Show that if $n > 1$ is not a $k$-th power, then its $k$-th root is irrational, i.e. not of the form $u/v$...












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  • How to prove: if $a,b in mathbb N$, then $a^{1/b}$ is an integer or an irrational number?

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any solutions or suggestions on this one?



An integer $n$ is called a $k$-th power if $n = m^k$ for some $m ∈ Z$.
Show that if $n > 1$ is not a $k$-th power, then its $k$-th root is irrational, i.e. not of the form $u/v$ for $u, v ∈ N$.










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marked as duplicate by Robert Israel, Lord Shark the Unknown, Lee David Chung Lin, Leucippus, Martin Argerami Jan 31 at 4:35


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.























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    This question already has an answer here:




    • How to prove: if $a,b in mathbb N$, then $a^{1/b}$ is an integer or an irrational number?

      12 answers




    any solutions or suggestions on this one?



    An integer $n$ is called a $k$-th power if $n = m^k$ for some $m ∈ Z$.
    Show that if $n > 1$ is not a $k$-th power, then its $k$-th root is irrational, i.e. not of the form $u/v$ for $u, v ∈ N$.










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    marked as duplicate by Robert Israel, Lord Shark the Unknown, Lee David Chung Lin, Leucippus, Martin Argerami Jan 31 at 4:35


    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.





















      0












      0








      0





      $begingroup$



      This question already has an answer here:




      • How to prove: if $a,b in mathbb N$, then $a^{1/b}$ is an integer or an irrational number?

        12 answers




      any solutions or suggestions on this one?



      An integer $n$ is called a $k$-th power if $n = m^k$ for some $m ∈ Z$.
      Show that if $n > 1$ is not a $k$-th power, then its $k$-th root is irrational, i.e. not of the form $u/v$ for $u, v ∈ N$.










      share|cite|improve this question









      $endgroup$





      This question already has an answer here:




      • How to prove: if $a,b in mathbb N$, then $a^{1/b}$ is an integer or an irrational number?

        12 answers




      any solutions or suggestions on this one?



      An integer $n$ is called a $k$-th power if $n = m^k$ for some $m ∈ Z$.
      Show that if $n > 1$ is not a $k$-th power, then its $k$-th root is irrational, i.e. not of the form $u/v$ for $u, v ∈ N$.





      This question already has an answer here:




      • How to prove: if $a,b in mathbb N$, then $a^{1/b}$ is an integer or an irrational number?

        12 answers








      discrete-mathematics






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      asked Jan 30 at 22:38









      SomethingSomething

      817




      817




      marked as duplicate by Robert Israel, Lord Shark the Unknown, Lee David Chung Lin, Leucippus, Martin Argerami Jan 31 at 4:35


      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.









      marked as duplicate by Robert Israel, Lord Shark the Unknown, Lee David Chung Lin, Leucippus, Martin Argerami Jan 31 at 4:35


      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.
























          6 Answers
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          $begingroup$

          Assume $frac{u}{v}$ is in its lowest terms, i.e. $u$ and $v$ have no common factors, then $frac{u^k}{v^k}$ is in its lowest terms, and will not be an integer unless $v=1$.






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            0












            $begingroup$

            If $sqrt[k]{n} = u/v$, then $v^k n = u^k$. Consider now the power of each prime $p$ that appears in the factorization on both sides. You get a number of the form $ak+c$ on the LHS and a number of the form $bk$ on the $RHS$. Therefore, $ak+c=bk$ and so $c$ is a multiple of $k$. This means that $n$ is a $k$-th power.






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              Claim 1: Let $c$ and $d$ be integers. If there is a prime $p ge 2$ that divides $d$ but not $c$, then $frac{c}{d}$ is not an integer.




              Suppose the $k$-th power of $n$ is rational but nonintegral. Then let $a$ and $b$ be relatively prime integers satisfying $(frac{a}{b})^k = m$. Then as $a$ and $b$ are relatively prime, there is a prime $p ge 2$ that divides $b$ but not $a$. Thus letting $c=a^k$ and $d=b^k$, that prime $p$ divides $d$ but not $c$ [Note that the prime factors of $m^k$ are the same as that of $m$ for every integer $m$ and every positive integer $k$.] Thus $frac{c}{d} = frac{a^k}{b^k} = (frac{a}{b})^k$ cannot be the integer $n$ after all by Claim 1.






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                0












                $begingroup$

                How about proving by the contrapositive. We only care about $min Bbb Q / {Bbb Z}$, so let:
                $$m=frac ab, a,b text{ coprime }, bneq 1$$



                Then $$m^k =frac{a^k}{b^k}$$
                Which is also irreducible. In other words.



                $$min Bbb Q / {Bbb Z} implies m^kin Bbb Q / {Bbb Z}$$



                We thusly rule out the possibility $m$ is a rational non-integer given integer $m^k$, so it must either be an integer or irrational.






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                  0












                  $begingroup$

                  Your claim is equivalent to this:



                  If the $k$-th root of $n$ is rational, then $n$ is a $k$-th power.



                  So let $(u/v)^k = n$, i.e. $u^k = n v^k$. We may assume that $text{gcd}(u,v) = 1$. Then $u^k$ must divide $n$ which means $n = m u^k$ for some $m in mathbb{N}$. This yields $u^k = m u^k v^k$, i.e. $1 = m v^k$ which is possible only when $m = 1$ and $v = 1$. Hence $u/v = u in mathbb{N}$.






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                    0












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                    By contrapositive, without using prime factorisation:



                    Set $x = sqrt[uproot{1}leftroot{-1}n]{a}$, and denote $y$ its decimal part. Suppose $x$ is rational.



                    Claim: $y = 0$ (so that $sqrt[uproot{1}leftroot{-1}n]{a}$ is an integer).



                    Let $q$ be the smallest positive integer such that each $qx^i$, for $i = 1, dots, n-1$, is an integer. Note that $q' = qy$ is an integer, since $qx$ is.



                    On the other hand,
                    $$ q'x^i = q( x-lfloor xrfloor)x^i = qx^{i + 1} - lfloor xrfloor qx^i.$$
                    By hypothesis, in this formula, $qx^i$ and $qx^{i + 1}$ are integers if $i < n-1$ – and if $i = n-1$, $qx^{i + 1} = qa$ is an integer too. There results that each $q'x^i$ is an integer.



                    However $q$ is the smallest positive integer which has this property, and $0 ≤ q' < q$,so necessarily $q' = 0$, whence $y = 0$.






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                      6 Answers
                      6






                      active

                      oldest

                      votes








                      6 Answers
                      6






                      active

                      oldest

                      votes









                      active

                      oldest

                      votes






                      active

                      oldest

                      votes









                      1












                      $begingroup$

                      Assume $frac{u}{v}$ is in its lowest terms, i.e. $u$ and $v$ have no common factors, then $frac{u^k}{v^k}$ is in its lowest terms, and will not be an integer unless $v=1$.






                      share|cite|improve this answer









                      $endgroup$


















                        1












                        $begingroup$

                        Assume $frac{u}{v}$ is in its lowest terms, i.e. $u$ and $v$ have no common factors, then $frac{u^k}{v^k}$ is in its lowest terms, and will not be an integer unless $v=1$.






                        share|cite|improve this answer









                        $endgroup$
















                          1












                          1








                          1





                          $begingroup$

                          Assume $frac{u}{v}$ is in its lowest terms, i.e. $u$ and $v$ have no common factors, then $frac{u^k}{v^k}$ is in its lowest terms, and will not be an integer unless $v=1$.






                          share|cite|improve this answer









                          $endgroup$



                          Assume $frac{u}{v}$ is in its lowest terms, i.e. $u$ and $v$ have no common factors, then $frac{u^k}{v^k}$ is in its lowest terms, and will not be an integer unless $v=1$.







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                          answered Jan 30 at 22:49









                          herb steinbergherb steinberg

                          3,1132311




                          3,1132311























                              0












                              $begingroup$

                              If $sqrt[k]{n} = u/v$, then $v^k n = u^k$. Consider now the power of each prime $p$ that appears in the factorization on both sides. You get a number of the form $ak+c$ on the LHS and a number of the form $bk$ on the $RHS$. Therefore, $ak+c=bk$ and so $c$ is a multiple of $k$. This means that $n$ is a $k$-th power.






                              share|cite|improve this answer









                              $endgroup$


















                                0












                                $begingroup$

                                If $sqrt[k]{n} = u/v$, then $v^k n = u^k$. Consider now the power of each prime $p$ that appears in the factorization on both sides. You get a number of the form $ak+c$ on the LHS and a number of the form $bk$ on the $RHS$. Therefore, $ak+c=bk$ and so $c$ is a multiple of $k$. This means that $n$ is a $k$-th power.






                                share|cite|improve this answer









                                $endgroup$
















                                  0












                                  0








                                  0





                                  $begingroup$

                                  If $sqrt[k]{n} = u/v$, then $v^k n = u^k$. Consider now the power of each prime $p$ that appears in the factorization on both sides. You get a number of the form $ak+c$ on the LHS and a number of the form $bk$ on the $RHS$. Therefore, $ak+c=bk$ and so $c$ is a multiple of $k$. This means that $n$ is a $k$-th power.






                                  share|cite|improve this answer









                                  $endgroup$



                                  If $sqrt[k]{n} = u/v$, then $v^k n = u^k$. Consider now the power of each prime $p$ that appears in the factorization on both sides. You get a number of the form $ak+c$ on the LHS and a number of the form $bk$ on the $RHS$. Therefore, $ak+c=bk$ and so $c$ is a multiple of $k$. This means that $n$ is a $k$-th power.







                                  share|cite|improve this answer












                                  share|cite|improve this answer



                                  share|cite|improve this answer










                                  answered Jan 30 at 22:47









                                  lhflhf

                                  167k11172404




                                  167k11172404























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                                      $begingroup$


                                      Claim 1: Let $c$ and $d$ be integers. If there is a prime $p ge 2$ that divides $d$ but not $c$, then $frac{c}{d}$ is not an integer.




                                      Suppose the $k$-th power of $n$ is rational but nonintegral. Then let $a$ and $b$ be relatively prime integers satisfying $(frac{a}{b})^k = m$. Then as $a$ and $b$ are relatively prime, there is a prime $p ge 2$ that divides $b$ but not $a$. Thus letting $c=a^k$ and $d=b^k$, that prime $p$ divides $d$ but not $c$ [Note that the prime factors of $m^k$ are the same as that of $m$ for every integer $m$ and every positive integer $k$.] Thus $frac{c}{d} = frac{a^k}{b^k} = (frac{a}{b})^k$ cannot be the integer $n$ after all by Claim 1.






                                      share|cite|improve this answer









                                      $endgroup$


















                                        0












                                        $begingroup$


                                        Claim 1: Let $c$ and $d$ be integers. If there is a prime $p ge 2$ that divides $d$ but not $c$, then $frac{c}{d}$ is not an integer.




                                        Suppose the $k$-th power of $n$ is rational but nonintegral. Then let $a$ and $b$ be relatively prime integers satisfying $(frac{a}{b})^k = m$. Then as $a$ and $b$ are relatively prime, there is a prime $p ge 2$ that divides $b$ but not $a$. Thus letting $c=a^k$ and $d=b^k$, that prime $p$ divides $d$ but not $c$ [Note that the prime factors of $m^k$ are the same as that of $m$ for every integer $m$ and every positive integer $k$.] Thus $frac{c}{d} = frac{a^k}{b^k} = (frac{a}{b})^k$ cannot be the integer $n$ after all by Claim 1.






                                        share|cite|improve this answer









                                        $endgroup$
















                                          0












                                          0








                                          0





                                          $begingroup$


                                          Claim 1: Let $c$ and $d$ be integers. If there is a prime $p ge 2$ that divides $d$ but not $c$, then $frac{c}{d}$ is not an integer.




                                          Suppose the $k$-th power of $n$ is rational but nonintegral. Then let $a$ and $b$ be relatively prime integers satisfying $(frac{a}{b})^k = m$. Then as $a$ and $b$ are relatively prime, there is a prime $p ge 2$ that divides $b$ but not $a$. Thus letting $c=a^k$ and $d=b^k$, that prime $p$ divides $d$ but not $c$ [Note that the prime factors of $m^k$ are the same as that of $m$ for every integer $m$ and every positive integer $k$.] Thus $frac{c}{d} = frac{a^k}{b^k} = (frac{a}{b})^k$ cannot be the integer $n$ after all by Claim 1.






                                          share|cite|improve this answer









                                          $endgroup$




                                          Claim 1: Let $c$ and $d$ be integers. If there is a prime $p ge 2$ that divides $d$ but not $c$, then $frac{c}{d}$ is not an integer.




                                          Suppose the $k$-th power of $n$ is rational but nonintegral. Then let $a$ and $b$ be relatively prime integers satisfying $(frac{a}{b})^k = m$. Then as $a$ and $b$ are relatively prime, there is a prime $p ge 2$ that divides $b$ but not $a$. Thus letting $c=a^k$ and $d=b^k$, that prime $p$ divides $d$ but not $c$ [Note that the prime factors of $m^k$ are the same as that of $m$ for every integer $m$ and every positive integer $k$.] Thus $frac{c}{d} = frac{a^k}{b^k} = (frac{a}{b})^k$ cannot be the integer $n$ after all by Claim 1.







                                          share|cite|improve this answer












                                          share|cite|improve this answer



                                          share|cite|improve this answer










                                          answered Jan 30 at 22:47









                                          MikeMike

                                          4,611512




                                          4,611512























                                              0












                                              $begingroup$

                                              How about proving by the contrapositive. We only care about $min Bbb Q / {Bbb Z}$, so let:
                                              $$m=frac ab, a,b text{ coprime }, bneq 1$$



                                              Then $$m^k =frac{a^k}{b^k}$$
                                              Which is also irreducible. In other words.



                                              $$min Bbb Q / {Bbb Z} implies m^kin Bbb Q / {Bbb Z}$$



                                              We thusly rule out the possibility $m$ is a rational non-integer given integer $m^k$, so it must either be an integer or irrational.






                                              share|cite|improve this answer









                                              $endgroup$


















                                                0












                                                $begingroup$

                                                How about proving by the contrapositive. We only care about $min Bbb Q / {Bbb Z}$, so let:
                                                $$m=frac ab, a,b text{ coprime }, bneq 1$$



                                                Then $$m^k =frac{a^k}{b^k}$$
                                                Which is also irreducible. In other words.



                                                $$min Bbb Q / {Bbb Z} implies m^kin Bbb Q / {Bbb Z}$$



                                                We thusly rule out the possibility $m$ is a rational non-integer given integer $m^k$, so it must either be an integer or irrational.






                                                share|cite|improve this answer









                                                $endgroup$
















                                                  0












                                                  0








                                                  0





                                                  $begingroup$

                                                  How about proving by the contrapositive. We only care about $min Bbb Q / {Bbb Z}$, so let:
                                                  $$m=frac ab, a,b text{ coprime }, bneq 1$$



                                                  Then $$m^k =frac{a^k}{b^k}$$
                                                  Which is also irreducible. In other words.



                                                  $$min Bbb Q / {Bbb Z} implies m^kin Bbb Q / {Bbb Z}$$



                                                  We thusly rule out the possibility $m$ is a rational non-integer given integer $m^k$, so it must either be an integer or irrational.






                                                  share|cite|improve this answer









                                                  $endgroup$



                                                  How about proving by the contrapositive. We only care about $min Bbb Q / {Bbb Z}$, so let:
                                                  $$m=frac ab, a,b text{ coprime }, bneq 1$$



                                                  Then $$m^k =frac{a^k}{b^k}$$
                                                  Which is also irreducible. In other words.



                                                  $$min Bbb Q / {Bbb Z} implies m^kin Bbb Q / {Bbb Z}$$



                                                  We thusly rule out the possibility $m$ is a rational non-integer given integer $m^k$, so it must either be an integer or irrational.







                                                  share|cite|improve this answer












                                                  share|cite|improve this answer



                                                  share|cite|improve this answer










                                                  answered Jan 30 at 22:50









                                                  Rhys HughesRhys Hughes

                                                  7,0501630




                                                  7,0501630























                                                      0












                                                      $begingroup$

                                                      Your claim is equivalent to this:



                                                      If the $k$-th root of $n$ is rational, then $n$ is a $k$-th power.



                                                      So let $(u/v)^k = n$, i.e. $u^k = n v^k$. We may assume that $text{gcd}(u,v) = 1$. Then $u^k$ must divide $n$ which means $n = m u^k$ for some $m in mathbb{N}$. This yields $u^k = m u^k v^k$, i.e. $1 = m v^k$ which is possible only when $m = 1$ and $v = 1$. Hence $u/v = u in mathbb{N}$.






                                                      share|cite|improve this answer









                                                      $endgroup$


















                                                        0












                                                        $begingroup$

                                                        Your claim is equivalent to this:



                                                        If the $k$-th root of $n$ is rational, then $n$ is a $k$-th power.



                                                        So let $(u/v)^k = n$, i.e. $u^k = n v^k$. We may assume that $text{gcd}(u,v) = 1$. Then $u^k$ must divide $n$ which means $n = m u^k$ for some $m in mathbb{N}$. This yields $u^k = m u^k v^k$, i.e. $1 = m v^k$ which is possible only when $m = 1$ and $v = 1$. Hence $u/v = u in mathbb{N}$.






                                                        share|cite|improve this answer









                                                        $endgroup$
















                                                          0












                                                          0








                                                          0





                                                          $begingroup$

                                                          Your claim is equivalent to this:



                                                          If the $k$-th root of $n$ is rational, then $n$ is a $k$-th power.



                                                          So let $(u/v)^k = n$, i.e. $u^k = n v^k$. We may assume that $text{gcd}(u,v) = 1$. Then $u^k$ must divide $n$ which means $n = m u^k$ for some $m in mathbb{N}$. This yields $u^k = m u^k v^k$, i.e. $1 = m v^k$ which is possible only when $m = 1$ and $v = 1$. Hence $u/v = u in mathbb{N}$.






                                                          share|cite|improve this answer









                                                          $endgroup$



                                                          Your claim is equivalent to this:



                                                          If the $k$-th root of $n$ is rational, then $n$ is a $k$-th power.



                                                          So let $(u/v)^k = n$, i.e. $u^k = n v^k$. We may assume that $text{gcd}(u,v) = 1$. Then $u^k$ must divide $n$ which means $n = m u^k$ for some $m in mathbb{N}$. This yields $u^k = m u^k v^k$, i.e. $1 = m v^k$ which is possible only when $m = 1$ and $v = 1$. Hence $u/v = u in mathbb{N}$.







                                                          share|cite|improve this answer












                                                          share|cite|improve this answer



                                                          share|cite|improve this answer










                                                          answered Jan 30 at 22:56









                                                          Paul FrostPaul Frost

                                                          12.2k3935




                                                          12.2k3935























                                                              0












                                                              $begingroup$

                                                              By contrapositive, without using prime factorisation:



                                                              Set $x = sqrt[uproot{1}leftroot{-1}n]{a}$, and denote $y$ its decimal part. Suppose $x$ is rational.



                                                              Claim: $y = 0$ (so that $sqrt[uproot{1}leftroot{-1}n]{a}$ is an integer).



                                                              Let $q$ be the smallest positive integer such that each $qx^i$, for $i = 1, dots, n-1$, is an integer. Note that $q' = qy$ is an integer, since $qx$ is.



                                                              On the other hand,
                                                              $$ q'x^i = q( x-lfloor xrfloor)x^i = qx^{i + 1} - lfloor xrfloor qx^i.$$
                                                              By hypothesis, in this formula, $qx^i$ and $qx^{i + 1}$ are integers if $i < n-1$ – and if $i = n-1$, $qx^{i + 1} = qa$ is an integer too. There results that each $q'x^i$ is an integer.



                                                              However $q$ is the smallest positive integer which has this property, and $0 ≤ q' < q$,so necessarily $q' = 0$, whence $y = 0$.






                                                              share|cite|improve this answer









                                                              $endgroup$


















                                                                0












                                                                $begingroup$

                                                                By contrapositive, without using prime factorisation:



                                                                Set $x = sqrt[uproot{1}leftroot{-1}n]{a}$, and denote $y$ its decimal part. Suppose $x$ is rational.



                                                                Claim: $y = 0$ (so that $sqrt[uproot{1}leftroot{-1}n]{a}$ is an integer).



                                                                Let $q$ be the smallest positive integer such that each $qx^i$, for $i = 1, dots, n-1$, is an integer. Note that $q' = qy$ is an integer, since $qx$ is.



                                                                On the other hand,
                                                                $$ q'x^i = q( x-lfloor xrfloor)x^i = qx^{i + 1} - lfloor xrfloor qx^i.$$
                                                                By hypothesis, in this formula, $qx^i$ and $qx^{i + 1}$ are integers if $i < n-1$ – and if $i = n-1$, $qx^{i + 1} = qa$ is an integer too. There results that each $q'x^i$ is an integer.



                                                                However $q$ is the smallest positive integer which has this property, and $0 ≤ q' < q$,so necessarily $q' = 0$, whence $y = 0$.






                                                                share|cite|improve this answer









                                                                $endgroup$
















                                                                  0












                                                                  0








                                                                  0





                                                                  $begingroup$

                                                                  By contrapositive, without using prime factorisation:



                                                                  Set $x = sqrt[uproot{1}leftroot{-1}n]{a}$, and denote $y$ its decimal part. Suppose $x$ is rational.



                                                                  Claim: $y = 0$ (so that $sqrt[uproot{1}leftroot{-1}n]{a}$ is an integer).



                                                                  Let $q$ be the smallest positive integer such that each $qx^i$, for $i = 1, dots, n-1$, is an integer. Note that $q' = qy$ is an integer, since $qx$ is.



                                                                  On the other hand,
                                                                  $$ q'x^i = q( x-lfloor xrfloor)x^i = qx^{i + 1} - lfloor xrfloor qx^i.$$
                                                                  By hypothesis, in this formula, $qx^i$ and $qx^{i + 1}$ are integers if $i < n-1$ – and if $i = n-1$, $qx^{i + 1} = qa$ is an integer too. There results that each $q'x^i$ is an integer.



                                                                  However $q$ is the smallest positive integer which has this property, and $0 ≤ q' < q$,so necessarily $q' = 0$, whence $y = 0$.






                                                                  share|cite|improve this answer









                                                                  $endgroup$



                                                                  By contrapositive, without using prime factorisation:



                                                                  Set $x = sqrt[uproot{1}leftroot{-1}n]{a}$, and denote $y$ its decimal part. Suppose $x$ is rational.



                                                                  Claim: $y = 0$ (so that $sqrt[uproot{1}leftroot{-1}n]{a}$ is an integer).



                                                                  Let $q$ be the smallest positive integer such that each $qx^i$, for $i = 1, dots, n-1$, is an integer. Note that $q' = qy$ is an integer, since $qx$ is.



                                                                  On the other hand,
                                                                  $$ q'x^i = q( x-lfloor xrfloor)x^i = qx^{i + 1} - lfloor xrfloor qx^i.$$
                                                                  By hypothesis, in this formula, $qx^i$ and $qx^{i + 1}$ are integers if $i < n-1$ – and if $i = n-1$, $qx^{i + 1} = qa$ is an integer too. There results that each $q'x^i$ is an integer.



                                                                  However $q$ is the smallest positive integer which has this property, and $0 ≤ q' < q$,so necessarily $q' = 0$, whence $y = 0$.







                                                                  share|cite|improve this answer












                                                                  share|cite|improve this answer



                                                                  share|cite|improve this answer










                                                                  answered Jan 30 at 23:39









                                                                  BernardBernard

                                                                  124k741116




                                                                  124k741116















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