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Difference between revisions of "Proof of the Existence of Primitive Roots"

(Created page with "This page is dedicated to proving the existence of primitive roots for certain integers. ==Statement== Let <math>n</math> be a positive integer. Then <math>n</math> have a p...")
 
(→‎Proof)
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==Proof==
 
==Proof==
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We start case by case, starting with the easiest case:
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===Case 1===
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In this case, we have <math>n=p</math>, where <math>p</math> is prime.
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We wish to show that <math>n</math> have a primitive root.
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To do so, we first need a few lemmas regarding the solutions of [[polynomial congruences]].
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'''Lemma 1 (Lagrange's Theorem):'''
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Let <math>p</math> be a prime and let
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<cmath>f(x)= \sum _{i=0}^{n} (a_i \cdot x^i)</cmath>
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be a polynomial with degree <math>n</math> (where <math>n</math> is an integer), with integer coeficients and leading coeficient <math>a_n</math> not divisible by <math>p</math>.
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Then <math>f(x)</math> have at most <math>n</math> incongruent roots modulo <math>p</math>.
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Note that this is a number theory analogue of the fundamental theorem of algebra.
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Note also that this theorem only holds for primes. For example, when <math>f(x)=x^2-x</math>, there is <math>4</math> incongruent roots (as the reader should verify) modulo <math>6</math>, but this does not contradicts Lagrange's theorem since <math>6</math> is composite.
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''Proof:'' We proceed by mathematical induction. When <math>n=1</math>, the statement is obvious; it merely states that a linear congruence have at most one solution modulo <math>p</math>, which is true since <math>\gcd (a_1,p)=1</math>.
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Now, suppose the theorem holds for a (n-1)th degree polynomial. Let <math>f(x)</math> be a nth degree polynomial with leading coeficient relatively prime to <math>p</math>. Then assume it have <math>n+1</math> incongruent roots modulo <math>p</math>, say <math>c_1, c_2, \dots , c_{n+1}</math>. Then,
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<cmath>f(x) - f(c_1) = \sum _{i=0}^{n} a_i \cdot (x^i- {c_1}^i)</cmath>
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<cmath>=\sum _{i=0}^{n} a_i \cdot (x-c_1) \cdot (\sum _{j=0}^{i-1} x^j \cdot {c_1}^{i-1-j})</cmath>
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<cmath>=(x-c_1) \cdot g(x)</cmath>
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for a (n-1)th degree polynomial <math>g(x)</math> with leading coeficient <math>a_{n}</math>. Then <math>g(x)</math> satisfy the conditions of the inductive hypothesis.
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Now, let <math>k</math> be an arbitary integer with <math>1 \le k \le n+1</math>. <math>f(c_k) \equiv 0 \equiv f(c_i) \pmod {p}</math>. Thus,
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<cmath>0 \equiv f(c_k) - f(c_1) = (c_k-c_1) g(c_k) \pmod {p}</cmath>
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<cmath>\implies g(c_k) \equiv 0 \pmod {p}</cmath>
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since
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<cmath>c_k \not\equiv c_1 \pmod {p}</cmath>
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Thus <math>c_2 , c_2 , \dots , c_{n+1}</math> are all roots of <math>g(x)</math>. This contradicts the inductive hypothesis, and thus establishes the lemma. <math>\square</math>.
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Now, we have a useful corollary:
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'''Corollary (Lemma 2):'''
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Let <math>p</math> be prime and let <math>d</math> be a divisor of <math>p-1</math>. Then the polynomial <math>x^d -1</math> have exactly <math>d</math> incongruent roots modulo <math>p</math>.
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''Proof:'' Let <math>p-1 =d \cdot e</math>. We have
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<cmath>x^{p-1} -1= (x^d-1) g(x)</cmath>
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for a <math>p-1-d</math> degree polynomial <math>g(x)</math>. By Fermat's Little Theorem, <math>x^{p-1}-1</math> have exactly <math>p-1</math> roots modulo <math>p</math>. By Lagrange's Theorem, <math>g(x)</math> have at most <math>p-d-1</math> incongruent roots modulo <math>p</math>. Thus, <math>x^d -1</math> have at least <math>d</math> roots modulo <math>p</math>. On the other hand, Lagrange's Theorem again implies <math>x^d -1</math> have at ''most'' <math>d</math> incongruent roots. Thus, <math>x^d -1</math> have exactly <math>d</math> roots. <math>\square</math>
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This result can be used to prove a result regarding the number of integer of a certain integer a prime <math>p</math> can have. Before we state and prove that, we present yet another lemma.
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'''Lemma 3:'''
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Let <math>p</math> be prime and let <math>d</math> be a divisor of <math>p-1</math>. Then the number of positive integers less than <math>p</math> of order <math>d</math> modulo <math>p</math> do not exceed <math>\phi (d)</math>.
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''Proof:'' Let <math>F(d)</math> denote the number of positive integers less than <math>p</math> of order <math>d</math> modulo <math>p</math>.
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Now, if <math>F(d)=0</math>, the result is trivial. Assume <math>F(d) \ne 0</math>, and that <math>a</math> is an integer with order <math>d</math> modulo <math>p</math>. Then the integers
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<cmath>a, a^2, \dots , a^d</cmath>
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are incongruent modulo <math>p</math>. Also, each of those integers are a root of <math>x^d-1</math>. By Lemma 3, those are the only roots. Thus, every possible integer with order <math>d</math> modulo <math>p</math> must be a power of <math>a</math>. Obviously, the exponent of <math>a</math> must be relatively prime to <math>d</math>, or otherwise there exist a smaller solution to <math>(a^k)^x \equiv 1 \pmod {p}</math> (for <math>x</math>) than <math>d</math>, which would cause <math>\text {ord} _d a^k</math> to be less than <math>d</math>. Therefore, the result follow by the definition of the Euler-Phi Function. <math>\square</math>.
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In fact, the number of positive integers less than <math>p</math> of order <math>d</math> modulo <math>p</math> is ''exactly'' <math>\phi (d)</math>, as the following lemma shows:
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'''Lemma 4:'''
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Let <math>p</math> be prime and let <math>d</math> be a divisor of <math>p-1</math>. Then the number of positive integers less than <math>p</math> of order <math>d</math> modulo <math>p</math> is exactly <math>\phi (d)</math>.
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''Proof:'' As the page [[Proof of Some Primitive Roots Facts]] proved, the order of an integer modulo <math>p</math> divides <math>p-1</math>, we have
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<cmath>\sum _{d|p-1} F(d) = p-1</cmath>
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By some Euler-Phi Function facts, we know that
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<cmath>\sum _{d|p-1} \phi (d) = p-1</cmath>
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Lemma 3 implies <math>F(d) \le \phi (d)</math> for each <math>d|p-1</math>, so it follows that <math>F(d)= \phi (d)</math> for every <math>d</math>. <math>\square</math>.
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An easy corollary of that is when we take <math>d=p-1</math>, and we get that every prime have <math>\phi (p-1)</math> primitive roots. We have thus shown that every prime have a primitive roots.
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Case 1 is completed.
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===Case 2===
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UNDER CONSTRUCTION

Revision as of 22:34, 19 January 2025

This page is dedicated to proving the existence of primitive roots for certain integers.

Statement

Let $n$ be a positive integer. Then $n$ have a primitive root if and only if

\[n=2,4,p^t, \text{or} , 2p^t\]

Proof

We start case by case, starting with the easiest case:

Case 1

In this case, we have $n=p$, where $p$ is prime.

We wish to show that $n$ have a primitive root.

To do so, we first need a few lemmas regarding the solutions of polynomial congruences.

Lemma 1 (Lagrange's Theorem):

Let $p$ be a prime and let

\[f(x)= \sum _{i=0}^{n} (a_i \cdot x^i)\]

be a polynomial with degree $n$ (where $n$ is an integer), with integer coeficients and leading coeficient $a_n$ not divisible by $p$.

Then $f(x)$ have at most $n$ incongruent roots modulo $p$.


Note that this is a number theory analogue of the fundamental theorem of algebra.

Note also that this theorem only holds for primes. For example, when $f(x)=x^2-x$, there is $4$ incongruent roots (as the reader should verify) modulo $6$, but this does not contradicts Lagrange's theorem since $6$ is composite.


Proof: We proceed by mathematical induction. When $n=1$, the statement is obvious; it merely states that a linear congruence have at most one solution modulo $p$, which is true since $\gcd (a_1,p)=1$.

Now, suppose the theorem holds for a (n-1)th degree polynomial. Let $f(x)$ be a nth degree polynomial with leading coeficient relatively prime to $p$. Then assume it have $n+1$ incongruent roots modulo $p$, say $c_1, c_2, \dots , c_{n+1}$. Then,

\[f(x) - f(c_1) = \sum _{i=0}^{n} a_i \cdot (x^i- {c_1}^i)\]

\[=\sum _{i=0}^{n} a_i \cdot (x-c_1) \cdot (\sum _{j=0}^{i-1} x^j \cdot {c_1}^{i-1-j})\]

\[=(x-c_1) \cdot g(x)\]

for a (n-1)th degree polynomial $g(x)$ with leading coeficient $a_{n}$. Then $g(x)$ satisfy the conditions of the inductive hypothesis.

Now, let $k$ be an arbitary integer with $1 \le k \le n+1$. $f(c_k) \equiv 0 \equiv f(c_i) \pmod {p}$. Thus,

\[0 \equiv f(c_k) - f(c_1) = (c_k-c_1) g(c_k) \pmod {p}\]

\[\implies g(c_k) \equiv 0 \pmod {p}\]

since

\[c_k \not\equiv c_1 \pmod {p}\]

Thus $c_2 , c_2 , \dots , c_{n+1}$ are all roots of $g(x)$. This contradicts the inductive hypothesis, and thus establishes the lemma. $\square$.


Now, we have a useful corollary:


Corollary (Lemma 2):

Let $p$ be prime and let $d$ be a divisor of $p-1$. Then the polynomial $x^d -1$ have exactly $d$ incongruent roots modulo $p$.


Proof: Let $p-1 =d \cdot e$. We have

\[x^{p-1} -1= (x^d-1) g(x)\]

for a $p-1-d$ degree polynomial $g(x)$. By Fermat's Little Theorem, $x^{p-1}-1$ have exactly $p-1$ roots modulo $p$. By Lagrange's Theorem, $g(x)$ have at most $p-d-1$ incongruent roots modulo $p$. Thus, $x^d -1$ have at least $d$ roots modulo $p$. On the other hand, Lagrange's Theorem again implies $x^d -1$ have at most $d$ incongruent roots. Thus, $x^d -1$ have exactly $d$ roots. $\square$


This result can be used to prove a result regarding the number of integer of a certain integer a prime $p$ can have. Before we state and prove that, we present yet another lemma.


Lemma 3:

Let $p$ be prime and let $d$ be a divisor of $p-1$. Then the number of positive integers less than $p$ of order $d$ modulo $p$ do not exceed $\phi (d)$.


Proof: Let $F(d)$ denote the number of positive integers less than $p$ of order $d$ modulo $p$.

Now, if $F(d)=0$, the result is trivial. Assume $F(d) \ne 0$, and that $a$ is an integer with order $d$ modulo $p$. Then the integers

\[a, a^2, \dots , a^d\]

are incongruent modulo $p$. Also, each of those integers are a root of $x^d-1$. By Lemma 3, those are the only roots. Thus, every possible integer with order $d$ modulo $p$ must be a power of $a$. Obviously, the exponent of $a$ must be relatively prime to $d$, or otherwise there exist a smaller solution to $(a^k)^x \equiv 1 \pmod {p}$ (for $x$) than $d$, which would cause $\text {ord} _d a^k$ to be less than $d$. Therefore, the result follow by the definition of the Euler-Phi Function. $\square$.


In fact, the number of positive integers less than $p$ of order $d$ modulo $p$ is exactly $\phi (d)$, as the following lemma shows:


Lemma 4:

Let $p$ be prime and let $d$ be a divisor of $p-1$. Then the number of positive integers less than $p$ of order $d$ modulo $p$ is exactly $\phi (d)$.


Proof: As the page Proof of Some Primitive Roots Facts proved, the order of an integer modulo $p$ divides $p-1$, we have

\[\sum _{d|p-1} F(d) = p-1\]

By some Euler-Phi Function facts, we know that

\[\sum _{d|p-1} \phi (d) = p-1\]

Lemma 3 implies $F(d) \le \phi (d)$ for each $d|p-1$, so it follows that $F(d)= \phi (d)$ for every $d$. $\square$.


An easy corollary of that is when we take $d=p-1$, and we get that every prime have $\phi (p-1)$ primitive roots. We have thus shown that every prime have a primitive roots.

Case 1 is completed.

Case 2

UNDER CONSTRUCTION

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