$mathbb{F}_{4096}$,$mathbb{F}_{16}$ and $mathbb{F}_{2}$












0












$begingroup$


I'm stuck at these two questions:



i)
How many distinct elements $a in mathbb{F}_{4096}$ exist, such that $mathbb{F}_{4096}=mathbb{F}_2 left[aright]$



ii)
Find an irreducible $g in mathbb{F}_4 left[xright]$ such that $mathbb{F}_{16}=mathbb{F}_4/(g)$



As for i) I know that the degree of the minimal polynomial of $a$ has to be $12$, since $2$ is prime and $4096=2^{12}$.



As for ii) I'm lost: Does $deg(g)$ equal to $2$? If so, how would I show that $mathbb{F}_4/(g)$ indeed is $mathbb{F}_{16}$? Is $g=x^2+1$ a viable guess?










share|cite|improve this question









$endgroup$

















    0












    $begingroup$


    I'm stuck at these two questions:



    i)
    How many distinct elements $a in mathbb{F}_{4096}$ exist, such that $mathbb{F}_{4096}=mathbb{F}_2 left[aright]$



    ii)
    Find an irreducible $g in mathbb{F}_4 left[xright]$ such that $mathbb{F}_{16}=mathbb{F}_4/(g)$



    As for i) I know that the degree of the minimal polynomial of $a$ has to be $12$, since $2$ is prime and $4096=2^{12}$.



    As for ii) I'm lost: Does $deg(g)$ equal to $2$? If so, how would I show that $mathbb{F}_4/(g)$ indeed is $mathbb{F}_{16}$? Is $g=x^2+1$ a viable guess?










    share|cite|improve this question









    $endgroup$















      0












      0








      0





      $begingroup$


      I'm stuck at these two questions:



      i)
      How many distinct elements $a in mathbb{F}_{4096}$ exist, such that $mathbb{F}_{4096}=mathbb{F}_2 left[aright]$



      ii)
      Find an irreducible $g in mathbb{F}_4 left[xright]$ such that $mathbb{F}_{16}=mathbb{F}_4/(g)$



      As for i) I know that the degree of the minimal polynomial of $a$ has to be $12$, since $2$ is prime and $4096=2^{12}$.



      As for ii) I'm lost: Does $deg(g)$ equal to $2$? If so, how would I show that $mathbb{F}_4/(g)$ indeed is $mathbb{F}_{16}$? Is $g=x^2+1$ a viable guess?










      share|cite|improve this question









      $endgroup$




      I'm stuck at these two questions:



      i)
      How many distinct elements $a in mathbb{F}_{4096}$ exist, such that $mathbb{F}_{4096}=mathbb{F}_2 left[aright]$



      ii)
      Find an irreducible $g in mathbb{F}_4 left[xright]$ such that $mathbb{F}_{16}=mathbb{F}_4/(g)$



      As for i) I know that the degree of the minimal polynomial of $a$ has to be $12$, since $2$ is prime and $4096=2^{12}$.



      As for ii) I'm lost: Does $deg(g)$ equal to $2$? If so, how would I show that $mathbb{F}_4/(g)$ indeed is $mathbb{F}_{16}$? Is $g=x^2+1$ a viable guess?







      finite-fields extension-field






      share|cite|improve this question













      share|cite|improve this question











      share|cite|improve this question




      share|cite|improve this question










      asked Feb 3 at 10:33









      C. BrendelC. Brendel

      638




      638






















          2 Answers
          2






          active

          oldest

          votes


















          1












          $begingroup$

          i) Hint: By a bad choice for $a$, you would end up with $Bbb F_{2048}$ or a sub-field thereof



          ii) Yes, we need $deg g=2$. However, $x^2+1$ is not irreducible: $x^2+1=(x+1)(x+1)$. In fact, any $x^2+ax+b$ with $a,binBbb F_2$ will fail to work as it can at most produce a degree 2 extension of $Bbb F_2$, i.e., it can already be factored in $Bbb F_4$.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            If $a$ is in a sub-ring $K$ of $mathbb{F}_{4096}$ isomorphic to $mathbb{F}_{2048}$ does that imply that $mathbb{F}_2 left[aright] cong mathbb{F}_{2048}$ ? If so: Are the elements that satisfy this condition exactly those of $mathbb{F}_{4096} setminus K$ ? Also: I don't see how $2x=0$ in $mathbb{F}_4$. Thank you for your help!
            $endgroup$
            – C. Brendel
            Feb 3 at 11:06












          • $begingroup$
            Figured it out myself! Thanks!
            $endgroup$
            – C. Brendel
            Feb 3 at 16:04










          • $begingroup$
            Hmm. $Bbb{F}_{2048}$ intersects trivially with $Bbb{F}_{4096}$.
            $endgroup$
            – Jyrki Lahtonen
            Feb 4 at 6:35



















          1












          $begingroup$

          Because $12=2^2cdot3$, the field $Bbb{F}_{4096}$ has the following subfields
          $$
          Bbb{F}_2,Bbb{F}_4,Bbb{F}_8,Bbb{F}_{16},Bbb{F}_{64}.
          $$

          The exponents $m$ of $2^m$ here are the proper factors of $12$, namely $1,2,3,4$ and $6$.



          Of the listed subfields $K_1=Bbb{F}_{16}$ and $K_2=Bbb{F}_{64}$ contain the smaller ones. We further observe that $K_1cap K_2=Bbb{F}_4$. This lets us answer your first question:




          $Bbb{F}_2[alpha]=Bbb{F}_{4096}$ unless $alphain K_1cup K_2$. We need to exclude $$|K_1cup K_2|=|K_1|+|K_2|-|K_1cap K_2|=64+16-4=76$$
          elements. Implying that $4096-76=4020$ choices of $alpha$ will work.




          To answer your second question see the bottom quarter of this answer I prepared for referrals. You will find that with
          $Bbb{F}_4={0,1,beta,beta+1=beta^2}$ an irreducible quadratic is $g(x)=x^2+x+beta$. There are a total of six irreducible monic quadratics in $Bbb{F}_4[x]$. Anyone of them can serve in the role of $g(x)$.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Oh! Neat! Thanks. Already handed the assignment in... I guess in i) I goofed a little :P as for ii) in the end I just went with multiplying every possible linear factor with another to rule out any reducibles of degree 2 :) Very much appreciated!
            $endgroup$
            – C. Brendel
            Feb 5 at 16:29












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          2 Answers
          2






          active

          oldest

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          2 Answers
          2






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          1












          $begingroup$

          i) Hint: By a bad choice for $a$, you would end up with $Bbb F_{2048}$ or a sub-field thereof



          ii) Yes, we need $deg g=2$. However, $x^2+1$ is not irreducible: $x^2+1=(x+1)(x+1)$. In fact, any $x^2+ax+b$ with $a,binBbb F_2$ will fail to work as it can at most produce a degree 2 extension of $Bbb F_2$, i.e., it can already be factored in $Bbb F_4$.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            If $a$ is in a sub-ring $K$ of $mathbb{F}_{4096}$ isomorphic to $mathbb{F}_{2048}$ does that imply that $mathbb{F}_2 left[aright] cong mathbb{F}_{2048}$ ? If so: Are the elements that satisfy this condition exactly those of $mathbb{F}_{4096} setminus K$ ? Also: I don't see how $2x=0$ in $mathbb{F}_4$. Thank you for your help!
            $endgroup$
            – C. Brendel
            Feb 3 at 11:06












          • $begingroup$
            Figured it out myself! Thanks!
            $endgroup$
            – C. Brendel
            Feb 3 at 16:04










          • $begingroup$
            Hmm. $Bbb{F}_{2048}$ intersects trivially with $Bbb{F}_{4096}$.
            $endgroup$
            – Jyrki Lahtonen
            Feb 4 at 6:35
















          1












          $begingroup$

          i) Hint: By a bad choice for $a$, you would end up with $Bbb F_{2048}$ or a sub-field thereof



          ii) Yes, we need $deg g=2$. However, $x^2+1$ is not irreducible: $x^2+1=(x+1)(x+1)$. In fact, any $x^2+ax+b$ with $a,binBbb F_2$ will fail to work as it can at most produce a degree 2 extension of $Bbb F_2$, i.e., it can already be factored in $Bbb F_4$.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            If $a$ is in a sub-ring $K$ of $mathbb{F}_{4096}$ isomorphic to $mathbb{F}_{2048}$ does that imply that $mathbb{F}_2 left[aright] cong mathbb{F}_{2048}$ ? If so: Are the elements that satisfy this condition exactly those of $mathbb{F}_{4096} setminus K$ ? Also: I don't see how $2x=0$ in $mathbb{F}_4$. Thank you for your help!
            $endgroup$
            – C. Brendel
            Feb 3 at 11:06












          • $begingroup$
            Figured it out myself! Thanks!
            $endgroup$
            – C. Brendel
            Feb 3 at 16:04










          • $begingroup$
            Hmm. $Bbb{F}_{2048}$ intersects trivially with $Bbb{F}_{4096}$.
            $endgroup$
            – Jyrki Lahtonen
            Feb 4 at 6:35














          1












          1








          1





          $begingroup$

          i) Hint: By a bad choice for $a$, you would end up with $Bbb F_{2048}$ or a sub-field thereof



          ii) Yes, we need $deg g=2$. However, $x^2+1$ is not irreducible: $x^2+1=(x+1)(x+1)$. In fact, any $x^2+ax+b$ with $a,binBbb F_2$ will fail to work as it can at most produce a degree 2 extension of $Bbb F_2$, i.e., it can already be factored in $Bbb F_4$.






          share|cite|improve this answer









          $endgroup$



          i) Hint: By a bad choice for $a$, you would end up with $Bbb F_{2048}$ or a sub-field thereof



          ii) Yes, we need $deg g=2$. However, $x^2+1$ is not irreducible: $x^2+1=(x+1)(x+1)$. In fact, any $x^2+ax+b$ with $a,binBbb F_2$ will fail to work as it can at most produce a degree 2 extension of $Bbb F_2$, i.e., it can already be factored in $Bbb F_4$.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Feb 3 at 10:40









          Hagen von EitzenHagen von Eitzen

          283k23273508




          283k23273508












          • $begingroup$
            If $a$ is in a sub-ring $K$ of $mathbb{F}_{4096}$ isomorphic to $mathbb{F}_{2048}$ does that imply that $mathbb{F}_2 left[aright] cong mathbb{F}_{2048}$ ? If so: Are the elements that satisfy this condition exactly those of $mathbb{F}_{4096} setminus K$ ? Also: I don't see how $2x=0$ in $mathbb{F}_4$. Thank you for your help!
            $endgroup$
            – C. Brendel
            Feb 3 at 11:06












          • $begingroup$
            Figured it out myself! Thanks!
            $endgroup$
            – C. Brendel
            Feb 3 at 16:04










          • $begingroup$
            Hmm. $Bbb{F}_{2048}$ intersects trivially with $Bbb{F}_{4096}$.
            $endgroup$
            – Jyrki Lahtonen
            Feb 4 at 6:35


















          • $begingroup$
            If $a$ is in a sub-ring $K$ of $mathbb{F}_{4096}$ isomorphic to $mathbb{F}_{2048}$ does that imply that $mathbb{F}_2 left[aright] cong mathbb{F}_{2048}$ ? If so: Are the elements that satisfy this condition exactly those of $mathbb{F}_{4096} setminus K$ ? Also: I don't see how $2x=0$ in $mathbb{F}_4$. Thank you for your help!
            $endgroup$
            – C. Brendel
            Feb 3 at 11:06












          • $begingroup$
            Figured it out myself! Thanks!
            $endgroup$
            – C. Brendel
            Feb 3 at 16:04










          • $begingroup$
            Hmm. $Bbb{F}_{2048}$ intersects trivially with $Bbb{F}_{4096}$.
            $endgroup$
            – Jyrki Lahtonen
            Feb 4 at 6:35
















          $begingroup$
          If $a$ is in a sub-ring $K$ of $mathbb{F}_{4096}$ isomorphic to $mathbb{F}_{2048}$ does that imply that $mathbb{F}_2 left[aright] cong mathbb{F}_{2048}$ ? If so: Are the elements that satisfy this condition exactly those of $mathbb{F}_{4096} setminus K$ ? Also: I don't see how $2x=0$ in $mathbb{F}_4$. Thank you for your help!
          $endgroup$
          – C. Brendel
          Feb 3 at 11:06






          $begingroup$
          If $a$ is in a sub-ring $K$ of $mathbb{F}_{4096}$ isomorphic to $mathbb{F}_{2048}$ does that imply that $mathbb{F}_2 left[aright] cong mathbb{F}_{2048}$ ? If so: Are the elements that satisfy this condition exactly those of $mathbb{F}_{4096} setminus K$ ? Also: I don't see how $2x=0$ in $mathbb{F}_4$. Thank you for your help!
          $endgroup$
          – C. Brendel
          Feb 3 at 11:06














          $begingroup$
          Figured it out myself! Thanks!
          $endgroup$
          – C. Brendel
          Feb 3 at 16:04




          $begingroup$
          Figured it out myself! Thanks!
          $endgroup$
          – C. Brendel
          Feb 3 at 16:04












          $begingroup$
          Hmm. $Bbb{F}_{2048}$ intersects trivially with $Bbb{F}_{4096}$.
          $endgroup$
          – Jyrki Lahtonen
          Feb 4 at 6:35




          $begingroup$
          Hmm. $Bbb{F}_{2048}$ intersects trivially with $Bbb{F}_{4096}$.
          $endgroup$
          – Jyrki Lahtonen
          Feb 4 at 6:35











          1












          $begingroup$

          Because $12=2^2cdot3$, the field $Bbb{F}_{4096}$ has the following subfields
          $$
          Bbb{F}_2,Bbb{F}_4,Bbb{F}_8,Bbb{F}_{16},Bbb{F}_{64}.
          $$

          The exponents $m$ of $2^m$ here are the proper factors of $12$, namely $1,2,3,4$ and $6$.



          Of the listed subfields $K_1=Bbb{F}_{16}$ and $K_2=Bbb{F}_{64}$ contain the smaller ones. We further observe that $K_1cap K_2=Bbb{F}_4$. This lets us answer your first question:




          $Bbb{F}_2[alpha]=Bbb{F}_{4096}$ unless $alphain K_1cup K_2$. We need to exclude $$|K_1cup K_2|=|K_1|+|K_2|-|K_1cap K_2|=64+16-4=76$$
          elements. Implying that $4096-76=4020$ choices of $alpha$ will work.




          To answer your second question see the bottom quarter of this answer I prepared for referrals. You will find that with
          $Bbb{F}_4={0,1,beta,beta+1=beta^2}$ an irreducible quadratic is $g(x)=x^2+x+beta$. There are a total of six irreducible monic quadratics in $Bbb{F}_4[x]$. Anyone of them can serve in the role of $g(x)$.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Oh! Neat! Thanks. Already handed the assignment in... I guess in i) I goofed a little :P as for ii) in the end I just went with multiplying every possible linear factor with another to rule out any reducibles of degree 2 :) Very much appreciated!
            $endgroup$
            – C. Brendel
            Feb 5 at 16:29
















          1












          $begingroup$

          Because $12=2^2cdot3$, the field $Bbb{F}_{4096}$ has the following subfields
          $$
          Bbb{F}_2,Bbb{F}_4,Bbb{F}_8,Bbb{F}_{16},Bbb{F}_{64}.
          $$

          The exponents $m$ of $2^m$ here are the proper factors of $12$, namely $1,2,3,4$ and $6$.



          Of the listed subfields $K_1=Bbb{F}_{16}$ and $K_2=Bbb{F}_{64}$ contain the smaller ones. We further observe that $K_1cap K_2=Bbb{F}_4$. This lets us answer your first question:




          $Bbb{F}_2[alpha]=Bbb{F}_{4096}$ unless $alphain K_1cup K_2$. We need to exclude $$|K_1cup K_2|=|K_1|+|K_2|-|K_1cap K_2|=64+16-4=76$$
          elements. Implying that $4096-76=4020$ choices of $alpha$ will work.




          To answer your second question see the bottom quarter of this answer I prepared for referrals. You will find that with
          $Bbb{F}_4={0,1,beta,beta+1=beta^2}$ an irreducible quadratic is $g(x)=x^2+x+beta$. There are a total of six irreducible monic quadratics in $Bbb{F}_4[x]$. Anyone of them can serve in the role of $g(x)$.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Oh! Neat! Thanks. Already handed the assignment in... I guess in i) I goofed a little :P as for ii) in the end I just went with multiplying every possible linear factor with another to rule out any reducibles of degree 2 :) Very much appreciated!
            $endgroup$
            – C. Brendel
            Feb 5 at 16:29














          1












          1








          1





          $begingroup$

          Because $12=2^2cdot3$, the field $Bbb{F}_{4096}$ has the following subfields
          $$
          Bbb{F}_2,Bbb{F}_4,Bbb{F}_8,Bbb{F}_{16},Bbb{F}_{64}.
          $$

          The exponents $m$ of $2^m$ here are the proper factors of $12$, namely $1,2,3,4$ and $6$.



          Of the listed subfields $K_1=Bbb{F}_{16}$ and $K_2=Bbb{F}_{64}$ contain the smaller ones. We further observe that $K_1cap K_2=Bbb{F}_4$. This lets us answer your first question:




          $Bbb{F}_2[alpha]=Bbb{F}_{4096}$ unless $alphain K_1cup K_2$. We need to exclude $$|K_1cup K_2|=|K_1|+|K_2|-|K_1cap K_2|=64+16-4=76$$
          elements. Implying that $4096-76=4020$ choices of $alpha$ will work.




          To answer your second question see the bottom quarter of this answer I prepared for referrals. You will find that with
          $Bbb{F}_4={0,1,beta,beta+1=beta^2}$ an irreducible quadratic is $g(x)=x^2+x+beta$. There are a total of six irreducible monic quadratics in $Bbb{F}_4[x]$. Anyone of them can serve in the role of $g(x)$.






          share|cite|improve this answer









          $endgroup$



          Because $12=2^2cdot3$, the field $Bbb{F}_{4096}$ has the following subfields
          $$
          Bbb{F}_2,Bbb{F}_4,Bbb{F}_8,Bbb{F}_{16},Bbb{F}_{64}.
          $$

          The exponents $m$ of $2^m$ here are the proper factors of $12$, namely $1,2,3,4$ and $6$.



          Of the listed subfields $K_1=Bbb{F}_{16}$ and $K_2=Bbb{F}_{64}$ contain the smaller ones. We further observe that $K_1cap K_2=Bbb{F}_4$. This lets us answer your first question:




          $Bbb{F}_2[alpha]=Bbb{F}_{4096}$ unless $alphain K_1cup K_2$. We need to exclude $$|K_1cup K_2|=|K_1|+|K_2|-|K_1cap K_2|=64+16-4=76$$
          elements. Implying that $4096-76=4020$ choices of $alpha$ will work.




          To answer your second question see the bottom quarter of this answer I prepared for referrals. You will find that with
          $Bbb{F}_4={0,1,beta,beta+1=beta^2}$ an irreducible quadratic is $g(x)=x^2+x+beta$. There are a total of six irreducible monic quadratics in $Bbb{F}_4[x]$. Anyone of them can serve in the role of $g(x)$.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Feb 4 at 6:34









          Jyrki LahtonenJyrki Lahtonen

          110k13172392




          110k13172392












          • $begingroup$
            Oh! Neat! Thanks. Already handed the assignment in... I guess in i) I goofed a little :P as for ii) in the end I just went with multiplying every possible linear factor with another to rule out any reducibles of degree 2 :) Very much appreciated!
            $endgroup$
            – C. Brendel
            Feb 5 at 16:29


















          • $begingroup$
            Oh! Neat! Thanks. Already handed the assignment in... I guess in i) I goofed a little :P as for ii) in the end I just went with multiplying every possible linear factor with another to rule out any reducibles of degree 2 :) Very much appreciated!
            $endgroup$
            – C. Brendel
            Feb 5 at 16:29
















          $begingroup$
          Oh! Neat! Thanks. Already handed the assignment in... I guess in i) I goofed a little :P as for ii) in the end I just went with multiplying every possible linear factor with another to rule out any reducibles of degree 2 :) Very much appreciated!
          $endgroup$
          – C. Brendel
          Feb 5 at 16:29




          $begingroup$
          Oh! Neat! Thanks. Already handed the assignment in... I guess in i) I goofed a little :P as for ii) in the end I just went with multiplying every possible linear factor with another to rule out any reducibles of degree 2 :) Very much appreciated!
          $endgroup$
          – C. Brendel
          Feb 5 at 16:29


















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