Linear independence of $sqrt{2}, sqrt[3]{2}, sqrt[5]{2}$ over $mathbb{Q}$












2












$begingroup$


In other words, show that for $a,b,cin mathbb{Q}$, $asqrt{2}+b sqrt[3]{2}+c sqrt[5]{2}=0$ implies that $a=b=c=0$.



There might be some questions on this forum that are similar to mine (e.g. Linear independence of $sqrt{2}$, $sqrt[3]{2}$, $sqrt[4]{2}$, . . .). However, I was given the following hint:



"Note: If organized smartly, this comes out as first the third order radical, then the second order is added to form an extension of degree 6 (since it can't belong to the first), and then, the radical of order 5 cannot be in the extension since it would violate the degree."



So I would love to see a solution to this using the field extensions, and how the hint makes sense. Huge thanks in advance!










share|cite|improve this question









$endgroup$

















    2












    $begingroup$


    In other words, show that for $a,b,cin mathbb{Q}$, $asqrt{2}+b sqrt[3]{2}+c sqrt[5]{2}=0$ implies that $a=b=c=0$.



    There might be some questions on this forum that are similar to mine (e.g. Linear independence of $sqrt{2}$, $sqrt[3]{2}$, $sqrt[4]{2}$, . . .). However, I was given the following hint:



    "Note: If organized smartly, this comes out as first the third order radical, then the second order is added to form an extension of degree 6 (since it can't belong to the first), and then, the radical of order 5 cannot be in the extension since it would violate the degree."



    So I would love to see a solution to this using the field extensions, and how the hint makes sense. Huge thanks in advance!










    share|cite|improve this question









    $endgroup$















      2












      2








      2





      $begingroup$


      In other words, show that for $a,b,cin mathbb{Q}$, $asqrt{2}+b sqrt[3]{2}+c sqrt[5]{2}=0$ implies that $a=b=c=0$.



      There might be some questions on this forum that are similar to mine (e.g. Linear independence of $sqrt{2}$, $sqrt[3]{2}$, $sqrt[4]{2}$, . . .). However, I was given the following hint:



      "Note: If organized smartly, this comes out as first the third order radical, then the second order is added to form an extension of degree 6 (since it can't belong to the first), and then, the radical of order 5 cannot be in the extension since it would violate the degree."



      So I would love to see a solution to this using the field extensions, and how the hint makes sense. Huge thanks in advance!










      share|cite|improve this question









      $endgroup$




      In other words, show that for $a,b,cin mathbb{Q}$, $asqrt{2}+b sqrt[3]{2}+c sqrt[5]{2}=0$ implies that $a=b=c=0$.



      There might be some questions on this forum that are similar to mine (e.g. Linear independence of $sqrt{2}$, $sqrt[3]{2}$, $sqrt[4]{2}$, . . .). However, I was given the following hint:



      "Note: If organized smartly, this comes out as first the third order radical, then the second order is added to form an extension of degree 6 (since it can't belong to the first), and then, the radical of order 5 cannot be in the extension since it would violate the degree."



      So I would love to see a solution to this using the field extensions, and how the hint makes sense. Huge thanks in advance!







      abstract-algebra field-theory






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      asked Dec 29 '18 at 2:08









      AlexAlex

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          5












          $begingroup$


          • $x^6-2$ and $x^5-2$ both are irreducible over $mathbb{Q}$ thus $mathbb{Q}(sqrt[6]{2}) cong mathbb{Q}[x]/(x^6-2)$ and $mathbb{Q}(sqrt[5]{2})cong mathbb{Q}[x]/(x^5-2)$ are $mathbb{Q}$ vector spaces of dimension $6$ and $5$.



          • Let $K = mathbb{Q}(sqrt[6]{2},sqrt[5]{2})$ a $mathbb{Q}$ vector space of dimension $n$.



            By the multiplicativity of degree of extensions we know that $5|n$ and $6|n$ thus $30| n$.




          • Assume that $asqrt{2}+b sqrt[3]{2}= sqrt[5]{2}$ with $a,b in mathbb{Q}$.



            Then $sqrt[5]{2} in mathbb{Q}(sqrt[6]{2})$ and $n= 6$ which is a contradiction.



          • The same idea applies to show that $sqrt[3]{2} not in mathbb{Q}(sqrt{2})$ so $asqrt{2}+bsqrt[3]{2} =0$ implies $a=b=0$.


          • An alternative is to develop a similar contradiction over some finite field $mathbb{F}_p$ instead of $mathbb{Q}$ and then to lift back the contradiction to $mathbb{Q}$.







          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Thanks very much for your answer! It made a lot more sense to me. So the idea is first to show that $sqrt[3]{2}notin mathbb{Q}(sqrt{2})$, showing that $sqrt{2}$ and $sqrt[3]{2}$ are linearly independent. Then by the same idea, show that $sqrt[5]{2}notin mathbb{Q}(sqrt[6]{2})$, so that $sqrt[5]{2}notin mathbb{Q}(sqrt[3]{2}), mathbb{Q}(sqrt[3]{2})$, since $mathbb{Q}(sqrt[3]{2}),mathbb{Q}(sqrt{2})subseteq mathbb{Q}(sqrt[6]{2})$.
            $endgroup$
            – Alex
            Dec 29 '18 at 2:55










          • $begingroup$
            @Alex And to show that $x^6-2,x^5-2$ are irreducible ! Factorize them, let $f_6,f_5$ be the minimal polynomials of $sqrt[6]{2},sqrt[5]{2}$ then look at $f_6(0),f_5(0)$
            $endgroup$
            – reuns
            Dec 29 '18 at 5:08








          • 1




            $begingroup$
            @renus Aren't they irreducible by Eisenstein's Criterion?
            $endgroup$
            – Alex
            Dec 29 '18 at 5:10











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          1 Answer
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          5












          $begingroup$


          • $x^6-2$ and $x^5-2$ both are irreducible over $mathbb{Q}$ thus $mathbb{Q}(sqrt[6]{2}) cong mathbb{Q}[x]/(x^6-2)$ and $mathbb{Q}(sqrt[5]{2})cong mathbb{Q}[x]/(x^5-2)$ are $mathbb{Q}$ vector spaces of dimension $6$ and $5$.



          • Let $K = mathbb{Q}(sqrt[6]{2},sqrt[5]{2})$ a $mathbb{Q}$ vector space of dimension $n$.



            By the multiplicativity of degree of extensions we know that $5|n$ and $6|n$ thus $30| n$.




          • Assume that $asqrt{2}+b sqrt[3]{2}= sqrt[5]{2}$ with $a,b in mathbb{Q}$.



            Then $sqrt[5]{2} in mathbb{Q}(sqrt[6]{2})$ and $n= 6$ which is a contradiction.



          • The same idea applies to show that $sqrt[3]{2} not in mathbb{Q}(sqrt{2})$ so $asqrt{2}+bsqrt[3]{2} =0$ implies $a=b=0$.


          • An alternative is to develop a similar contradiction over some finite field $mathbb{F}_p$ instead of $mathbb{Q}$ and then to lift back the contradiction to $mathbb{Q}$.







          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Thanks very much for your answer! It made a lot more sense to me. So the idea is first to show that $sqrt[3]{2}notin mathbb{Q}(sqrt{2})$, showing that $sqrt{2}$ and $sqrt[3]{2}$ are linearly independent. Then by the same idea, show that $sqrt[5]{2}notin mathbb{Q}(sqrt[6]{2})$, so that $sqrt[5]{2}notin mathbb{Q}(sqrt[3]{2}), mathbb{Q}(sqrt[3]{2})$, since $mathbb{Q}(sqrt[3]{2}),mathbb{Q}(sqrt{2})subseteq mathbb{Q}(sqrt[6]{2})$.
            $endgroup$
            – Alex
            Dec 29 '18 at 2:55










          • $begingroup$
            @Alex And to show that $x^6-2,x^5-2$ are irreducible ! Factorize them, let $f_6,f_5$ be the minimal polynomials of $sqrt[6]{2},sqrt[5]{2}$ then look at $f_6(0),f_5(0)$
            $endgroup$
            – reuns
            Dec 29 '18 at 5:08








          • 1




            $begingroup$
            @renus Aren't they irreducible by Eisenstein's Criterion?
            $endgroup$
            – Alex
            Dec 29 '18 at 5:10
















          5












          $begingroup$


          • $x^6-2$ and $x^5-2$ both are irreducible over $mathbb{Q}$ thus $mathbb{Q}(sqrt[6]{2}) cong mathbb{Q}[x]/(x^6-2)$ and $mathbb{Q}(sqrt[5]{2})cong mathbb{Q}[x]/(x^5-2)$ are $mathbb{Q}$ vector spaces of dimension $6$ and $5$.



          • Let $K = mathbb{Q}(sqrt[6]{2},sqrt[5]{2})$ a $mathbb{Q}$ vector space of dimension $n$.



            By the multiplicativity of degree of extensions we know that $5|n$ and $6|n$ thus $30| n$.




          • Assume that $asqrt{2}+b sqrt[3]{2}= sqrt[5]{2}$ with $a,b in mathbb{Q}$.



            Then $sqrt[5]{2} in mathbb{Q}(sqrt[6]{2})$ and $n= 6$ which is a contradiction.



          • The same idea applies to show that $sqrt[3]{2} not in mathbb{Q}(sqrt{2})$ so $asqrt{2}+bsqrt[3]{2} =0$ implies $a=b=0$.


          • An alternative is to develop a similar contradiction over some finite field $mathbb{F}_p$ instead of $mathbb{Q}$ and then to lift back the contradiction to $mathbb{Q}$.







          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Thanks very much for your answer! It made a lot more sense to me. So the idea is first to show that $sqrt[3]{2}notin mathbb{Q}(sqrt{2})$, showing that $sqrt{2}$ and $sqrt[3]{2}$ are linearly independent. Then by the same idea, show that $sqrt[5]{2}notin mathbb{Q}(sqrt[6]{2})$, so that $sqrt[5]{2}notin mathbb{Q}(sqrt[3]{2}), mathbb{Q}(sqrt[3]{2})$, since $mathbb{Q}(sqrt[3]{2}),mathbb{Q}(sqrt{2})subseteq mathbb{Q}(sqrt[6]{2})$.
            $endgroup$
            – Alex
            Dec 29 '18 at 2:55










          • $begingroup$
            @Alex And to show that $x^6-2,x^5-2$ are irreducible ! Factorize them, let $f_6,f_5$ be the minimal polynomials of $sqrt[6]{2},sqrt[5]{2}$ then look at $f_6(0),f_5(0)$
            $endgroup$
            – reuns
            Dec 29 '18 at 5:08








          • 1




            $begingroup$
            @renus Aren't they irreducible by Eisenstein's Criterion?
            $endgroup$
            – Alex
            Dec 29 '18 at 5:10














          5












          5








          5





          $begingroup$


          • $x^6-2$ and $x^5-2$ both are irreducible over $mathbb{Q}$ thus $mathbb{Q}(sqrt[6]{2}) cong mathbb{Q}[x]/(x^6-2)$ and $mathbb{Q}(sqrt[5]{2})cong mathbb{Q}[x]/(x^5-2)$ are $mathbb{Q}$ vector spaces of dimension $6$ and $5$.



          • Let $K = mathbb{Q}(sqrt[6]{2},sqrt[5]{2})$ a $mathbb{Q}$ vector space of dimension $n$.



            By the multiplicativity of degree of extensions we know that $5|n$ and $6|n$ thus $30| n$.




          • Assume that $asqrt{2}+b sqrt[3]{2}= sqrt[5]{2}$ with $a,b in mathbb{Q}$.



            Then $sqrt[5]{2} in mathbb{Q}(sqrt[6]{2})$ and $n= 6$ which is a contradiction.



          • The same idea applies to show that $sqrt[3]{2} not in mathbb{Q}(sqrt{2})$ so $asqrt{2}+bsqrt[3]{2} =0$ implies $a=b=0$.


          • An alternative is to develop a similar contradiction over some finite field $mathbb{F}_p$ instead of $mathbb{Q}$ and then to lift back the contradiction to $mathbb{Q}$.







          share|cite|improve this answer











          $endgroup$




          • $x^6-2$ and $x^5-2$ both are irreducible over $mathbb{Q}$ thus $mathbb{Q}(sqrt[6]{2}) cong mathbb{Q}[x]/(x^6-2)$ and $mathbb{Q}(sqrt[5]{2})cong mathbb{Q}[x]/(x^5-2)$ are $mathbb{Q}$ vector spaces of dimension $6$ and $5$.



          • Let $K = mathbb{Q}(sqrt[6]{2},sqrt[5]{2})$ a $mathbb{Q}$ vector space of dimension $n$.



            By the multiplicativity of degree of extensions we know that $5|n$ and $6|n$ thus $30| n$.




          • Assume that $asqrt{2}+b sqrt[3]{2}= sqrt[5]{2}$ with $a,b in mathbb{Q}$.



            Then $sqrt[5]{2} in mathbb{Q}(sqrt[6]{2})$ and $n= 6$ which is a contradiction.



          • The same idea applies to show that $sqrt[3]{2} not in mathbb{Q}(sqrt{2})$ so $asqrt{2}+bsqrt[3]{2} =0$ implies $a=b=0$.


          • An alternative is to develop a similar contradiction over some finite field $mathbb{F}_p$ instead of $mathbb{Q}$ and then to lift back the contradiction to $mathbb{Q}$.








          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Dec 29 '18 at 2:34

























          answered Dec 29 '18 at 2:26









          reunsreuns

          21.5k21352




          21.5k21352












          • $begingroup$
            Thanks very much for your answer! It made a lot more sense to me. So the idea is first to show that $sqrt[3]{2}notin mathbb{Q}(sqrt{2})$, showing that $sqrt{2}$ and $sqrt[3]{2}$ are linearly independent. Then by the same idea, show that $sqrt[5]{2}notin mathbb{Q}(sqrt[6]{2})$, so that $sqrt[5]{2}notin mathbb{Q}(sqrt[3]{2}), mathbb{Q}(sqrt[3]{2})$, since $mathbb{Q}(sqrt[3]{2}),mathbb{Q}(sqrt{2})subseteq mathbb{Q}(sqrt[6]{2})$.
            $endgroup$
            – Alex
            Dec 29 '18 at 2:55










          • $begingroup$
            @Alex And to show that $x^6-2,x^5-2$ are irreducible ! Factorize them, let $f_6,f_5$ be the minimal polynomials of $sqrt[6]{2},sqrt[5]{2}$ then look at $f_6(0),f_5(0)$
            $endgroup$
            – reuns
            Dec 29 '18 at 5:08








          • 1




            $begingroup$
            @renus Aren't they irreducible by Eisenstein's Criterion?
            $endgroup$
            – Alex
            Dec 29 '18 at 5:10


















          • $begingroup$
            Thanks very much for your answer! It made a lot more sense to me. So the idea is first to show that $sqrt[3]{2}notin mathbb{Q}(sqrt{2})$, showing that $sqrt{2}$ and $sqrt[3]{2}$ are linearly independent. Then by the same idea, show that $sqrt[5]{2}notin mathbb{Q}(sqrt[6]{2})$, so that $sqrt[5]{2}notin mathbb{Q}(sqrt[3]{2}), mathbb{Q}(sqrt[3]{2})$, since $mathbb{Q}(sqrt[3]{2}),mathbb{Q}(sqrt{2})subseteq mathbb{Q}(sqrt[6]{2})$.
            $endgroup$
            – Alex
            Dec 29 '18 at 2:55










          • $begingroup$
            @Alex And to show that $x^6-2,x^5-2$ are irreducible ! Factorize them, let $f_6,f_5$ be the minimal polynomials of $sqrt[6]{2},sqrt[5]{2}$ then look at $f_6(0),f_5(0)$
            $endgroup$
            – reuns
            Dec 29 '18 at 5:08








          • 1




            $begingroup$
            @renus Aren't they irreducible by Eisenstein's Criterion?
            $endgroup$
            – Alex
            Dec 29 '18 at 5:10
















          $begingroup$
          Thanks very much for your answer! It made a lot more sense to me. So the idea is first to show that $sqrt[3]{2}notin mathbb{Q}(sqrt{2})$, showing that $sqrt{2}$ and $sqrt[3]{2}$ are linearly independent. Then by the same idea, show that $sqrt[5]{2}notin mathbb{Q}(sqrt[6]{2})$, so that $sqrt[5]{2}notin mathbb{Q}(sqrt[3]{2}), mathbb{Q}(sqrt[3]{2})$, since $mathbb{Q}(sqrt[3]{2}),mathbb{Q}(sqrt{2})subseteq mathbb{Q}(sqrt[6]{2})$.
          $endgroup$
          – Alex
          Dec 29 '18 at 2:55




          $begingroup$
          Thanks very much for your answer! It made a lot more sense to me. So the idea is first to show that $sqrt[3]{2}notin mathbb{Q}(sqrt{2})$, showing that $sqrt{2}$ and $sqrt[3]{2}$ are linearly independent. Then by the same idea, show that $sqrt[5]{2}notin mathbb{Q}(sqrt[6]{2})$, so that $sqrt[5]{2}notin mathbb{Q}(sqrt[3]{2}), mathbb{Q}(sqrt[3]{2})$, since $mathbb{Q}(sqrt[3]{2}),mathbb{Q}(sqrt{2})subseteq mathbb{Q}(sqrt[6]{2})$.
          $endgroup$
          – Alex
          Dec 29 '18 at 2:55












          $begingroup$
          @Alex And to show that $x^6-2,x^5-2$ are irreducible ! Factorize them, let $f_6,f_5$ be the minimal polynomials of $sqrt[6]{2},sqrt[5]{2}$ then look at $f_6(0),f_5(0)$
          $endgroup$
          – reuns
          Dec 29 '18 at 5:08






          $begingroup$
          @Alex And to show that $x^6-2,x^5-2$ are irreducible ! Factorize them, let $f_6,f_5$ be the minimal polynomials of $sqrt[6]{2},sqrt[5]{2}$ then look at $f_6(0),f_5(0)$
          $endgroup$
          – reuns
          Dec 29 '18 at 5:08






          1




          1




          $begingroup$
          @renus Aren't they irreducible by Eisenstein's Criterion?
          $endgroup$
          – Alex
          Dec 29 '18 at 5:10




          $begingroup$
          @renus Aren't they irreducible by Eisenstein's Criterion?
          $endgroup$
          – Alex
          Dec 29 '18 at 5:10


















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