Prove $x^2=y^3$ where $x,yin mathbb{Z}$ implies $x=a^3$ and $y=b^2$ where $a,binmathbb{Z}$ [duplicate]












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  • Are there any integer solutions to $a^3=b^2$?

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I feel like I'm meant to use the uniqueness of prime factorization and show that since $text{lcm}(3,2)=6$ each prime should appear 6 times, but how would I justify this?










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Dec 21 '18 at 4:02


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    Can you prove the simpler statement that if $n = x^2$ then the maximum exponent of any prime factor of $n$ is even?
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    – JMoravitz
    Dec 18 '18 at 23:34
















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  • Are there any integer solutions to $a^3=b^2$?

    3 answers




I feel like I'm meant to use the uniqueness of prime factorization and show that since $text{lcm}(3,2)=6$ each prime should appear 6 times, but how would I justify this?










share|cite|improve this question











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Dec 21 '18 at 4:02


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.


















  • $begingroup$
    Can you prove the simpler statement that if $n = x^2$ then the maximum exponent of any prime factor of $n$ is even?
    $endgroup$
    – JMoravitz
    Dec 18 '18 at 23:34














1












1








1





$begingroup$



This question already has an answer here:




  • Are there any integer solutions to $a^3=b^2$?

    3 answers




I feel like I'm meant to use the uniqueness of prime factorization and show that since $text{lcm}(3,2)=6$ each prime should appear 6 times, but how would I justify this?










share|cite|improve this question











$endgroup$





This question already has an answer here:




  • Are there any integer solutions to $a^3=b^2$?

    3 answers




I feel like I'm meant to use the uniqueness of prime factorization and show that since $text{lcm}(3,2)=6$ each prime should appear 6 times, but how would I justify this?





This question already has an answer here:




  • Are there any integer solutions to $a^3=b^2$?

    3 answers








elementary-number-theory






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edited Dec 19 '18 at 1:03









Will Fisher

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4,0381032










asked Dec 18 '18 at 23:29









4M4D3U5 M0Z4RT4M4D3U5 M0Z4RT

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Dec 21 '18 at 4:02


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.














  • $begingroup$
    Can you prove the simpler statement that if $n = x^2$ then the maximum exponent of any prime factor of $n$ is even?
    $endgroup$
    – JMoravitz
    Dec 18 '18 at 23:34


















  • $begingroup$
    Can you prove the simpler statement that if $n = x^2$ then the maximum exponent of any prime factor of $n$ is even?
    $endgroup$
    – JMoravitz
    Dec 18 '18 at 23:34
















$begingroup$
Can you prove the simpler statement that if $n = x^2$ then the maximum exponent of any prime factor of $n$ is even?
$endgroup$
– JMoravitz
Dec 18 '18 at 23:34




$begingroup$
Can you prove the simpler statement that if $n = x^2$ then the maximum exponent of any prime factor of $n$ is even?
$endgroup$
– JMoravitz
Dec 18 '18 at 23:34










4 Answers
4






active

oldest

votes


















1












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Use the unique prime factorization.



First not that if $p$ is a prime factor of $x$ then $p|x^2$ so $p|y^3$ so $p|y$. And if $q$ is a prime factor of $y$ then $q|y^3$ so $q|x^2$ so $q|x$. So $x$ and $y$ have the exact same prime factors.



So if the distinct prime factors of $x$ are $P={p_1, p_2, ..... p_n}$ the $x = prodlimits_{k=1}^n p_k^{a_i}$ for some $a_i ge 1$ and $y = prodlimits_{k=1}^n p_k^{b_i}$ for some $b_i ge 1$.



Now $x^2 = y^3$ so



$(prodlimits_{k=1}^n p_k^{a_i})^2 = (prodlimits_{k=1}^n p_k^{b_i})^3$ so



$prodlimits_{k=1}^n p_k^{2a_i}=prodlimits_{k=1}^n p_k^{3b_i}$ so (because the is a unique prime factorization of the value $x^2$ which is $y^3$) we have:



$2a_i = 3b_i$ which means $2|3b_i$ for each $b_i$ and $3|2a_i$ for each $a_i$. And as $2,3$ are primes that means $2|b_i$ and $3|a_i$ for each $b_i$ and each $a_i$.



So each $b_i = 2c_i$ for some integer $c_i$. So $y = prodlimits_{k=1}^n p_k^{b_i} = prodlimits_{k=1}^n p_k^{2c_i}= (prodlimits_{k=1}^n p_k^{c_i})^2$. So $y$ is a perfect square.



So each $a_i = 3d_i$ for some integer $d_i$. So $x = prodlimits_{k=1}^n p_k^{a_i} = prodlimits_{k=1}^n p_k^{3d_i}= (prodlimits_{k=1}^n p_k^{d_i})^3$. So $x$ is a perfect square.



That's it.






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    Write $x^2 = prod_{i=1}^k p_i^{l_i}$, for some positive finite integer $k$ where $p_i$ is the $i$-th prime and $l_i$ is a nonegative integer. Then as $prod_{i=1}^k p_i^{l_i}$ is a square [because $x$ is an integer] each $l_i$ must be a multiple of 2.



    As $prod^k_{i=1} p_i^{l_i}$ is also a cube [because $y$ is an integer] each $l_i$ must also be a multiple of 3. Thus each $l_i$ must be a multiple of 6.



    Then $x = prod_{i=1}^k p_i^{frac{l_i}{2}}$ [because $x^2 = prod_{i=1}^k p_i^{l_i}$] and $y = prod_{i=1}^k p_i^{frac{l_i}{3}}$ [because $y^3 = prod_{i=1}^k p_i^{l_i}$], which as $l_i$ is a multiple of 6, implies that $frac{l_i}{2}$ is a multiple of 3, which implies that $x$ is a cube; namely
    $x= left(prod_{i=1}^k p_i^{frac{l_i}{6}}right)^3$ with each $frac{l_i}{6}$ an integer. Likewise $y$ is a square, namely $y= left(prod_{i=1}^k p_i^{frac{l_i}{6}}right)^2$ with each $frac{l_i}{6}$ an integer..






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    • $begingroup$
      It's also possible for $x$ to be negative (whereas $y$ must be positive since its cube is positive).
      $endgroup$
      – Daniel Schepler
      Dec 19 '18 at 1:05



















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    Here's an alternate proof which uses unique factorization in a different way: Given $x^2 = y^3$ with $x,y ne 0$, define $a := frac{x}{y}$. Then $a^2 = frac{x^2}{y^2} = frac{y^3}{y^2} = y$ and $a^3 = frac{x^3}{y^3} = frac{x^3}{x^2} = x$.



    It remains to show that $a in mathbb{Z}$. We do know that $a in mathbb{Q}$ and that $a^2 = y in mathbb{Z}$. Now, look at one of the standard proofs that $sqrt{2}$ is irrational using unique factorization; this will generalize in a fairly straightforward manner to show the desired fact that if $a in mathbb{Q}$ and $a^2 in mathbb{Z}$, then $a in mathbb{Z}$.



    (This can also be viewed as a corollary of the fact that $mathbb{Z}$ is integrally closed, since $mathbb{Z}$ is a unique factorization domain and any unique factorization domain is integrally closed.)






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      Here's a proof by infinite descent that $x$ is necessarily a perfect cube. The proof that $y$ is a perfect square is of the same exact spirit.



      Without loss of generality assume that $x,y$ are positive. Suppose there were to exist pairs of positive integers $(x,y)$ with $x^2=y^3$ but $x$ not a perfect cube. Choose the pair $(x_0,y_0)$ with $x$ minimal. Clearly $x_0,y_0>1$. Consider some prime $p$ dividing $x_0$ and write $x_0=px_1$. Then $pmid y_0^3implies y_0=py_1$ for some $y_1inmathbb Z_{geq 0}$. So $$p^2x_1^2=p^3y_0^3implies x_1^2=py_1^3implies pmid x_1$$ Write $x_1=px_2$, again with $x_2$ a positive integer. Then $$p^2x_2^2=py_1^3implies px_2^2=y_1^3implies pmid y_1$$ Next let $y_1=py_2$. We have that $$px_2^2=p^3y_2^3implies x_2^2=p^2y_3^3implies pmid x_2$$ Finally, take $x_2=px_3$. Our equation becomes $$p^2x_2^2=p^2y_2^3implies x_3^2=y_2^3$$
      So $(x_3,y_2)$ is a pair of positive integers satisfying $x_3^2=y_2^3$, but clearly $x_3<p^3x_3=x_0$. A contradiction $blacksquare$






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        4 Answers
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        4 Answers
        4






        active

        oldest

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        active

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        active

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        $begingroup$

        Use the unique prime factorization.



        First not that if $p$ is a prime factor of $x$ then $p|x^2$ so $p|y^3$ so $p|y$. And if $q$ is a prime factor of $y$ then $q|y^3$ so $q|x^2$ so $q|x$. So $x$ and $y$ have the exact same prime factors.



        So if the distinct prime factors of $x$ are $P={p_1, p_2, ..... p_n}$ the $x = prodlimits_{k=1}^n p_k^{a_i}$ for some $a_i ge 1$ and $y = prodlimits_{k=1}^n p_k^{b_i}$ for some $b_i ge 1$.



        Now $x^2 = y^3$ so



        $(prodlimits_{k=1}^n p_k^{a_i})^2 = (prodlimits_{k=1}^n p_k^{b_i})^3$ so



        $prodlimits_{k=1}^n p_k^{2a_i}=prodlimits_{k=1}^n p_k^{3b_i}$ so (because the is a unique prime factorization of the value $x^2$ which is $y^3$) we have:



        $2a_i = 3b_i$ which means $2|3b_i$ for each $b_i$ and $3|2a_i$ for each $a_i$. And as $2,3$ are primes that means $2|b_i$ and $3|a_i$ for each $b_i$ and each $a_i$.



        So each $b_i = 2c_i$ for some integer $c_i$. So $y = prodlimits_{k=1}^n p_k^{b_i} = prodlimits_{k=1}^n p_k^{2c_i}= (prodlimits_{k=1}^n p_k^{c_i})^2$. So $y$ is a perfect square.



        So each $a_i = 3d_i$ for some integer $d_i$. So $x = prodlimits_{k=1}^n p_k^{a_i} = prodlimits_{k=1}^n p_k^{3d_i}= (prodlimits_{k=1}^n p_k^{d_i})^3$. So $x$ is a perfect square.



        That's it.






        share|cite|improve this answer









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          1












          $begingroup$

          Use the unique prime factorization.



          First not that if $p$ is a prime factor of $x$ then $p|x^2$ so $p|y^3$ so $p|y$. And if $q$ is a prime factor of $y$ then $q|y^3$ so $q|x^2$ so $q|x$. So $x$ and $y$ have the exact same prime factors.



          So if the distinct prime factors of $x$ are $P={p_1, p_2, ..... p_n}$ the $x = prodlimits_{k=1}^n p_k^{a_i}$ for some $a_i ge 1$ and $y = prodlimits_{k=1}^n p_k^{b_i}$ for some $b_i ge 1$.



          Now $x^2 = y^3$ so



          $(prodlimits_{k=1}^n p_k^{a_i})^2 = (prodlimits_{k=1}^n p_k^{b_i})^3$ so



          $prodlimits_{k=1}^n p_k^{2a_i}=prodlimits_{k=1}^n p_k^{3b_i}$ so (because the is a unique prime factorization of the value $x^2$ which is $y^3$) we have:



          $2a_i = 3b_i$ which means $2|3b_i$ for each $b_i$ and $3|2a_i$ for each $a_i$. And as $2,3$ are primes that means $2|b_i$ and $3|a_i$ for each $b_i$ and each $a_i$.



          So each $b_i = 2c_i$ for some integer $c_i$. So $y = prodlimits_{k=1}^n p_k^{b_i} = prodlimits_{k=1}^n p_k^{2c_i}= (prodlimits_{k=1}^n p_k^{c_i})^2$. So $y$ is a perfect square.



          So each $a_i = 3d_i$ for some integer $d_i$. So $x = prodlimits_{k=1}^n p_k^{a_i} = prodlimits_{k=1}^n p_k^{3d_i}= (prodlimits_{k=1}^n p_k^{d_i})^3$. So $x$ is a perfect square.



          That's it.






          share|cite|improve this answer









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            1












            1








            1





            $begingroup$

            Use the unique prime factorization.



            First not that if $p$ is a prime factor of $x$ then $p|x^2$ so $p|y^3$ so $p|y$. And if $q$ is a prime factor of $y$ then $q|y^3$ so $q|x^2$ so $q|x$. So $x$ and $y$ have the exact same prime factors.



            So if the distinct prime factors of $x$ are $P={p_1, p_2, ..... p_n}$ the $x = prodlimits_{k=1}^n p_k^{a_i}$ for some $a_i ge 1$ and $y = prodlimits_{k=1}^n p_k^{b_i}$ for some $b_i ge 1$.



            Now $x^2 = y^3$ so



            $(prodlimits_{k=1}^n p_k^{a_i})^2 = (prodlimits_{k=1}^n p_k^{b_i})^3$ so



            $prodlimits_{k=1}^n p_k^{2a_i}=prodlimits_{k=1}^n p_k^{3b_i}$ so (because the is a unique prime factorization of the value $x^2$ which is $y^3$) we have:



            $2a_i = 3b_i$ which means $2|3b_i$ for each $b_i$ and $3|2a_i$ for each $a_i$. And as $2,3$ are primes that means $2|b_i$ and $3|a_i$ for each $b_i$ and each $a_i$.



            So each $b_i = 2c_i$ for some integer $c_i$. So $y = prodlimits_{k=1}^n p_k^{b_i} = prodlimits_{k=1}^n p_k^{2c_i}= (prodlimits_{k=1}^n p_k^{c_i})^2$. So $y$ is a perfect square.



            So each $a_i = 3d_i$ for some integer $d_i$. So $x = prodlimits_{k=1}^n p_k^{a_i} = prodlimits_{k=1}^n p_k^{3d_i}= (prodlimits_{k=1}^n p_k^{d_i})^3$. So $x$ is a perfect square.



            That's it.






            share|cite|improve this answer









            $endgroup$



            Use the unique prime factorization.



            First not that if $p$ is a prime factor of $x$ then $p|x^2$ so $p|y^3$ so $p|y$. And if $q$ is a prime factor of $y$ then $q|y^3$ so $q|x^2$ so $q|x$. So $x$ and $y$ have the exact same prime factors.



            So if the distinct prime factors of $x$ are $P={p_1, p_2, ..... p_n}$ the $x = prodlimits_{k=1}^n p_k^{a_i}$ for some $a_i ge 1$ and $y = prodlimits_{k=1}^n p_k^{b_i}$ for some $b_i ge 1$.



            Now $x^2 = y^3$ so



            $(prodlimits_{k=1}^n p_k^{a_i})^2 = (prodlimits_{k=1}^n p_k^{b_i})^3$ so



            $prodlimits_{k=1}^n p_k^{2a_i}=prodlimits_{k=1}^n p_k^{3b_i}$ so (because the is a unique prime factorization of the value $x^2$ which is $y^3$) we have:



            $2a_i = 3b_i$ which means $2|3b_i$ for each $b_i$ and $3|2a_i$ for each $a_i$. And as $2,3$ are primes that means $2|b_i$ and $3|a_i$ for each $b_i$ and each $a_i$.



            So each $b_i = 2c_i$ for some integer $c_i$. So $y = prodlimits_{k=1}^n p_k^{b_i} = prodlimits_{k=1}^n p_k^{2c_i}= (prodlimits_{k=1}^n p_k^{c_i})^2$. So $y$ is a perfect square.



            So each $a_i = 3d_i$ for some integer $d_i$. So $x = prodlimits_{k=1}^n p_k^{a_i} = prodlimits_{k=1}^n p_k^{3d_i}= (prodlimits_{k=1}^n p_k^{d_i})^3$. So $x$ is a perfect square.



            That's it.







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            answered Dec 19 '18 at 0:37









            fleabloodfleablood

            70.8k22686




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                0












                $begingroup$

                Write $x^2 = prod_{i=1}^k p_i^{l_i}$, for some positive finite integer $k$ where $p_i$ is the $i$-th prime and $l_i$ is a nonegative integer. Then as $prod_{i=1}^k p_i^{l_i}$ is a square [because $x$ is an integer] each $l_i$ must be a multiple of 2.



                As $prod^k_{i=1} p_i^{l_i}$ is also a cube [because $y$ is an integer] each $l_i$ must also be a multiple of 3. Thus each $l_i$ must be a multiple of 6.



                Then $x = prod_{i=1}^k p_i^{frac{l_i}{2}}$ [because $x^2 = prod_{i=1}^k p_i^{l_i}$] and $y = prod_{i=1}^k p_i^{frac{l_i}{3}}$ [because $y^3 = prod_{i=1}^k p_i^{l_i}$], which as $l_i$ is a multiple of 6, implies that $frac{l_i}{2}$ is a multiple of 3, which implies that $x$ is a cube; namely
                $x= left(prod_{i=1}^k p_i^{frac{l_i}{6}}right)^3$ with each $frac{l_i}{6}$ an integer. Likewise $y$ is a square, namely $y= left(prod_{i=1}^k p_i^{frac{l_i}{6}}right)^2$ with each $frac{l_i}{6}$ an integer..






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                • $begingroup$
                  It's also possible for $x$ to be negative (whereas $y$ must be positive since its cube is positive).
                  $endgroup$
                  – Daniel Schepler
                  Dec 19 '18 at 1:05
















                0












                $begingroup$

                Write $x^2 = prod_{i=1}^k p_i^{l_i}$, for some positive finite integer $k$ where $p_i$ is the $i$-th prime and $l_i$ is a nonegative integer. Then as $prod_{i=1}^k p_i^{l_i}$ is a square [because $x$ is an integer] each $l_i$ must be a multiple of 2.



                As $prod^k_{i=1} p_i^{l_i}$ is also a cube [because $y$ is an integer] each $l_i$ must also be a multiple of 3. Thus each $l_i$ must be a multiple of 6.



                Then $x = prod_{i=1}^k p_i^{frac{l_i}{2}}$ [because $x^2 = prod_{i=1}^k p_i^{l_i}$] and $y = prod_{i=1}^k p_i^{frac{l_i}{3}}$ [because $y^3 = prod_{i=1}^k p_i^{l_i}$], which as $l_i$ is a multiple of 6, implies that $frac{l_i}{2}$ is a multiple of 3, which implies that $x$ is a cube; namely
                $x= left(prod_{i=1}^k p_i^{frac{l_i}{6}}right)^3$ with each $frac{l_i}{6}$ an integer. Likewise $y$ is a square, namely $y= left(prod_{i=1}^k p_i^{frac{l_i}{6}}right)^2$ with each $frac{l_i}{6}$ an integer..






                share|cite|improve this answer











                $endgroup$













                • $begingroup$
                  It's also possible for $x$ to be negative (whereas $y$ must be positive since its cube is positive).
                  $endgroup$
                  – Daniel Schepler
                  Dec 19 '18 at 1:05














                0












                0








                0





                $begingroup$

                Write $x^2 = prod_{i=1}^k p_i^{l_i}$, for some positive finite integer $k$ where $p_i$ is the $i$-th prime and $l_i$ is a nonegative integer. Then as $prod_{i=1}^k p_i^{l_i}$ is a square [because $x$ is an integer] each $l_i$ must be a multiple of 2.



                As $prod^k_{i=1} p_i^{l_i}$ is also a cube [because $y$ is an integer] each $l_i$ must also be a multiple of 3. Thus each $l_i$ must be a multiple of 6.



                Then $x = prod_{i=1}^k p_i^{frac{l_i}{2}}$ [because $x^2 = prod_{i=1}^k p_i^{l_i}$] and $y = prod_{i=1}^k p_i^{frac{l_i}{3}}$ [because $y^3 = prod_{i=1}^k p_i^{l_i}$], which as $l_i$ is a multiple of 6, implies that $frac{l_i}{2}$ is a multiple of 3, which implies that $x$ is a cube; namely
                $x= left(prod_{i=1}^k p_i^{frac{l_i}{6}}right)^3$ with each $frac{l_i}{6}$ an integer. Likewise $y$ is a square, namely $y= left(prod_{i=1}^k p_i^{frac{l_i}{6}}right)^2$ with each $frac{l_i}{6}$ an integer..






                share|cite|improve this answer











                $endgroup$



                Write $x^2 = prod_{i=1}^k p_i^{l_i}$, for some positive finite integer $k$ where $p_i$ is the $i$-th prime and $l_i$ is a nonegative integer. Then as $prod_{i=1}^k p_i^{l_i}$ is a square [because $x$ is an integer] each $l_i$ must be a multiple of 2.



                As $prod^k_{i=1} p_i^{l_i}$ is also a cube [because $y$ is an integer] each $l_i$ must also be a multiple of 3. Thus each $l_i$ must be a multiple of 6.



                Then $x = prod_{i=1}^k p_i^{frac{l_i}{2}}$ [because $x^2 = prod_{i=1}^k p_i^{l_i}$] and $y = prod_{i=1}^k p_i^{frac{l_i}{3}}$ [because $y^3 = prod_{i=1}^k p_i^{l_i}$], which as $l_i$ is a multiple of 6, implies that $frac{l_i}{2}$ is a multiple of 3, which implies that $x$ is a cube; namely
                $x= left(prod_{i=1}^k p_i^{frac{l_i}{6}}right)^3$ with each $frac{l_i}{6}$ an integer. Likewise $y$ is a square, namely $y= left(prod_{i=1}^k p_i^{frac{l_i}{6}}right)^2$ with each $frac{l_i}{6}$ an integer..







                share|cite|improve this answer














                share|cite|improve this answer



                share|cite|improve this answer








                edited Dec 19 '18 at 0:00

























                answered Dec 18 '18 at 23:48









                MikeMike

                4,056412




                4,056412












                • $begingroup$
                  It's also possible for $x$ to be negative (whereas $y$ must be positive since its cube is positive).
                  $endgroup$
                  – Daniel Schepler
                  Dec 19 '18 at 1:05


















                • $begingroup$
                  It's also possible for $x$ to be negative (whereas $y$ must be positive since its cube is positive).
                  $endgroup$
                  – Daniel Schepler
                  Dec 19 '18 at 1:05
















                $begingroup$
                It's also possible for $x$ to be negative (whereas $y$ must be positive since its cube is positive).
                $endgroup$
                – Daniel Schepler
                Dec 19 '18 at 1:05




                $begingroup$
                It's also possible for $x$ to be negative (whereas $y$ must be positive since its cube is positive).
                $endgroup$
                – Daniel Schepler
                Dec 19 '18 at 1:05











                0












                $begingroup$

                Here's an alternate proof which uses unique factorization in a different way: Given $x^2 = y^3$ with $x,y ne 0$, define $a := frac{x}{y}$. Then $a^2 = frac{x^2}{y^2} = frac{y^3}{y^2} = y$ and $a^3 = frac{x^3}{y^3} = frac{x^3}{x^2} = x$.



                It remains to show that $a in mathbb{Z}$. We do know that $a in mathbb{Q}$ and that $a^2 = y in mathbb{Z}$. Now, look at one of the standard proofs that $sqrt{2}$ is irrational using unique factorization; this will generalize in a fairly straightforward manner to show the desired fact that if $a in mathbb{Q}$ and $a^2 in mathbb{Z}$, then $a in mathbb{Z}$.



                (This can also be viewed as a corollary of the fact that $mathbb{Z}$ is integrally closed, since $mathbb{Z}$ is a unique factorization domain and any unique factorization domain is integrally closed.)






                share|cite|improve this answer









                $endgroup$


















                  0












                  $begingroup$

                  Here's an alternate proof which uses unique factorization in a different way: Given $x^2 = y^3$ with $x,y ne 0$, define $a := frac{x}{y}$. Then $a^2 = frac{x^2}{y^2} = frac{y^3}{y^2} = y$ and $a^3 = frac{x^3}{y^3} = frac{x^3}{x^2} = x$.



                  It remains to show that $a in mathbb{Z}$. We do know that $a in mathbb{Q}$ and that $a^2 = y in mathbb{Z}$. Now, look at one of the standard proofs that $sqrt{2}$ is irrational using unique factorization; this will generalize in a fairly straightforward manner to show the desired fact that if $a in mathbb{Q}$ and $a^2 in mathbb{Z}$, then $a in mathbb{Z}$.



                  (This can also be viewed as a corollary of the fact that $mathbb{Z}$ is integrally closed, since $mathbb{Z}$ is a unique factorization domain and any unique factorization domain is integrally closed.)






                  share|cite|improve this answer









                  $endgroup$
















                    0












                    0








                    0





                    $begingroup$

                    Here's an alternate proof which uses unique factorization in a different way: Given $x^2 = y^3$ with $x,y ne 0$, define $a := frac{x}{y}$. Then $a^2 = frac{x^2}{y^2} = frac{y^3}{y^2} = y$ and $a^3 = frac{x^3}{y^3} = frac{x^3}{x^2} = x$.



                    It remains to show that $a in mathbb{Z}$. We do know that $a in mathbb{Q}$ and that $a^2 = y in mathbb{Z}$. Now, look at one of the standard proofs that $sqrt{2}$ is irrational using unique factorization; this will generalize in a fairly straightforward manner to show the desired fact that if $a in mathbb{Q}$ and $a^2 in mathbb{Z}$, then $a in mathbb{Z}$.



                    (This can also be viewed as a corollary of the fact that $mathbb{Z}$ is integrally closed, since $mathbb{Z}$ is a unique factorization domain and any unique factorization domain is integrally closed.)






                    share|cite|improve this answer









                    $endgroup$



                    Here's an alternate proof which uses unique factorization in a different way: Given $x^2 = y^3$ with $x,y ne 0$, define $a := frac{x}{y}$. Then $a^2 = frac{x^2}{y^2} = frac{y^3}{y^2} = y$ and $a^3 = frac{x^3}{y^3} = frac{x^3}{x^2} = x$.



                    It remains to show that $a in mathbb{Z}$. We do know that $a in mathbb{Q}$ and that $a^2 = y in mathbb{Z}$. Now, look at one of the standard proofs that $sqrt{2}$ is irrational using unique factorization; this will generalize in a fairly straightforward manner to show the desired fact that if $a in mathbb{Q}$ and $a^2 in mathbb{Z}$, then $a in mathbb{Z}$.



                    (This can also be viewed as a corollary of the fact that $mathbb{Z}$ is integrally closed, since $mathbb{Z}$ is a unique factorization domain and any unique factorization domain is integrally closed.)







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered Dec 19 '18 at 0:15









                    Daniel ScheplerDaniel Schepler

                    8,8091620




                    8,8091620























                        0












                        $begingroup$

                        Here's a proof by infinite descent that $x$ is necessarily a perfect cube. The proof that $y$ is a perfect square is of the same exact spirit.



                        Without loss of generality assume that $x,y$ are positive. Suppose there were to exist pairs of positive integers $(x,y)$ with $x^2=y^3$ but $x$ not a perfect cube. Choose the pair $(x_0,y_0)$ with $x$ minimal. Clearly $x_0,y_0>1$. Consider some prime $p$ dividing $x_0$ and write $x_0=px_1$. Then $pmid y_0^3implies y_0=py_1$ for some $y_1inmathbb Z_{geq 0}$. So $$p^2x_1^2=p^3y_0^3implies x_1^2=py_1^3implies pmid x_1$$ Write $x_1=px_2$, again with $x_2$ a positive integer. Then $$p^2x_2^2=py_1^3implies px_2^2=y_1^3implies pmid y_1$$ Next let $y_1=py_2$. We have that $$px_2^2=p^3y_2^3implies x_2^2=p^2y_3^3implies pmid x_2$$ Finally, take $x_2=px_3$. Our equation becomes $$p^2x_2^2=p^2y_2^3implies x_3^2=y_2^3$$
                        So $(x_3,y_2)$ is a pair of positive integers satisfying $x_3^2=y_2^3$, but clearly $x_3<p^3x_3=x_0$. A contradiction $blacksquare$






                        share|cite|improve this answer









                        $endgroup$


















                          0












                          $begingroup$

                          Here's a proof by infinite descent that $x$ is necessarily a perfect cube. The proof that $y$ is a perfect square is of the same exact spirit.



                          Without loss of generality assume that $x,y$ are positive. Suppose there were to exist pairs of positive integers $(x,y)$ with $x^2=y^3$ but $x$ not a perfect cube. Choose the pair $(x_0,y_0)$ with $x$ minimal. Clearly $x_0,y_0>1$. Consider some prime $p$ dividing $x_0$ and write $x_0=px_1$. Then $pmid y_0^3implies y_0=py_1$ for some $y_1inmathbb Z_{geq 0}$. So $$p^2x_1^2=p^3y_0^3implies x_1^2=py_1^3implies pmid x_1$$ Write $x_1=px_2$, again with $x_2$ a positive integer. Then $$p^2x_2^2=py_1^3implies px_2^2=y_1^3implies pmid y_1$$ Next let $y_1=py_2$. We have that $$px_2^2=p^3y_2^3implies x_2^2=p^2y_3^3implies pmid x_2$$ Finally, take $x_2=px_3$. Our equation becomes $$p^2x_2^2=p^2y_2^3implies x_3^2=y_2^3$$
                          So $(x_3,y_2)$ is a pair of positive integers satisfying $x_3^2=y_2^3$, but clearly $x_3<p^3x_3=x_0$. A contradiction $blacksquare$






                          share|cite|improve this answer









                          $endgroup$
















                            0












                            0








                            0





                            $begingroup$

                            Here's a proof by infinite descent that $x$ is necessarily a perfect cube. The proof that $y$ is a perfect square is of the same exact spirit.



                            Without loss of generality assume that $x,y$ are positive. Suppose there were to exist pairs of positive integers $(x,y)$ with $x^2=y^3$ but $x$ not a perfect cube. Choose the pair $(x_0,y_0)$ with $x$ minimal. Clearly $x_0,y_0>1$. Consider some prime $p$ dividing $x_0$ and write $x_0=px_1$. Then $pmid y_0^3implies y_0=py_1$ for some $y_1inmathbb Z_{geq 0}$. So $$p^2x_1^2=p^3y_0^3implies x_1^2=py_1^3implies pmid x_1$$ Write $x_1=px_2$, again with $x_2$ a positive integer. Then $$p^2x_2^2=py_1^3implies px_2^2=y_1^3implies pmid y_1$$ Next let $y_1=py_2$. We have that $$px_2^2=p^3y_2^3implies x_2^2=p^2y_3^3implies pmid x_2$$ Finally, take $x_2=px_3$. Our equation becomes $$p^2x_2^2=p^2y_2^3implies x_3^2=y_2^3$$
                            So $(x_3,y_2)$ is a pair of positive integers satisfying $x_3^2=y_2^3$, but clearly $x_3<p^3x_3=x_0$. A contradiction $blacksquare$






                            share|cite|improve this answer









                            $endgroup$



                            Here's a proof by infinite descent that $x$ is necessarily a perfect cube. The proof that $y$ is a perfect square is of the same exact spirit.



                            Without loss of generality assume that $x,y$ are positive. Suppose there were to exist pairs of positive integers $(x,y)$ with $x^2=y^3$ but $x$ not a perfect cube. Choose the pair $(x_0,y_0)$ with $x$ minimal. Clearly $x_0,y_0>1$. Consider some prime $p$ dividing $x_0$ and write $x_0=px_1$. Then $pmid y_0^3implies y_0=py_1$ for some $y_1inmathbb Z_{geq 0}$. So $$p^2x_1^2=p^3y_0^3implies x_1^2=py_1^3implies pmid x_1$$ Write $x_1=px_2$, again with $x_2$ a positive integer. Then $$p^2x_2^2=py_1^3implies px_2^2=y_1^3implies pmid y_1$$ Next let $y_1=py_2$. We have that $$px_2^2=p^3y_2^3implies x_2^2=p^2y_3^3implies pmid x_2$$ Finally, take $x_2=px_3$. Our equation becomes $$p^2x_2^2=p^2y_2^3implies x_3^2=y_2^3$$
                            So $(x_3,y_2)$ is a pair of positive integers satisfying $x_3^2=y_2^3$, but clearly $x_3<p^3x_3=x_0$. A contradiction $blacksquare$







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered Dec 19 '18 at 1:16









                            Rafay AsharyRafay Ashary

                            83618




                            83618















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