Showing the lemma $operatorname{ord}_p(1+ζ_p)=0$ if $p>2$












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.










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$endgroup$








  • 1




    $begingroup$
    That $|zeta_p|_p = 1$ is obvious from $zeta_p^p = 1$
    $endgroup$
    – reuns
    Dec 7 '18 at 20:15
















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.










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    That $|zeta_p|_p = 1$ is obvious from $zeta_p^p = 1$
    $endgroup$
    – reuns
    Dec 7 '18 at 20:15














2












2








2


1



$begingroup$


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.










share|cite|improve this question











$endgroup$




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






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edited Dec 7 '18 at 19:41









Bernard

119k740113




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asked Dec 7 '18 at 19:28









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357315








  • 1




    $begingroup$
    That $|zeta_p|_p = 1$ is obvious from $zeta_p^p = 1$
    $endgroup$
    – reuns
    Dec 7 '18 at 20:15














  • 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










3 Answers
3






active

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0












$begingroup$

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 $|1+zeta_p|_p = 1$.






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$endgroup$





















    0












    $begingroup$

    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.






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    $endgroup$





















      0












      $begingroup$

      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}






      share|cite|improve this answer









      $endgroup$













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        3 Answers
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        3 Answers
        3






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        0












        $begingroup$

        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 $|1+zeta_p|_p = 1$.






        share|cite|improve this answer











        $endgroup$


















          0












          $begingroup$

          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 $|1+zeta_p|_p = 1$.






          share|cite|improve this answer











          $endgroup$
















            0












            0








            0





            $begingroup$

            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 $|1+zeta_p|_p = 1$.






            share|cite|improve this answer











            $endgroup$



            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 $|1+zeta_p|_p = 1$.







            share|cite|improve this answer














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            share|cite|improve this answer








            edited Dec 7 '18 at 20:16

























            answered Dec 7 '18 at 20:10









            reunsreuns

            19.9k21148




            19.9k21148























                0












                $begingroup$

                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.






                share|cite|improve this answer









                $endgroup$


















                  0












                  $begingroup$

                  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.






                  share|cite|improve this answer









                  $endgroup$
















                    0












                    0








                    0





                    $begingroup$

                    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.






                    share|cite|improve this answer









                    $endgroup$



                    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.







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                    share|cite|improve this answer



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                    answered Dec 7 '18 at 21:11









                    Pascal's WagerPascal's Wager

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                        0












                        $begingroup$

                        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}






                        share|cite|improve this answer









                        $endgroup$


















                          0












                          $begingroup$

                          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}






                          share|cite|improve this answer









                          $endgroup$
















                            0












                            0








                            0





                            $begingroup$

                            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}






                            share|cite|improve this answer









                            $endgroup$



                            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}







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered Dec 8 '18 at 5:45









                            LubinLubin

                            44.2k44585




                            44.2k44585






























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