minimal polynomial of $1+alpha^2$ where $alpha$ is a root of $x^3-x-1$











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Suppose $alpha$ is a root of $x^3-x-1$, and $gamma=1+alpha^2$.



What is the minimal polynomial of $gamma$ over $mathbb Q$?










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    What have you done so far? If you show your working then people are more willing and able to help on the parts you are stuck on
    – lioness99a
    Apr 7 '17 at 12:00










  • Hint: first find the minimum polynomial of $alpha^2$ by writing $alpha^3 - alpha = 1$ and squaring both sides.
    – Magdiragdag
    Apr 8 '17 at 7:34















up vote
0
down vote

favorite












Suppose $alpha$ is a root of $x^3-x-1$, and $gamma=1+alpha^2$.



What is the minimal polynomial of $gamma$ over $mathbb Q$?










share|cite|improve this question




















  • 1




    What have you done so far? If you show your working then people are more willing and able to help on the parts you are stuck on
    – lioness99a
    Apr 7 '17 at 12:00










  • Hint: first find the minimum polynomial of $alpha^2$ by writing $alpha^3 - alpha = 1$ and squaring both sides.
    – Magdiragdag
    Apr 8 '17 at 7:34













up vote
0
down vote

favorite









up vote
0
down vote

favorite











Suppose $alpha$ is a root of $x^3-x-1$, and $gamma=1+alpha^2$.



What is the minimal polynomial of $gamma$ over $mathbb Q$?










share|cite|improve this question















Suppose $alpha$ is a root of $x^3-x-1$, and $gamma=1+alpha^2$.



What is the minimal polynomial of $gamma$ over $mathbb Q$?







abstract-algebra roots minimal-polynomials






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edited Apr 8 '17 at 7:28









Marc van Leeuwen

86.2k5105218




86.2k5105218










asked Apr 7 '17 at 11:56









Lehostal11

214




214








  • 1




    What have you done so far? If you show your working then people are more willing and able to help on the parts you are stuck on
    – lioness99a
    Apr 7 '17 at 12:00










  • Hint: first find the minimum polynomial of $alpha^2$ by writing $alpha^3 - alpha = 1$ and squaring both sides.
    – Magdiragdag
    Apr 8 '17 at 7:34














  • 1




    What have you done so far? If you show your working then people are more willing and able to help on the parts you are stuck on
    – lioness99a
    Apr 7 '17 at 12:00










  • Hint: first find the minimum polynomial of $alpha^2$ by writing $alpha^3 - alpha = 1$ and squaring both sides.
    – Magdiragdag
    Apr 8 '17 at 7:34








1




1




What have you done so far? If you show your working then people are more willing and able to help on the parts you are stuck on
– lioness99a
Apr 7 '17 at 12:00




What have you done so far? If you show your working then people are more willing and able to help on the parts you are stuck on
– lioness99a
Apr 7 '17 at 12:00












Hint: first find the minimum polynomial of $alpha^2$ by writing $alpha^3 - alpha = 1$ and squaring both sides.
– Magdiragdag
Apr 8 '17 at 7:34




Hint: first find the minimum polynomial of $alpha^2$ by writing $alpha^3 - alpha = 1$ and squaring both sides.
– Magdiragdag
Apr 8 '17 at 7:34










4 Answers
4






active

oldest

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up vote
3
down vote













Write the matrix of the map $x mapsto gamma x$ in the basis $1,alpha,alpha^2$.



The minimal polynomial of this matrix is the minimal polynomial of $gamma$.






share|cite|improve this answer




























    up vote
    2
    down vote













    A bit of linear algebra makes this very straightforward. To expand on the answer by lhf, the minimal polynomial over$defQ{Bbb Q}~Q$ of an element $beta$ is also the minimal polynomial of the $Q$-linear map of mulitpliciation by$~beta$. The (sub)field $Q[alpha]$ is isomorphic to $Q[X]/(X^3-X-1)$, and in that $Q$-vector space the matrix, on the basis $[1,alpha,alpha^2]$ of images of $1,X,X^2$ is the companion matrix of $X^3-X-1$, which is
    $$ M=pmatrix{0&0&1\1&0&1\0&1&0}.
    $$

    The minimal polynomial of $gamma=alpha^2+1$ is of degree$~3$ (the only other possibility of an element of $Q[alpha]$ is degree$~1$, but that clearly is not the case here since $X^2+1$ is not divisible by $X^3-X-1$), so it will also be the characteristic polynomial of multiplication by$~gamma$, which has matrix $$ M^2+I=pmatrix{1&1&0\0&2&1\1&0&2}.$$
    The characteristic polynomial of that matrix is easily computed to be $X^3-5X^2+8X-5$.






    share|cite|improve this answer






























      up vote
      1
      down vote













      Hints:



      Observe that $;alpha^3=alpha+1;$ , and from here



      $$gamma^2=1+2alpha^2+alpha^2+alpha=gamma+3(gamma-1);ldots$$






      share|cite|improve this answer




























        up vote
        1
        down vote













        Hint:



        Find the minimal polynomial $p(x)$ of $alpha^2$, using Vieta's relations between the roots of $x^3-x-1$.



        The minimal polynomial of $1+alpha^2$ will be $q(x)=p(x-1)$.



        Some details:



        Explicitely, denotin $beta$ and $gamma$ the other roots of $x^3-x-1$, you have to calculate $$S=alpha^2+beta^2+gamma^2,quad S_2=alpha^2beta^2+beta^2gamma^2+gamma^2alpha^2,quad P=alpha^2beta^2gamma^2,$$ knowing
        $$s=alpha+beta+gamma,quad s_2=alphabeta+betagamma+gammaalpha,quad p=alphabetagamma.$$
        This is possible by Newton's theorem, since $S, S_2$ and $P$ are symmetric functions in $alpha,beta,gamma$.



        Example:
        $$S=(alpha+beta+gamma)^2-2(alphabeta+betagamma+gammaalpha)=s^2-2s_2$$






        share|cite|improve this answer























        • I try it this way. But it is still hard. Can you give me more hints?
          – Lehostal11
          Apr 7 '17 at 12:33










        • @Lehostal: I've added some details.
          – Bernard
          Apr 7 '17 at 13:10











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        4 Answers
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        4 Answers
        4






        active

        oldest

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        active

        oldest

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        active

        oldest

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        up vote
        3
        down vote













        Write the matrix of the map $x mapsto gamma x$ in the basis $1,alpha,alpha^2$.



        The minimal polynomial of this matrix is the minimal polynomial of $gamma$.






        share|cite|improve this answer

























          up vote
          3
          down vote













          Write the matrix of the map $x mapsto gamma x$ in the basis $1,alpha,alpha^2$.



          The minimal polynomial of this matrix is the minimal polynomial of $gamma$.






          share|cite|improve this answer























            up vote
            3
            down vote










            up vote
            3
            down vote









            Write the matrix of the map $x mapsto gamma x$ in the basis $1,alpha,alpha^2$.



            The minimal polynomial of this matrix is the minimal polynomial of $gamma$.






            share|cite|improve this answer












            Write the matrix of the map $x mapsto gamma x$ in the basis $1,alpha,alpha^2$.



            The minimal polynomial of this matrix is the minimal polynomial of $gamma$.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Apr 7 '17 at 13:25









            lhf

            162k9166385




            162k9166385






















                up vote
                2
                down vote













                A bit of linear algebra makes this very straightforward. To expand on the answer by lhf, the minimal polynomial over$defQ{Bbb Q}~Q$ of an element $beta$ is also the minimal polynomial of the $Q$-linear map of mulitpliciation by$~beta$. The (sub)field $Q[alpha]$ is isomorphic to $Q[X]/(X^3-X-1)$, and in that $Q$-vector space the matrix, on the basis $[1,alpha,alpha^2]$ of images of $1,X,X^2$ is the companion matrix of $X^3-X-1$, which is
                $$ M=pmatrix{0&0&1\1&0&1\0&1&0}.
                $$

                The minimal polynomial of $gamma=alpha^2+1$ is of degree$~3$ (the only other possibility of an element of $Q[alpha]$ is degree$~1$, but that clearly is not the case here since $X^2+1$ is not divisible by $X^3-X-1$), so it will also be the characteristic polynomial of multiplication by$~gamma$, which has matrix $$ M^2+I=pmatrix{1&1&0\0&2&1\1&0&2}.$$
                The characteristic polynomial of that matrix is easily computed to be $X^3-5X^2+8X-5$.






                share|cite|improve this answer



























                  up vote
                  2
                  down vote













                  A bit of linear algebra makes this very straightforward. To expand on the answer by lhf, the minimal polynomial over$defQ{Bbb Q}~Q$ of an element $beta$ is also the minimal polynomial of the $Q$-linear map of mulitpliciation by$~beta$. The (sub)field $Q[alpha]$ is isomorphic to $Q[X]/(X^3-X-1)$, and in that $Q$-vector space the matrix, on the basis $[1,alpha,alpha^2]$ of images of $1,X,X^2$ is the companion matrix of $X^3-X-1$, which is
                  $$ M=pmatrix{0&0&1\1&0&1\0&1&0}.
                  $$

                  The minimal polynomial of $gamma=alpha^2+1$ is of degree$~3$ (the only other possibility of an element of $Q[alpha]$ is degree$~1$, but that clearly is not the case here since $X^2+1$ is not divisible by $X^3-X-1$), so it will also be the characteristic polynomial of multiplication by$~gamma$, which has matrix $$ M^2+I=pmatrix{1&1&0\0&2&1\1&0&2}.$$
                  The characteristic polynomial of that matrix is easily computed to be $X^3-5X^2+8X-5$.






                  share|cite|improve this answer

























                    up vote
                    2
                    down vote










                    up vote
                    2
                    down vote









                    A bit of linear algebra makes this very straightforward. To expand on the answer by lhf, the minimal polynomial over$defQ{Bbb Q}~Q$ of an element $beta$ is also the minimal polynomial of the $Q$-linear map of mulitpliciation by$~beta$. The (sub)field $Q[alpha]$ is isomorphic to $Q[X]/(X^3-X-1)$, and in that $Q$-vector space the matrix, on the basis $[1,alpha,alpha^2]$ of images of $1,X,X^2$ is the companion matrix of $X^3-X-1$, which is
                    $$ M=pmatrix{0&0&1\1&0&1\0&1&0}.
                    $$

                    The minimal polynomial of $gamma=alpha^2+1$ is of degree$~3$ (the only other possibility of an element of $Q[alpha]$ is degree$~1$, but that clearly is not the case here since $X^2+1$ is not divisible by $X^3-X-1$), so it will also be the characteristic polynomial of multiplication by$~gamma$, which has matrix $$ M^2+I=pmatrix{1&1&0\0&2&1\1&0&2}.$$
                    The characteristic polynomial of that matrix is easily computed to be $X^3-5X^2+8X-5$.






                    share|cite|improve this answer














                    A bit of linear algebra makes this very straightforward. To expand on the answer by lhf, the minimal polynomial over$defQ{Bbb Q}~Q$ of an element $beta$ is also the minimal polynomial of the $Q$-linear map of mulitpliciation by$~beta$. The (sub)field $Q[alpha]$ is isomorphic to $Q[X]/(X^3-X-1)$, and in that $Q$-vector space the matrix, on the basis $[1,alpha,alpha^2]$ of images of $1,X,X^2$ is the companion matrix of $X^3-X-1$, which is
                    $$ M=pmatrix{0&0&1\1&0&1\0&1&0}.
                    $$

                    The minimal polynomial of $gamma=alpha^2+1$ is of degree$~3$ (the only other possibility of an element of $Q[alpha]$ is degree$~1$, but that clearly is not the case here since $X^2+1$ is not divisible by $X^3-X-1$), so it will also be the characteristic polynomial of multiplication by$~gamma$, which has matrix $$ M^2+I=pmatrix{1&1&0\0&2&1\1&0&2}.$$
                    The characteristic polynomial of that matrix is easily computed to be $X^3-5X^2+8X-5$.







                    share|cite|improve this answer














                    share|cite|improve this answer



                    share|cite|improve this answer








                    edited Nov 22 at 10:25

























                    answered Apr 8 '17 at 10:07









                    Marc van Leeuwen

                    86.2k5105218




                    86.2k5105218






















                        up vote
                        1
                        down vote













                        Hints:



                        Observe that $;alpha^3=alpha+1;$ , and from here



                        $$gamma^2=1+2alpha^2+alpha^2+alpha=gamma+3(gamma-1);ldots$$






                        share|cite|improve this answer

























                          up vote
                          1
                          down vote













                          Hints:



                          Observe that $;alpha^3=alpha+1;$ , and from here



                          $$gamma^2=1+2alpha^2+alpha^2+alpha=gamma+3(gamma-1);ldots$$






                          share|cite|improve this answer























                            up vote
                            1
                            down vote










                            up vote
                            1
                            down vote









                            Hints:



                            Observe that $;alpha^3=alpha+1;$ , and from here



                            $$gamma^2=1+2alpha^2+alpha^2+alpha=gamma+3(gamma-1);ldots$$






                            share|cite|improve this answer












                            Hints:



                            Observe that $;alpha^3=alpha+1;$ , and from here



                            $$gamma^2=1+2alpha^2+alpha^2+alpha=gamma+3(gamma-1);ldots$$







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered Apr 7 '17 at 12:06









                            DonAntonio

                            176k1491224




                            176k1491224






















                                up vote
                                1
                                down vote













                                Hint:



                                Find the minimal polynomial $p(x)$ of $alpha^2$, using Vieta's relations between the roots of $x^3-x-1$.



                                The minimal polynomial of $1+alpha^2$ will be $q(x)=p(x-1)$.



                                Some details:



                                Explicitely, denotin $beta$ and $gamma$ the other roots of $x^3-x-1$, you have to calculate $$S=alpha^2+beta^2+gamma^2,quad S_2=alpha^2beta^2+beta^2gamma^2+gamma^2alpha^2,quad P=alpha^2beta^2gamma^2,$$ knowing
                                $$s=alpha+beta+gamma,quad s_2=alphabeta+betagamma+gammaalpha,quad p=alphabetagamma.$$
                                This is possible by Newton's theorem, since $S, S_2$ and $P$ are symmetric functions in $alpha,beta,gamma$.



                                Example:
                                $$S=(alpha+beta+gamma)^2-2(alphabeta+betagamma+gammaalpha)=s^2-2s_2$$






                                share|cite|improve this answer























                                • I try it this way. But it is still hard. Can you give me more hints?
                                  – Lehostal11
                                  Apr 7 '17 at 12:33










                                • @Lehostal: I've added some details.
                                  – Bernard
                                  Apr 7 '17 at 13:10















                                up vote
                                1
                                down vote













                                Hint:



                                Find the minimal polynomial $p(x)$ of $alpha^2$, using Vieta's relations between the roots of $x^3-x-1$.



                                The minimal polynomial of $1+alpha^2$ will be $q(x)=p(x-1)$.



                                Some details:



                                Explicitely, denotin $beta$ and $gamma$ the other roots of $x^3-x-1$, you have to calculate $$S=alpha^2+beta^2+gamma^2,quad S_2=alpha^2beta^2+beta^2gamma^2+gamma^2alpha^2,quad P=alpha^2beta^2gamma^2,$$ knowing
                                $$s=alpha+beta+gamma,quad s_2=alphabeta+betagamma+gammaalpha,quad p=alphabetagamma.$$
                                This is possible by Newton's theorem, since $S, S_2$ and $P$ are symmetric functions in $alpha,beta,gamma$.



                                Example:
                                $$S=(alpha+beta+gamma)^2-2(alphabeta+betagamma+gammaalpha)=s^2-2s_2$$






                                share|cite|improve this answer























                                • I try it this way. But it is still hard. Can you give me more hints?
                                  – Lehostal11
                                  Apr 7 '17 at 12:33










                                • @Lehostal: I've added some details.
                                  – Bernard
                                  Apr 7 '17 at 13:10













                                up vote
                                1
                                down vote










                                up vote
                                1
                                down vote









                                Hint:



                                Find the minimal polynomial $p(x)$ of $alpha^2$, using Vieta's relations between the roots of $x^3-x-1$.



                                The minimal polynomial of $1+alpha^2$ will be $q(x)=p(x-1)$.



                                Some details:



                                Explicitely, denotin $beta$ and $gamma$ the other roots of $x^3-x-1$, you have to calculate $$S=alpha^2+beta^2+gamma^2,quad S_2=alpha^2beta^2+beta^2gamma^2+gamma^2alpha^2,quad P=alpha^2beta^2gamma^2,$$ knowing
                                $$s=alpha+beta+gamma,quad s_2=alphabeta+betagamma+gammaalpha,quad p=alphabetagamma.$$
                                This is possible by Newton's theorem, since $S, S_2$ and $P$ are symmetric functions in $alpha,beta,gamma$.



                                Example:
                                $$S=(alpha+beta+gamma)^2-2(alphabeta+betagamma+gammaalpha)=s^2-2s_2$$






                                share|cite|improve this answer














                                Hint:



                                Find the minimal polynomial $p(x)$ of $alpha^2$, using Vieta's relations between the roots of $x^3-x-1$.



                                The minimal polynomial of $1+alpha^2$ will be $q(x)=p(x-1)$.



                                Some details:



                                Explicitely, denotin $beta$ and $gamma$ the other roots of $x^3-x-1$, you have to calculate $$S=alpha^2+beta^2+gamma^2,quad S_2=alpha^2beta^2+beta^2gamma^2+gamma^2alpha^2,quad P=alpha^2beta^2gamma^2,$$ knowing
                                $$s=alpha+beta+gamma,quad s_2=alphabeta+betagamma+gammaalpha,quad p=alphabetagamma.$$
                                This is possible by Newton's theorem, since $S, S_2$ and $P$ are symmetric functions in $alpha,beta,gamma$.



                                Example:
                                $$S=(alpha+beta+gamma)^2-2(alphabeta+betagamma+gammaalpha)=s^2-2s_2$$







                                share|cite|improve this answer














                                share|cite|improve this answer



                                share|cite|improve this answer








                                edited Apr 7 '17 at 13:10

























                                answered Apr 7 '17 at 12:06









                                Bernard

                                117k637109




                                117k637109












                                • I try it this way. But it is still hard. Can you give me more hints?
                                  – Lehostal11
                                  Apr 7 '17 at 12:33










                                • @Lehostal: I've added some details.
                                  – Bernard
                                  Apr 7 '17 at 13:10


















                                • I try it this way. But it is still hard. Can you give me more hints?
                                  – Lehostal11
                                  Apr 7 '17 at 12:33










                                • @Lehostal: I've added some details.
                                  – Bernard
                                  Apr 7 '17 at 13:10
















                                I try it this way. But it is still hard. Can you give me more hints?
                                – Lehostal11
                                Apr 7 '17 at 12:33




                                I try it this way. But it is still hard. Can you give me more hints?
                                – Lehostal11
                                Apr 7 '17 at 12:33












                                @Lehostal: I've added some details.
                                – Bernard
                                Apr 7 '17 at 13:10




                                @Lehostal: I've added some details.
                                – Bernard
                                Apr 7 '17 at 13:10


















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