Finite fields are structures arising in abstract algebra. The order of a finite field is always a prime power, and for each prime power $q$ there is a single isomorphism type. It is usually denoted by $\mathbb{F}_q$ or $\operatorname{GF}(q)$.

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Proving the “freshman's dream”

$(x+y)^p = x^p + y^p$ holds in any field of characteristic $p$. However all the proofs I have seen use induction and some relatively nasty algebra despite how fundamental this fact seems. What is the ...
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Given $f=x^4+x+1 \in \mathbb Z_{2}[x]$ is primitive, write down an $m$-sequence ${a_n}$ associated to $f$

I'm not sure how to solve this question exactly. I know that the period will be 15 but I don't know how to construct the $m$-sequence.
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36 views

T is multliplication by a primitive element

Consider $\Bbb F_{p^n}$ as a vector space over $\Bbb F_{p}$. Now let $T$ be a non-zero $\Bbb F_{p}$-linear map from $\Bbb F_{p^n}$ to $\Bbb F_{p^n}$. Now if $0$ and $\Bbb F_{p^n}$ are the only ...
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1answer
36 views

About ring $( \frac{F[t]}{t^2})$

I was trying to study some properties of the group $GL_2 ( \frac{F[t]}{t^2})$ where $F$ is a field with $p$ elements. $p$ is a prime. I found that $ \frac{F[t]}{t^2}$ is a discrete Valuation ring. ...
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the Galois closure of $\mathbb{F}_4(x,y) / \mathbb{F}_4(x)$

I want to show that $\mathbb{F}_4(x,y) / \mathbb{F}_4(x)$ is NOT a Galois extension. Consider the field extension $\mathbb{F}_4(x,y) / \mathbb{F}_4(x)$ where $y$ is root of the polynomial $g(T) = ...
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about hyperplanes in finite fields

I was reading an article that said: if I have a finite field, say $\mathbb{F}_q^k$ where $q=p^n$ and $p$ is a prime; and a (k-2)-dimensional subspace, say $U\subset \mathbb{F}_q^k$ given by the span ...
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1answer
49 views

Rijndael S-Box algorithm: Can someone please explain how this code calculates the multiplicative inverse?

Wikipedia's explanation of the Rijndael Cipher's S-Box gives c code for calculating the S-Box. I've been able to calculate the S-Box values using exponent and log look-up tables to calculate the ...
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1answer
28 views

Isomorphisms between finite fields of same characteristic [duplicate]

Here I am taking the definition of isomorphism to be an injective homomorphism. Suppose we have two finite fields of the same characteristic, $\mathbb{F}_{p^n}$ and $\mathbb{F}_{p^m}$ with $m<n$. ...
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143 views
+100

Reduction modulo p of a linear group over the rational numbers

A paper (http://arxiv.org/pdf/1407.3158v2.pdf) contains the following theorem: Suppose $\mathbb{G}$ is a connected, simply connected, semisimple algebraic group defined over $\mathbb{Q}$, and let ...
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About Cayley graphs on finite fields

[I had yesterday asked a badly framed question about this. I would like to rewrite it again here.] Let $p$ be a prime number. (1) I take any set of $m$ vectors (say $S$) in $\mathbb{F}_p^d$ and ...
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23 views

“Consecutive” square residues in odd-order finite fields

Let $\mathbb F = \mathrm{GF}(p^r)$ be finite field for $p$ an odd prime, and define $$ \begin{align} Q &= \bigl\{ u^2 \,\big|\, u \in \mathbb F^\times \bigr\} \\ N &= \mathbb F^\times ...
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1answer
59 views

$\mathbb{F}_4(x,y) / \mathbb{F}_4(x)$ is a Galois extension, with $\mathbb{F}_4(x,y)$ a function field of an algebraic curve

Consider the field extension $\mathbb{F}_4(x,y) / \mathbb{F}_4(x)$ where $y$ is root of the polynomial $$ f(T)= x^4 + x^2T^2 + x^2T + x^2 + xT^2 + xT + T^4 + T^2 + 1 \in \mathbb{F}_4(x)[T]. $$ I ...
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+50

Differentiate polynomials in $\mathbb{Z}_2[x]$

It seems suggested that the differential of a polynomial in $\mathbb{Z}_2$ is as I would expect: $$\begin{align} &f = x^6 + x^3 + x + 1 \\ &f' = 6x^5 + 3x^2 +1 \mod 2 \\ &f'= x^2 + 1 \\ ...
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Subset of numbers analogous to primitive polynomials over finite fields

It is well known that many problems in number theory have an analogue on the ring of polynomials over finite fields and vice versa, the primes in $\mathbb{F}_q[x]$ being the irreducible polynomials. ...
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24 views

Prove that $F[x]/(f(x))$ has $q^n$ elements [duplicate]

let $F$ be a finite field of order $q$ and let $f(x)$ be a polynomial in $F[x]$ of degree $n \geq 1$. Prove that $F[x]/(f(x))$ has $q^n$ elements. I know that if a field is finite then the order is ...
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47 views

About Cayley graphs on finite fields.

If one is given $n$ vectors of length $n$ $\in \mathbb{F}_{p^k}^n$ for some prime number $p$ and $k \in \mathbb{Z}^+$ then how can one check if they are linearly independent? (the issue is if there ...
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1answer
28 views

Extensions of fields and dimension

The following is a homework question: Let M: N be a field extension, with a ∈ M algebraic over N. Show every element of N(a) is algebraic over N. Can anyone give me a strategy to approach this?
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Finite Fields (Factoring)

I am trying to factor the polynomial $x^3 + x^2 - x - 1$ in the quotient ring $\dfrac{\mathbb{F}_3[x]}{x^6 - 1}$ but I am having troubles finding a reduction. Something I have is $(x-1)^3 + x^2 - x = ...
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hyperplanes in finite vector spaces

how is an hyperplane in a finite vector space? I know a hyperplane is the kernel of a linear map, and the dimension of the hyperplane is n-1 if dimension of the vector space is n. So if I have, for ...
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75 views

Find a ring homomorphism $\tau: \mathbb{F} \rightarrow \mathbb{F}$

Just working on some exam prep questions, and I'm a bit stuck on this one. Let $ \mathbb{F} = \{ a + bX + cX^2 | a,b,c \in \mathbb{F}_2 = \{0,1\} \} $ be a ring with the operations: Addition, ...
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Algebra: Field Theory Question

I am being asked to find the following: Let $F$ denote the field $\dfrac{\mathbb{F}_2[\alpha]}{(\alpha^3 + \alpha + 1)}$. Simplify $\alpha(\alpha + 1)(\alpha + 1)$ in $F$ and calculate ...
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Generating elements of large group of units of a field.

Suppose $F$ is a large but finite field, and $F^\text{x}$ is the group of non-zero elements of the field. It is known that this group is always cyclic, and we could ask a natural question "what are ...
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Orthogonal matrices over GF(2^t) with the first column fixed

I'm interested in the existence of certain orthogonal transformations over $\mathbb F = \mathrm{GF}(2^t)$: matrices $M: \mathbb F^d \to \mathbb F^d$ for which $M^{\!\!\;\mathsf T\!\!\;} M = I_d$ over ...
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What is the best known lower bound for: $\max_{2\leq i\leq p-1}(ord_n(i)) $?

Given an in integer $n$, and let $p$ be its smallest prime divisor (you can assume that $p$ is very large ). Let $ord_n(i)$ denotes the order of $i$ as an element of $\Bbb Z_n^*$ the multiplicative ...
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40 views

Greatest common divisor of polynomial in Finite Field(256), AES

Have assigment and will use it as example, found solution computationaly, want to understand idea. It is about SubBytes procedure in AES, particulary about finding inverse of polynomial. Suppose we ...
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79 views

Equivalence of $\mathbb{F}_p$ and $\mathbb{Z}/p\mathbb{Z}$

I feel like I have some fundamental misunderstanding and I'm not really sure how to phrase this question, but here's a first attempt. In Child's Concrete Introduction to Higher Algebra (ISBN: ...
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Suppose that $a$ and $b$ belong to a field of order $8$ and that $a^2 + ab + b^2 =0$ then $a=0$ and $b=0$ . [duplicate]

Suppose that $a$ and $b$ belong to a field of order $8$ and $a^2 + ab + b^2 =0$. Then $a=0$ and $b=0$. Do the same when the field has order $2^n$ with $n$ odd? If one of the term is zero, i.e. ...
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I have to show this polynomial is irreducible. [closed]

Suppose that $p(x)=x^9+x^8+x^4+x^2+1 \in \mathbb{Z}_2[x]$. I have to show this polynomial is irreducible.
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Determine with proof the no of monic irreducible polynomials of prime degree $p$ over $F$,

Let $F$ be a field with $|F|=q$. Determine with proof the no of monic irreducible polynomials of prime degree $p$ over $F$, where $p$ need not be the characteristic of $F$. I know that $x^{p^n} -x$ ...
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Finding normal basis for GF(q^m) over GF(q)

Could you kindly explain, how can one find a normal basis for GF$(3^6)$ over the GF$(3^2)$? As I understood, I should start with finding the polynomial in a form $$a(x^2) + (a^9)x + a^{81},$$ which ...
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A question about the additive and multiplicative group of a finite field

Suppose $(F, +,.)$ is the finite field with $9$ elements. Let $G = (F,+)$ and $H=(F-\{0\},.)$ denote the underlying additive and multiplicative groups respectively. Then $G \cong \mathbb Z_3 \times ...
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Shortened Reed-Solomon proving p(D) is primitive

Assume we have a shortened $(n=18, k=12, t=3)$ Reed Solomon code in $GF(2^{8})$.Let $\alpha$ be a primitive element of $GF(2^{8})$. Consider the primitive polynomial given by: $p(D) = D^{8} + D^{4} + ...
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Why are the elements of GF(q), whose characteristic is 2, all squares? [duplicate]

If $a\in GF(2^n)$, then there is the element in $GF(2^n)$ such that $x^2 = a$. Why?
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Is there any definition of such semi-bilinear?

$K$ is a finite field which not equal to its base field $F_2$. Let $f: V \rightarrow K$ be a function and $B(x,y)=f(x+y)+f(x)+f(y)$ such that $B(x+y,z)=B(x,z)+B(y,z)$ and $B(z,x+y)=B(z,x)+B(z,y)$ for ...
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Trace over $\mathbf{F}_2$ of $1/(\alpha + \alpha^{-1})$ where $\alpha^{2^n+1} = 1$

Let $n$ be an integer $\ge 2$. Put $K = \mathbf{F}_{2^n}$ and $L = \mathbf{F}_{2^{2n}}$. Let $\alpha$ be an element of $L$ such that $\alpha^{2^n+1} = 1$ and $\alpha \ne 1$. Using Sage, I have noticed ...
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Why is $E[l]\cong\mathbb Z/l\mathbb Z\times\mathbb Z/l\mathbb Z$ for an elliptic curve $E$?

René Schoof's 1995 paper contains the following statement about an elliptic curve $E$ (at the bottom of page 233): [...], we use the subgroup $E[l]$ of $l$-torsion points of $E(\overline{\mathbb ...
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Counting total number of monic irreducible polynomials of all degrees $k$ that divide $m$.

Why is the following relation counting monic irreducible polynomials of all degrees $d$ that divide $m$ true? \begin{equation} \sum_{d\ |\ m}\left(\frac{1}{d} \sum_{c\ |\ d} \mu(d/c)\ p^{c}\right) = ...
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Explicitly construct a field with 729 elements

I would like to construct a field with 729 elements. I know that $729 = 3^6$, and that I have to find an irreducible monic polynomial of degree 6 over $GF(3)$. I chose the polynomial $x^6 + 2x^2 + 1$, ...
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What happens to symplectic basis if bilinearity condition is weak

Let $B:V\times V \rightarrow K$ be a (weak!) bilinear form where $K$ is a finite field with base field $F$ and $V$ a vector space over $K$. Let $u,v \in V$ and $\lambda \in F (!)$ $B(u + v, w) = B(u, ...
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Let $c_0, \cdots, c_{n-1}$ be elements of $\mathbb{F_{2^k}}$, find the sum $\sum\limits_{0\leq i < j <n}c_ic_j$

Let $c_0, \cdots, c_{n-1}$ be elements of $\mathbb{F_{2^k}}$, find the sum $\sum\limits_{0\leq i < j <n}c_ic_j$. It is true that if $k=1$ and $d$ be the number of non-zero elements, then ...
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Elementary questions about polynomials and field extensions

Let $$f(x)=x^2+x+1.$$ This is irreducible in $\mathbb{Z_2}[x]$, and thus $\mathbb{Z_2}[x]/(f(x))$ is a field $K$ where $(f(x))$ is a principle ideal. I don't quite understand how I find that ...
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Prove $(F,+)$ isomorphic to $(\mathbb{Z}_{p}, +) \times (\mathbb{Z}_{p}, +) \times … \times (\mathbb{Z}_{p},+)$ ( $n$ times)

Let $F$ be a field of order $p^n$ for some prime $p$ and positive integer $n$, and let $\mathbb{Z}_{p} \subset F$ be a prime field of $F$. Prove the additive group of $F$, that is the group ...
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139 views

Can we rewrite $Tr(ab^2)$ as $Tr(f(a)b)$?

Can we rewrite $Tr(ab^2)$ as $Tr(f(a)b)$ where $Tr:F_{2^{kn}}\rightarrow F_{2^{k}} $ is trace map, $k \neq 1$, $f$ is a function just depends to $a$.
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Linear equations - how to find the solution over the boolean field closest to zero

I want to solve a system of linear equations over the field of $F_2$, in a way such that the solution vector is as close to the zero vector as possible. For example, suppose I have a system of ...
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1answer
45 views

When is the Frobenius the identity?

If $f$ is an irreducible polynomial of degree $n$ over $\mathbb{F}_{p}$, then $\mathbb{F}_{p}[x]/(f)$ is the finite field $\mathbb{F}_{p^{n}}$ and the map $a \mapsto a^{p}$ is the Frobenius ...
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Polynomials in Finite Field

Given $\Bbb F_{32}=\Bbb F_2[X]/(X^5+X^2+1)$ where the polynomial is irreducible over $\Bbb F_2$, how would I compute $(a^4+a^2)*(a^3+a+1)$ given $a=[X]$ is the congruence class of [X]? Multiplying ...
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1answer
49 views

Does this imply that $F_1$ and $F_2$ are isomorphic as fields?

Suppose $F_1$ and $F_2$ be two finite fields such that additive groups of $F_1$ and $F_2$ are isomorphic and also multiplicative groups of $F_1$ and $F_2$ are isomorphic.Does this imply that $F_1$ ...
4
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35 views

On diagonizability of commutating matrices

Let $A$ and $B$ be $n\times n$ matrices over the Galois Field of order $p$ ($p$ is a prime). Suppose that $A$ and $B$ are diagonizable matrices and that they commutate. Is it possible to make them ...
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How to guarantee existence of a finite field [duplicate]

Existence of a finite field: Solution: I can understand that if I have a finite field $F$ of characteristic $p$ where $p$ is prime then I can consider $\mathbb Z_p$ as its prime field and hence $F$ ...
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1answer
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How to do multiplication in $GF(2^8)$?

I am taking an Internet Security Class and we received some practice problems and answers, but I do not know how to do these problems , an explanation would be greatly appreciated Try to compute the ...