Showing the lemma $operatorname{ord}_p(1+ζ_p)=0$ if $p>2$
$begingroup$
Just to give some background regarding my motivation, I'm trying to prove a lemma to help me solve How do we prove p-order of $g_k$ is $frac {k} {p-1}$?
Let $Z_p$ denote the p-adic integers, and let us adjoin a primitive p-th root of unity $ζ_p$. Assume $p>2DeclareMathOperator{ord}{ord}$.
I want to show that $ord_p(1-ζ_p)=ord_p(1-ζ_p^2)$, but I see that it is enough to prove $ord_p( frac {1-ζ_p^2} {1-ζ_p})=0$ or, equivalently, $ord_p(1+ζ_p)=0$.
I have tried applying the properties from here. (Even though they are for rational numbers, I'm going to assume for now that they hold in $Z_p(ζ_p)$ also. If not, please correct me.)
I see that, if $ord_p(ζ_p) neq ord_p(1)=0$, we will have $ord_p(1+ζ_p)=inf {ord_p(1), ord_p(ζ_p)} leq 0$ and we will get a contradiction if $<0$
.
But if instead $ord_p(ζ_p)=0,$ then I don't know what to do. We will have $ord_p(1+ζ_p) geq inf {ord_p(1), ord_p(ζ_p)}=0$, but I don't see how $>0$ would give us a contradiction.
abstract-algebra field-theory prime-numbers p-adic-number-theory roots-of-unity
$endgroup$
add a comment |
$begingroup$
Just to give some background regarding my motivation, I'm trying to prove a lemma to help me solve How do we prove p-order of $g_k$ is $frac {k} {p-1}$?
Let $Z_p$ denote the p-adic integers, and let us adjoin a primitive p-th root of unity $ζ_p$. Assume $p>2DeclareMathOperator{ord}{ord}$.
I want to show that $ord_p(1-ζ_p)=ord_p(1-ζ_p^2)$, but I see that it is enough to prove $ord_p( frac {1-ζ_p^2} {1-ζ_p})=0$ or, equivalently, $ord_p(1+ζ_p)=0$.
I have tried applying the properties from here. (Even though they are for rational numbers, I'm going to assume for now that they hold in $Z_p(ζ_p)$ also. If not, please correct me.)
I see that, if $ord_p(ζ_p) neq ord_p(1)=0$, we will have $ord_p(1+ζ_p)=inf {ord_p(1), ord_p(ζ_p)} leq 0$ and we will get a contradiction if $<0$
.
But if instead $ord_p(ζ_p)=0,$ then I don't know what to do. We will have $ord_p(1+ζ_p) geq inf {ord_p(1), ord_p(ζ_p)}=0$, but I don't see how $>0$ would give us a contradiction.
abstract-algebra field-theory prime-numbers p-adic-number-theory roots-of-unity
$endgroup$
1
$begingroup$
That $|zeta_p|_p = 1$ is obvious from $zeta_p^p = 1$
$endgroup$
– reuns
Dec 7 '18 at 20:15
add a comment |
$begingroup$
Just to give some background regarding my motivation, I'm trying to prove a lemma to help me solve How do we prove p-order of $g_k$ is $frac {k} {p-1}$?
Let $Z_p$ denote the p-adic integers, and let us adjoin a primitive p-th root of unity $ζ_p$. Assume $p>2DeclareMathOperator{ord}{ord}$.
I want to show that $ord_p(1-ζ_p)=ord_p(1-ζ_p^2)$, but I see that it is enough to prove $ord_p( frac {1-ζ_p^2} {1-ζ_p})=0$ or, equivalently, $ord_p(1+ζ_p)=0$.
I have tried applying the properties from here. (Even though they are for rational numbers, I'm going to assume for now that they hold in $Z_p(ζ_p)$ also. If not, please correct me.)
I see that, if $ord_p(ζ_p) neq ord_p(1)=0$, we will have $ord_p(1+ζ_p)=inf {ord_p(1), ord_p(ζ_p)} leq 0$ and we will get a contradiction if $<0$
.
But if instead $ord_p(ζ_p)=0,$ then I don't know what to do. We will have $ord_p(1+ζ_p) geq inf {ord_p(1), ord_p(ζ_p)}=0$, but I don't see how $>0$ would give us a contradiction.
abstract-algebra field-theory prime-numbers p-adic-number-theory roots-of-unity
$endgroup$
Just to give some background regarding my motivation, I'm trying to prove a lemma to help me solve How do we prove p-order of $g_k$ is $frac {k} {p-1}$?
Let $Z_p$ denote the p-adic integers, and let us adjoin a primitive p-th root of unity $ζ_p$. Assume $p>2DeclareMathOperator{ord}{ord}$.
I want to show that $ord_p(1-ζ_p)=ord_p(1-ζ_p^2)$, but I see that it is enough to prove $ord_p( frac {1-ζ_p^2} {1-ζ_p})=0$ or, equivalently, $ord_p(1+ζ_p)=0$.
I have tried applying the properties from here. (Even though they are for rational numbers, I'm going to assume for now that they hold in $Z_p(ζ_p)$ also. If not, please correct me.)
I see that, if $ord_p(ζ_p) neq ord_p(1)=0$, we will have $ord_p(1+ζ_p)=inf {ord_p(1), ord_p(ζ_p)} leq 0$ and we will get a contradiction if $<0$
.
But if instead $ord_p(ζ_p)=0,$ then I don't know what to do. We will have $ord_p(1+ζ_p) geq inf {ord_p(1), ord_p(ζ_p)}=0$, but I don't see how $>0$ would give us a contradiction.
abstract-algebra field-theory prime-numbers p-adic-number-theory roots-of-unity
abstract-algebra field-theory prime-numbers p-adic-number-theory roots-of-unity
edited Dec 7 '18 at 19:41
Bernard
119k740113
119k740113
asked Dec 7 '18 at 19:28
Pascal's WagerPascal's Wager
357315
357315
1
$begingroup$
That $|zeta_p|_p = 1$ is obvious from $zeta_p^p = 1$
$endgroup$
– reuns
Dec 7 '18 at 20:15
add a comment |
1
$begingroup$
That $|zeta_p|_p = 1$ is obvious from $zeta_p^p = 1$
$endgroup$
– reuns
Dec 7 '18 at 20:15
1
1
$begingroup$
That $|zeta_p|_p = 1$ is obvious from $zeta_p^p = 1$
$endgroup$
– reuns
Dec 7 '18 at 20:15
$begingroup$
That $|zeta_p|_p = 1$ is obvious from $zeta_p^p = 1$
$endgroup$
– reuns
Dec 7 '18 at 20:15
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
Let $$(x-1)^p-1 = f(x)g(x) in mathbf{Z}_p[x]$$ where $f(x)$ is the minimal polynomial of $1+zeta_p$ so $$f(x) = prod_{alpha in Gal(overline{mathbf{Q}_p}/mathbf{Q}_p)cdot (1+zeta_p)} (x-alpha)$$
by definition $|.|_p$ is $Gal(overline{mathbf{Q}_p}/mathbf{Q}_p)$ invariant and$$|1+zeta_p|_p=|f(0)|_p^{1/deg(f)}$$
and since $g(0) in mathbf{Z}_p$
$$1 ge |f(0)|_p ge |f(0)g(0)|_p = |(-1)^p-1|_p = 1$$
whence $|f(0)|_p= 1$ and $|1+zeta_p|_p = 1$.
$endgroup$
add a comment |
$begingroup$
Note that $frac {ζ^2-1} {ζ-1}=1+ζ$ and $frac {ζ-1} {ζ^2-1} =1+ζ^2+ζ^4+...+ζ^{p-1}$.
In the case that $ord_p(ζ)=0$, we will have $ord_p(ζ^2)=ord_p(ζ^3)=...=0$.
This would imply $ord_p(ζ^2-1)-οrd_p(ζ-1)=ord_p(1+ζ) geq inf {ord_p(1),ord_p(ζ)}=0 $ and also $οrd_p(ζ-1)-ord_p(ζ^2-1)=ord_p(1+ζ^2+ζ^4+...+ζ^{p-1})geq inf{ord_p(1),...,ord_p(ζ^{p-1})}=0$.
From these two inequalities, it should be easy to see $ord_p(ζ^2-1)=οrd_p(ζ-1)$ as needed.
$endgroup$
add a comment |
$begingroup$
Here’s yet another argument:
Start with the minimal polynomial for $zeta_p$ ,
begin{align}
text{Irr}(zeta_p,Bbb Q_p)&=frac{X^p-1}{X-1}\
f=text{Irr}(zeta_p-1,Bbb Q_p)&=frac{(X+1)^p-1}X&text{($p$-Eisenstein, root $pi$)}\
v_p(pi)=v_p(zeta_p-1)&=frac1{p-1}\
v_p(pi+2)=v_p(zeta_p+1)&=v_p(2)=0&text{(’cause $pne2$)},.
end{align}
$endgroup$
add a comment |
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Let $$(x-1)^p-1 = f(x)g(x) in mathbf{Z}_p[x]$$ where $f(x)$ is the minimal polynomial of $1+zeta_p$ so $$f(x) = prod_{alpha in Gal(overline{mathbf{Q}_p}/mathbf{Q}_p)cdot (1+zeta_p)} (x-alpha)$$
by definition $|.|_p$ is $Gal(overline{mathbf{Q}_p}/mathbf{Q}_p)$ invariant and$$|1+zeta_p|_p=|f(0)|_p^{1/deg(f)}$$
and since $g(0) in mathbf{Z}_p$
$$1 ge |f(0)|_p ge |f(0)g(0)|_p = |(-1)^p-1|_p = 1$$
whence $|f(0)|_p= 1$ and $|1+zeta_p|_p = 1$.
$endgroup$
add a comment |
$begingroup$
Let $$(x-1)^p-1 = f(x)g(x) in mathbf{Z}_p[x]$$ where $f(x)$ is the minimal polynomial of $1+zeta_p$ so $$f(x) = prod_{alpha in Gal(overline{mathbf{Q}_p}/mathbf{Q}_p)cdot (1+zeta_p)} (x-alpha)$$
by definition $|.|_p$ is $Gal(overline{mathbf{Q}_p}/mathbf{Q}_p)$ invariant and$$|1+zeta_p|_p=|f(0)|_p^{1/deg(f)}$$
and since $g(0) in mathbf{Z}_p$
$$1 ge |f(0)|_p ge |f(0)g(0)|_p = |(-1)^p-1|_p = 1$$
whence $|f(0)|_p= 1$ and $|1+zeta_p|_p = 1$.
$endgroup$
add a comment |
$begingroup$
Let $$(x-1)^p-1 = f(x)g(x) in mathbf{Z}_p[x]$$ where $f(x)$ is the minimal polynomial of $1+zeta_p$ so $$f(x) = prod_{alpha in Gal(overline{mathbf{Q}_p}/mathbf{Q}_p)cdot (1+zeta_p)} (x-alpha)$$
by definition $|.|_p$ is $Gal(overline{mathbf{Q}_p}/mathbf{Q}_p)$ invariant and$$|1+zeta_p|_p=|f(0)|_p^{1/deg(f)}$$
and since $g(0) in mathbf{Z}_p$
$$1 ge |f(0)|_p ge |f(0)g(0)|_p = |(-1)^p-1|_p = 1$$
whence $|f(0)|_p= 1$ and $|1+zeta_p|_p = 1$.
$endgroup$
Let $$(x-1)^p-1 = f(x)g(x) in mathbf{Z}_p[x]$$ where $f(x)$ is the minimal polynomial of $1+zeta_p$ so $$f(x) = prod_{alpha in Gal(overline{mathbf{Q}_p}/mathbf{Q}_p)cdot (1+zeta_p)} (x-alpha)$$
by definition $|.|_p$ is $Gal(overline{mathbf{Q}_p}/mathbf{Q}_p)$ invariant and$$|1+zeta_p|_p=|f(0)|_p^{1/deg(f)}$$
and since $g(0) in mathbf{Z}_p$
$$1 ge |f(0)|_p ge |f(0)g(0)|_p = |(-1)^p-1|_p = 1$$
whence $|f(0)|_p= 1$ and $|1+zeta_p|_p = 1$.
edited Dec 7 '18 at 20:16
answered Dec 7 '18 at 20:10
reunsreuns
19.9k21148
19.9k21148
add a comment |
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$begingroup$
Note that $frac {ζ^2-1} {ζ-1}=1+ζ$ and $frac {ζ-1} {ζ^2-1} =1+ζ^2+ζ^4+...+ζ^{p-1}$.
In the case that $ord_p(ζ)=0$, we will have $ord_p(ζ^2)=ord_p(ζ^3)=...=0$.
This would imply $ord_p(ζ^2-1)-οrd_p(ζ-1)=ord_p(1+ζ) geq inf {ord_p(1),ord_p(ζ)}=0 $ and also $οrd_p(ζ-1)-ord_p(ζ^2-1)=ord_p(1+ζ^2+ζ^4+...+ζ^{p-1})geq inf{ord_p(1),...,ord_p(ζ^{p-1})}=0$.
From these two inequalities, it should be easy to see $ord_p(ζ^2-1)=οrd_p(ζ-1)$ as needed.
$endgroup$
add a comment |
$begingroup$
Note that $frac {ζ^2-1} {ζ-1}=1+ζ$ and $frac {ζ-1} {ζ^2-1} =1+ζ^2+ζ^4+...+ζ^{p-1}$.
In the case that $ord_p(ζ)=0$, we will have $ord_p(ζ^2)=ord_p(ζ^3)=...=0$.
This would imply $ord_p(ζ^2-1)-οrd_p(ζ-1)=ord_p(1+ζ) geq inf {ord_p(1),ord_p(ζ)}=0 $ and also $οrd_p(ζ-1)-ord_p(ζ^2-1)=ord_p(1+ζ^2+ζ^4+...+ζ^{p-1})geq inf{ord_p(1),...,ord_p(ζ^{p-1})}=0$.
From these two inequalities, it should be easy to see $ord_p(ζ^2-1)=οrd_p(ζ-1)$ as needed.
$endgroup$
add a comment |
$begingroup$
Note that $frac {ζ^2-1} {ζ-1}=1+ζ$ and $frac {ζ-1} {ζ^2-1} =1+ζ^2+ζ^4+...+ζ^{p-1}$.
In the case that $ord_p(ζ)=0$, we will have $ord_p(ζ^2)=ord_p(ζ^3)=...=0$.
This would imply $ord_p(ζ^2-1)-οrd_p(ζ-1)=ord_p(1+ζ) geq inf {ord_p(1),ord_p(ζ)}=0 $ and also $οrd_p(ζ-1)-ord_p(ζ^2-1)=ord_p(1+ζ^2+ζ^4+...+ζ^{p-1})geq inf{ord_p(1),...,ord_p(ζ^{p-1})}=0$.
From these two inequalities, it should be easy to see $ord_p(ζ^2-1)=οrd_p(ζ-1)$ as needed.
$endgroup$
Note that $frac {ζ^2-1} {ζ-1}=1+ζ$ and $frac {ζ-1} {ζ^2-1} =1+ζ^2+ζ^4+...+ζ^{p-1}$.
In the case that $ord_p(ζ)=0$, we will have $ord_p(ζ^2)=ord_p(ζ^3)=...=0$.
This would imply $ord_p(ζ^2-1)-οrd_p(ζ-1)=ord_p(1+ζ) geq inf {ord_p(1),ord_p(ζ)}=0 $ and also $οrd_p(ζ-1)-ord_p(ζ^2-1)=ord_p(1+ζ^2+ζ^4+...+ζ^{p-1})geq inf{ord_p(1),...,ord_p(ζ^{p-1})}=0$.
From these two inequalities, it should be easy to see $ord_p(ζ^2-1)=οrd_p(ζ-1)$ as needed.
answered Dec 7 '18 at 21:11
Pascal's WagerPascal's Wager
357315
357315
add a comment |
add a comment |
$begingroup$
Here’s yet another argument:
Start with the minimal polynomial for $zeta_p$ ,
begin{align}
text{Irr}(zeta_p,Bbb Q_p)&=frac{X^p-1}{X-1}\
f=text{Irr}(zeta_p-1,Bbb Q_p)&=frac{(X+1)^p-1}X&text{($p$-Eisenstein, root $pi$)}\
v_p(pi)=v_p(zeta_p-1)&=frac1{p-1}\
v_p(pi+2)=v_p(zeta_p+1)&=v_p(2)=0&text{(’cause $pne2$)},.
end{align}
$endgroup$
add a comment |
$begingroup$
Here’s yet another argument:
Start with the minimal polynomial for $zeta_p$ ,
begin{align}
text{Irr}(zeta_p,Bbb Q_p)&=frac{X^p-1}{X-1}\
f=text{Irr}(zeta_p-1,Bbb Q_p)&=frac{(X+1)^p-1}X&text{($p$-Eisenstein, root $pi$)}\
v_p(pi)=v_p(zeta_p-1)&=frac1{p-1}\
v_p(pi+2)=v_p(zeta_p+1)&=v_p(2)=0&text{(’cause $pne2$)},.
end{align}
$endgroup$
add a comment |
$begingroup$
Here’s yet another argument:
Start with the minimal polynomial for $zeta_p$ ,
begin{align}
text{Irr}(zeta_p,Bbb Q_p)&=frac{X^p-1}{X-1}\
f=text{Irr}(zeta_p-1,Bbb Q_p)&=frac{(X+1)^p-1}X&text{($p$-Eisenstein, root $pi$)}\
v_p(pi)=v_p(zeta_p-1)&=frac1{p-1}\
v_p(pi+2)=v_p(zeta_p+1)&=v_p(2)=0&text{(’cause $pne2$)},.
end{align}
$endgroup$
Here’s yet another argument:
Start with the minimal polynomial for $zeta_p$ ,
begin{align}
text{Irr}(zeta_p,Bbb Q_p)&=frac{X^p-1}{X-1}\
f=text{Irr}(zeta_p-1,Bbb Q_p)&=frac{(X+1)^p-1}X&text{($p$-Eisenstein, root $pi$)}\
v_p(pi)=v_p(zeta_p-1)&=frac1{p-1}\
v_p(pi+2)=v_p(zeta_p+1)&=v_p(2)=0&text{(’cause $pne2$)},.
end{align}
answered Dec 8 '18 at 5:45
LubinLubin
44.2k44585
44.2k44585
add a comment |
add a comment |
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$begingroup$
That $|zeta_p|_p = 1$ is obvious from $zeta_p^p = 1$
$endgroup$
– reuns
Dec 7 '18 at 20:15