Finding the Normal Basis of Cyclotomic field












2












$begingroup$


So let $p$ be a prime number and $zeta_p$ the p-th roots of unity. I want to proof that $ B = { zeta_p, zeta_{p}^{2}, dots, zeta_{p}^{p-1} }$ is the normal basis of $mathbb{Q}(zeta_p)/mathbb{Q}$ (which I hope is true ...).





First of all, I know that $B$ is a $mathbb{Q}$-basis of $mathbb{Q}(zeta_p)$ since




  1. the elements of $B$ are $mathbb{Q}$-linearly independent, because $zeta_p$ is a $p$-th primitive roots of unity,

  2. and $B$ has exactly $p-1$ elements and the degree of $mathbb{Q}(zeta_p)$ over $mathbb{Q}$ is $p-1$ too, because the Galois group of $mathbb{Q}(zeta_p)/mathbb{Q}$ is isomorphic to $(mathbb{Z}/pmathbb{Z})^times$ which has $p-1$ elements.


Now, I have trouble proving that $B$ is indeed a normal basis. According to the definition of a normal basis, if we let $sigma_i in text{Gal}(mathbb{Q}(zeta_p)/mathbb{Q})$, I would have to find an $a in mathbb{Q}(zeta_p)$ such that



$$
{sigma_1(a),sigma_2(a), dots, sigma_{p-1}(a)}
$$



forms a $mathbb{Q}$-basis of $mathbb{Q}(zeta_p)$.



Since $text{Gal}(mathbb{Q}(zeta_p)/mathbb{Q})$ is cyclic, there is a generating automorphism which I will just call $sigma$. Then, just reformulating the definition, we would have



$$
{sigma(a),sigma^{2}(a), dots, sigma^{p-1}(a)} text{.}
$$



Now, I have troubles determining this element $a$.





Right now, I believe that $a = zeta_p$ in which case, we would have



$${sigma(zeta_p),sigma^{2}(zeta_p), dots, sigma^{p-1}(zeta_p)}text{.}$$



So, my questions are




  1. Am I right with my assumption to set $a = zeta_p$?

  2. And if so, how do I proceed with my proof in order to show that $ B = { zeta_p, zeta_{p}^{2}, dots, zeta_{p}^{p-1} }$ is a normal basis?










share|cite|improve this question









$endgroup$

















    2












    $begingroup$


    So let $p$ be a prime number and $zeta_p$ the p-th roots of unity. I want to proof that $ B = { zeta_p, zeta_{p}^{2}, dots, zeta_{p}^{p-1} }$ is the normal basis of $mathbb{Q}(zeta_p)/mathbb{Q}$ (which I hope is true ...).





    First of all, I know that $B$ is a $mathbb{Q}$-basis of $mathbb{Q}(zeta_p)$ since




    1. the elements of $B$ are $mathbb{Q}$-linearly independent, because $zeta_p$ is a $p$-th primitive roots of unity,

    2. and $B$ has exactly $p-1$ elements and the degree of $mathbb{Q}(zeta_p)$ over $mathbb{Q}$ is $p-1$ too, because the Galois group of $mathbb{Q}(zeta_p)/mathbb{Q}$ is isomorphic to $(mathbb{Z}/pmathbb{Z})^times$ which has $p-1$ elements.


    Now, I have trouble proving that $B$ is indeed a normal basis. According to the definition of a normal basis, if we let $sigma_i in text{Gal}(mathbb{Q}(zeta_p)/mathbb{Q})$, I would have to find an $a in mathbb{Q}(zeta_p)$ such that



    $$
    {sigma_1(a),sigma_2(a), dots, sigma_{p-1}(a)}
    $$



    forms a $mathbb{Q}$-basis of $mathbb{Q}(zeta_p)$.



    Since $text{Gal}(mathbb{Q}(zeta_p)/mathbb{Q})$ is cyclic, there is a generating automorphism which I will just call $sigma$. Then, just reformulating the definition, we would have



    $$
    {sigma(a),sigma^{2}(a), dots, sigma^{p-1}(a)} text{.}
    $$



    Now, I have troubles determining this element $a$.





    Right now, I believe that $a = zeta_p$ in which case, we would have



    $${sigma(zeta_p),sigma^{2}(zeta_p), dots, sigma^{p-1}(zeta_p)}text{.}$$



    So, my questions are




    1. Am I right with my assumption to set $a = zeta_p$?

    2. And if so, how do I proceed with my proof in order to show that $ B = { zeta_p, zeta_{p}^{2}, dots, zeta_{p}^{p-1} }$ is a normal basis?










    share|cite|improve this question









    $endgroup$















      2












      2








      2





      $begingroup$


      So let $p$ be a prime number and $zeta_p$ the p-th roots of unity. I want to proof that $ B = { zeta_p, zeta_{p}^{2}, dots, zeta_{p}^{p-1} }$ is the normal basis of $mathbb{Q}(zeta_p)/mathbb{Q}$ (which I hope is true ...).





      First of all, I know that $B$ is a $mathbb{Q}$-basis of $mathbb{Q}(zeta_p)$ since




      1. the elements of $B$ are $mathbb{Q}$-linearly independent, because $zeta_p$ is a $p$-th primitive roots of unity,

      2. and $B$ has exactly $p-1$ elements and the degree of $mathbb{Q}(zeta_p)$ over $mathbb{Q}$ is $p-1$ too, because the Galois group of $mathbb{Q}(zeta_p)/mathbb{Q}$ is isomorphic to $(mathbb{Z}/pmathbb{Z})^times$ which has $p-1$ elements.


      Now, I have trouble proving that $B$ is indeed a normal basis. According to the definition of a normal basis, if we let $sigma_i in text{Gal}(mathbb{Q}(zeta_p)/mathbb{Q})$, I would have to find an $a in mathbb{Q}(zeta_p)$ such that



      $$
      {sigma_1(a),sigma_2(a), dots, sigma_{p-1}(a)}
      $$



      forms a $mathbb{Q}$-basis of $mathbb{Q}(zeta_p)$.



      Since $text{Gal}(mathbb{Q}(zeta_p)/mathbb{Q})$ is cyclic, there is a generating automorphism which I will just call $sigma$. Then, just reformulating the definition, we would have



      $$
      {sigma(a),sigma^{2}(a), dots, sigma^{p-1}(a)} text{.}
      $$



      Now, I have troubles determining this element $a$.





      Right now, I believe that $a = zeta_p$ in which case, we would have



      $${sigma(zeta_p),sigma^{2}(zeta_p), dots, sigma^{p-1}(zeta_p)}text{.}$$



      So, my questions are




      1. Am I right with my assumption to set $a = zeta_p$?

      2. And if so, how do I proceed with my proof in order to show that $ B = { zeta_p, zeta_{p}^{2}, dots, zeta_{p}^{p-1} }$ is a normal basis?










      share|cite|improve this question









      $endgroup$




      So let $p$ be a prime number and $zeta_p$ the p-th roots of unity. I want to proof that $ B = { zeta_p, zeta_{p}^{2}, dots, zeta_{p}^{p-1} }$ is the normal basis of $mathbb{Q}(zeta_p)/mathbb{Q}$ (which I hope is true ...).





      First of all, I know that $B$ is a $mathbb{Q}$-basis of $mathbb{Q}(zeta_p)$ since




      1. the elements of $B$ are $mathbb{Q}$-linearly independent, because $zeta_p$ is a $p$-th primitive roots of unity,

      2. and $B$ has exactly $p-1$ elements and the degree of $mathbb{Q}(zeta_p)$ over $mathbb{Q}$ is $p-1$ too, because the Galois group of $mathbb{Q}(zeta_p)/mathbb{Q}$ is isomorphic to $(mathbb{Z}/pmathbb{Z})^times$ which has $p-1$ elements.


      Now, I have trouble proving that $B$ is indeed a normal basis. According to the definition of a normal basis, if we let $sigma_i in text{Gal}(mathbb{Q}(zeta_p)/mathbb{Q})$, I would have to find an $a in mathbb{Q}(zeta_p)$ such that



      $$
      {sigma_1(a),sigma_2(a), dots, sigma_{p-1}(a)}
      $$



      forms a $mathbb{Q}$-basis of $mathbb{Q}(zeta_p)$.



      Since $text{Gal}(mathbb{Q}(zeta_p)/mathbb{Q})$ is cyclic, there is a generating automorphism which I will just call $sigma$. Then, just reformulating the definition, we would have



      $$
      {sigma(a),sigma^{2}(a), dots, sigma^{p-1}(a)} text{.}
      $$



      Now, I have troubles determining this element $a$.





      Right now, I believe that $a = zeta_p$ in which case, we would have



      $${sigma(zeta_p),sigma^{2}(zeta_p), dots, sigma^{p-1}(zeta_p)}text{.}$$



      So, my questions are




      1. Am I right with my assumption to set $a = zeta_p$?

      2. And if so, how do I proceed with my proof in order to show that $ B = { zeta_p, zeta_{p}^{2}, dots, zeta_{p}^{p-1} }$ is a normal basis?







      abstract-algebra field-theory roots-of-unity cyclotomic-fields






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      share|cite|improve this question











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      share|cite|improve this question










      asked Jan 17 at 21:36









      mattmatt

      857




      857






















          1 Answer
          1






          active

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          2












          $begingroup$

          The $zeta_p^j$ ($1le jle p-1$) are the zeros of the $p$-th cyclotomic
          polynomial $Phi_p(X)=X^{p-1}+X^{p-2}+cdots+X+1$, which is well-known to be
          irreducible over $Bbb Q$. Thus the Galois group of $Bbb Q(zeta_p)$ acts
          transitively on the zeros of $Phi_p(X)$. Thus there is a Galois group
          element $sigma_j$ with $sigma_j(zeta_p)=zeta_p^j$. This is unique:
          its action on $zeta_p$ determines its action on all of $Bbb Q(zeta_p)$.
          So $B={sigma_1(zeta_p),sigma_2(zeta_p),cdots,sigma_{p-1}(zeta_p)}$
          really is a normal basis.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thank you for your answer! However, I have some troubles following you. When you say the Galois group acts transitively, you mean that $sigma(zeta_p)$ is also a root of $Phi_p(X)$, right? And, if possible, could you elaborate on the part where you say its action on $zeta_p$ determines its action on all of $mathbb{Q}(zeta_p)$? Thank you.
            $endgroup$
            – matt
            Jan 17 at 23:48












          • $begingroup$
            I mean that $sigma(zeta)$ is also a zero of $Phi_p$, and all such zeroes arise. If $sigma$ takes $zeta$ to $zeta'$, then it takes $a_0+a_1zeta+a_2zeta^2+cdots$ to $a_0+a_1zeta'+a_2zeta'^2+cdots$ where the $a_iinBbb Q$.
            $endgroup$
            – Lord Shark the Unknown
            Jan 18 at 4:49










          • $begingroup$
            Did you guys deal with the linear independence of this set of conjugates? Anyway, if it were linearly dependent this would imply the existence of a polynomial (i) of degree $le p-1$, (ii) with rational coefficients, (iii) zero constant term, and (iv) $zeta_p$ as a zero, contradicting irreducibility of $Phi_p(X)$.
            $endgroup$
            – Jyrki Lahtonen
            Jan 18 at 21:52













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          1 Answer
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          1 Answer
          1






          active

          oldest

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          active

          oldest

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          active

          oldest

          votes









          2












          $begingroup$

          The $zeta_p^j$ ($1le jle p-1$) are the zeros of the $p$-th cyclotomic
          polynomial $Phi_p(X)=X^{p-1}+X^{p-2}+cdots+X+1$, which is well-known to be
          irreducible over $Bbb Q$. Thus the Galois group of $Bbb Q(zeta_p)$ acts
          transitively on the zeros of $Phi_p(X)$. Thus there is a Galois group
          element $sigma_j$ with $sigma_j(zeta_p)=zeta_p^j$. This is unique:
          its action on $zeta_p$ determines its action on all of $Bbb Q(zeta_p)$.
          So $B={sigma_1(zeta_p),sigma_2(zeta_p),cdots,sigma_{p-1}(zeta_p)}$
          really is a normal basis.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thank you for your answer! However, I have some troubles following you. When you say the Galois group acts transitively, you mean that $sigma(zeta_p)$ is also a root of $Phi_p(X)$, right? And, if possible, could you elaborate on the part where you say its action on $zeta_p$ determines its action on all of $mathbb{Q}(zeta_p)$? Thank you.
            $endgroup$
            – matt
            Jan 17 at 23:48












          • $begingroup$
            I mean that $sigma(zeta)$ is also a zero of $Phi_p$, and all such zeroes arise. If $sigma$ takes $zeta$ to $zeta'$, then it takes $a_0+a_1zeta+a_2zeta^2+cdots$ to $a_0+a_1zeta'+a_2zeta'^2+cdots$ where the $a_iinBbb Q$.
            $endgroup$
            – Lord Shark the Unknown
            Jan 18 at 4:49










          • $begingroup$
            Did you guys deal with the linear independence of this set of conjugates? Anyway, if it were linearly dependent this would imply the existence of a polynomial (i) of degree $le p-1$, (ii) with rational coefficients, (iii) zero constant term, and (iv) $zeta_p$ as a zero, contradicting irreducibility of $Phi_p(X)$.
            $endgroup$
            – Jyrki Lahtonen
            Jan 18 at 21:52


















          2












          $begingroup$

          The $zeta_p^j$ ($1le jle p-1$) are the zeros of the $p$-th cyclotomic
          polynomial $Phi_p(X)=X^{p-1}+X^{p-2}+cdots+X+1$, which is well-known to be
          irreducible over $Bbb Q$. Thus the Galois group of $Bbb Q(zeta_p)$ acts
          transitively on the zeros of $Phi_p(X)$. Thus there is a Galois group
          element $sigma_j$ with $sigma_j(zeta_p)=zeta_p^j$. This is unique:
          its action on $zeta_p$ determines its action on all of $Bbb Q(zeta_p)$.
          So $B={sigma_1(zeta_p),sigma_2(zeta_p),cdots,sigma_{p-1}(zeta_p)}$
          really is a normal basis.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thank you for your answer! However, I have some troubles following you. When you say the Galois group acts transitively, you mean that $sigma(zeta_p)$ is also a root of $Phi_p(X)$, right? And, if possible, could you elaborate on the part where you say its action on $zeta_p$ determines its action on all of $mathbb{Q}(zeta_p)$? Thank you.
            $endgroup$
            – matt
            Jan 17 at 23:48












          • $begingroup$
            I mean that $sigma(zeta)$ is also a zero of $Phi_p$, and all such zeroes arise. If $sigma$ takes $zeta$ to $zeta'$, then it takes $a_0+a_1zeta+a_2zeta^2+cdots$ to $a_0+a_1zeta'+a_2zeta'^2+cdots$ where the $a_iinBbb Q$.
            $endgroup$
            – Lord Shark the Unknown
            Jan 18 at 4:49










          • $begingroup$
            Did you guys deal with the linear independence of this set of conjugates? Anyway, if it were linearly dependent this would imply the existence of a polynomial (i) of degree $le p-1$, (ii) with rational coefficients, (iii) zero constant term, and (iv) $zeta_p$ as a zero, contradicting irreducibility of $Phi_p(X)$.
            $endgroup$
            – Jyrki Lahtonen
            Jan 18 at 21:52
















          2












          2








          2





          $begingroup$

          The $zeta_p^j$ ($1le jle p-1$) are the zeros of the $p$-th cyclotomic
          polynomial $Phi_p(X)=X^{p-1}+X^{p-2}+cdots+X+1$, which is well-known to be
          irreducible over $Bbb Q$. Thus the Galois group of $Bbb Q(zeta_p)$ acts
          transitively on the zeros of $Phi_p(X)$. Thus there is a Galois group
          element $sigma_j$ with $sigma_j(zeta_p)=zeta_p^j$. This is unique:
          its action on $zeta_p$ determines its action on all of $Bbb Q(zeta_p)$.
          So $B={sigma_1(zeta_p),sigma_2(zeta_p),cdots,sigma_{p-1}(zeta_p)}$
          really is a normal basis.






          share|cite|improve this answer









          $endgroup$



          The $zeta_p^j$ ($1le jle p-1$) are the zeros of the $p$-th cyclotomic
          polynomial $Phi_p(X)=X^{p-1}+X^{p-2}+cdots+X+1$, which is well-known to be
          irreducible over $Bbb Q$. Thus the Galois group of $Bbb Q(zeta_p)$ acts
          transitively on the zeros of $Phi_p(X)$. Thus there is a Galois group
          element $sigma_j$ with $sigma_j(zeta_p)=zeta_p^j$. This is unique:
          its action on $zeta_p$ determines its action on all of $Bbb Q(zeta_p)$.
          So $B={sigma_1(zeta_p),sigma_2(zeta_p),cdots,sigma_{p-1}(zeta_p)}$
          really is a normal basis.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Jan 17 at 22:30









          Lord Shark the UnknownLord Shark the Unknown

          105k1160133




          105k1160133












          • $begingroup$
            Thank you for your answer! However, I have some troubles following you. When you say the Galois group acts transitively, you mean that $sigma(zeta_p)$ is also a root of $Phi_p(X)$, right? And, if possible, could you elaborate on the part where you say its action on $zeta_p$ determines its action on all of $mathbb{Q}(zeta_p)$? Thank you.
            $endgroup$
            – matt
            Jan 17 at 23:48












          • $begingroup$
            I mean that $sigma(zeta)$ is also a zero of $Phi_p$, and all such zeroes arise. If $sigma$ takes $zeta$ to $zeta'$, then it takes $a_0+a_1zeta+a_2zeta^2+cdots$ to $a_0+a_1zeta'+a_2zeta'^2+cdots$ where the $a_iinBbb Q$.
            $endgroup$
            – Lord Shark the Unknown
            Jan 18 at 4:49










          • $begingroup$
            Did you guys deal with the linear independence of this set of conjugates? Anyway, if it were linearly dependent this would imply the existence of a polynomial (i) of degree $le p-1$, (ii) with rational coefficients, (iii) zero constant term, and (iv) $zeta_p$ as a zero, contradicting irreducibility of $Phi_p(X)$.
            $endgroup$
            – Jyrki Lahtonen
            Jan 18 at 21:52




















          • $begingroup$
            Thank you for your answer! However, I have some troubles following you. When you say the Galois group acts transitively, you mean that $sigma(zeta_p)$ is also a root of $Phi_p(X)$, right? And, if possible, could you elaborate on the part where you say its action on $zeta_p$ determines its action on all of $mathbb{Q}(zeta_p)$? Thank you.
            $endgroup$
            – matt
            Jan 17 at 23:48












          • $begingroup$
            I mean that $sigma(zeta)$ is also a zero of $Phi_p$, and all such zeroes arise. If $sigma$ takes $zeta$ to $zeta'$, then it takes $a_0+a_1zeta+a_2zeta^2+cdots$ to $a_0+a_1zeta'+a_2zeta'^2+cdots$ where the $a_iinBbb Q$.
            $endgroup$
            – Lord Shark the Unknown
            Jan 18 at 4:49










          • $begingroup$
            Did you guys deal with the linear independence of this set of conjugates? Anyway, if it were linearly dependent this would imply the existence of a polynomial (i) of degree $le p-1$, (ii) with rational coefficients, (iii) zero constant term, and (iv) $zeta_p$ as a zero, contradicting irreducibility of $Phi_p(X)$.
            $endgroup$
            – Jyrki Lahtonen
            Jan 18 at 21:52


















          $begingroup$
          Thank you for your answer! However, I have some troubles following you. When you say the Galois group acts transitively, you mean that $sigma(zeta_p)$ is also a root of $Phi_p(X)$, right? And, if possible, could you elaborate on the part where you say its action on $zeta_p$ determines its action on all of $mathbb{Q}(zeta_p)$? Thank you.
          $endgroup$
          – matt
          Jan 17 at 23:48






          $begingroup$
          Thank you for your answer! However, I have some troubles following you. When you say the Galois group acts transitively, you mean that $sigma(zeta_p)$ is also a root of $Phi_p(X)$, right? And, if possible, could you elaborate on the part where you say its action on $zeta_p$ determines its action on all of $mathbb{Q}(zeta_p)$? Thank you.
          $endgroup$
          – matt
          Jan 17 at 23:48














          $begingroup$
          I mean that $sigma(zeta)$ is also a zero of $Phi_p$, and all such zeroes arise. If $sigma$ takes $zeta$ to $zeta'$, then it takes $a_0+a_1zeta+a_2zeta^2+cdots$ to $a_0+a_1zeta'+a_2zeta'^2+cdots$ where the $a_iinBbb Q$.
          $endgroup$
          – Lord Shark the Unknown
          Jan 18 at 4:49




          $begingroup$
          I mean that $sigma(zeta)$ is also a zero of $Phi_p$, and all such zeroes arise. If $sigma$ takes $zeta$ to $zeta'$, then it takes $a_0+a_1zeta+a_2zeta^2+cdots$ to $a_0+a_1zeta'+a_2zeta'^2+cdots$ where the $a_iinBbb Q$.
          $endgroup$
          – Lord Shark the Unknown
          Jan 18 at 4:49












          $begingroup$
          Did you guys deal with the linear independence of this set of conjugates? Anyway, if it were linearly dependent this would imply the existence of a polynomial (i) of degree $le p-1$, (ii) with rational coefficients, (iii) zero constant term, and (iv) $zeta_p$ as a zero, contradicting irreducibility of $Phi_p(X)$.
          $endgroup$
          – Jyrki Lahtonen
          Jan 18 at 21:52






          $begingroup$
          Did you guys deal with the linear independence of this set of conjugates? Anyway, if it were linearly dependent this would imply the existence of a polynomial (i) of degree $le p-1$, (ii) with rational coefficients, (iii) zero constant term, and (iv) $zeta_p$ as a zero, contradicting irreducibility of $Phi_p(X)$.
          $endgroup$
          – Jyrki Lahtonen
          Jan 18 at 21:52




















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