials
Irreducible polynomials over a finite field are the building blocks of field extensions. An irreducible polynomial cannot be factored into polynomials of lower degree with coefficients in the base field. In constructing GF(16) and determining minimal polynomials of its elements, recognizing and using irreducible polynomials ensures that the extension field has the correct structure and the minimal polynomial accurately reflects the element’s properties.
Frobenius Automorphism
The Frobenius automorphism is a crucial tool in the study of finite fields. It maps each element to its q-th power (where q is the size of the base field) and is an automorphism of any finite field. In determining the minimal polynomial of an element, this automorphism is used to generate all the distinct conjugates of the element, which are then used to construct the minimal polynomial as the product of linear factors corresponding to each conjugate.
Conjugate Elements
Conjugate elements in a finite field extension are the images of an element under the various automorphisms of the field (usually under iterations of the Frobenius automorphism). These conjugates are the roots of the element’s minimal polynomial. Understanding conjugates is essential for constructing minimal polynomials because the set of all distinct conjugates directly determines the polynomial’s degree and its factorization over the base field.

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