Degree of $a+b$ over a field $k$, where $a$ and $b$ are distinct roots of the same polynomial












6












$begingroup$


Let $K/k$ be an algebraic extension of fields with $a$ and $b$ distinct roots in $K$ of the same irreducible polynomial $f(x) in k[x]$ of degree $n$. Show that the degree of $k(a+b)/k$ is less than or equal to $frac{n(n-1)}{2}$.



Also, how does one construct fields $k$ and $K$ together with roots $a,bin K$ so that the preceding inequality is actually an equality?



I'm pretty sure I can get that $k(a+b)/k$ has degree less than or equal to $n(n-1)$ since the minimal polynomial of $b$ over $k(a)$ has degree less than or equal to $n-1$, but I'm not sure how to reduce this by a factor of $1/2$. I've also seen that there are computational techniques for computing the minimal polynomial of a sum, but a proof that avoids things such as resolvents would be ideal.










share|cite|improve this question











$endgroup$

















    6












    $begingroup$


    Let $K/k$ be an algebraic extension of fields with $a$ and $b$ distinct roots in $K$ of the same irreducible polynomial $f(x) in k[x]$ of degree $n$. Show that the degree of $k(a+b)/k$ is less than or equal to $frac{n(n-1)}{2}$.



    Also, how does one construct fields $k$ and $K$ together with roots $a,bin K$ so that the preceding inequality is actually an equality?



    I'm pretty sure I can get that $k(a+b)/k$ has degree less than or equal to $n(n-1)$ since the minimal polynomial of $b$ over $k(a)$ has degree less than or equal to $n-1$, but I'm not sure how to reduce this by a factor of $1/2$. I've also seen that there are computational techniques for computing the minimal polynomial of a sum, but a proof that avoids things such as resolvents would be ideal.










    share|cite|improve this question











    $endgroup$















      6












      6








      6


      2



      $begingroup$


      Let $K/k$ be an algebraic extension of fields with $a$ and $b$ distinct roots in $K$ of the same irreducible polynomial $f(x) in k[x]$ of degree $n$. Show that the degree of $k(a+b)/k$ is less than or equal to $frac{n(n-1)}{2}$.



      Also, how does one construct fields $k$ and $K$ together with roots $a,bin K$ so that the preceding inequality is actually an equality?



      I'm pretty sure I can get that $k(a+b)/k$ has degree less than or equal to $n(n-1)$ since the minimal polynomial of $b$ over $k(a)$ has degree less than or equal to $n-1$, but I'm not sure how to reduce this by a factor of $1/2$. I've also seen that there are computational techniques for computing the minimal polynomial of a sum, but a proof that avoids things such as resolvents would be ideal.










      share|cite|improve this question











      $endgroup$




      Let $K/k$ be an algebraic extension of fields with $a$ and $b$ distinct roots in $K$ of the same irreducible polynomial $f(x) in k[x]$ of degree $n$. Show that the degree of $k(a+b)/k$ is less than or equal to $frac{n(n-1)}{2}$.



      Also, how does one construct fields $k$ and $K$ together with roots $a,bin K$ so that the preceding inequality is actually an equality?



      I'm pretty sure I can get that $k(a+b)/k$ has degree less than or equal to $n(n-1)$ since the minimal polynomial of $b$ over $k(a)$ has degree less than or equal to $n-1$, but I'm not sure how to reduce this by a factor of $1/2$. I've also seen that there are computational techniques for computing the minimal polynomial of a sum, but a proof that avoids things such as resolvents would be ideal.







      abstract-algebra field-theory galois-theory






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Dec 25 '18 at 19:56







      leibnewtz

















      asked Dec 25 '18 at 19:43









      leibnewtzleibnewtz

      2,6111717




      2,6111717






















          1 Answer
          1






          active

          oldest

          votes


















          5












          $begingroup$

          If $f$ is inseparable, then $k$ has prime characteristic $p$ and $f(X) = X^{p^l} - t$ for some $tin k$ and $lgeq 0$. Then $a+b$ satisfies the polynomial $X^{p^l} - 2t$, which has degree $p^l=n$, which is $leq {n choose 2}$ when $ngeq 3$. $n=2$ is a special case, since $p=2$ and so $a+b=0$, while $n=1$ is impossible.



          On the other hand, suppose that $f$ is separable. Then every conjugate of $a+b$ is a sum of two distinct roots of $f$, and there are ${nchoose 2}$ of these. Since $prod_{gamma sim a+b} (X-gamma)$ is fixed by the Galois group, it has coefficients in $k$, and it follows that $a+b$ satisfies a polynomial of degree $leq {nchoose 2}$.



          To see that the inequality is tight, let $f$ be any polynomial with doubly-transitive Galois group, e.g. $S_n$, such that the pairwise sums of roots are distinct. (These should be common, but is there a good way to construct them?) Then $a+b$ has exactly ${nchoose 2}$ conjugates.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Very nice! I'm refraining from accepting for the moment just in case anyone is able to offer an explicit example
            $endgroup$
            – leibnewtz
            Dec 25 '18 at 20:58










          • $begingroup$
            @leibnewtz For starters, I'd guess that $x^n - sx - s$ is an example for most $sin mathbb{Q}$. See here for a discussion of why this has full Galois group.
            $endgroup$
            – Slade
            Dec 25 '18 at 21:52






          • 1




            $begingroup$
            Good job! Commenting on the last paragraph. If we let $K$ be the field of fractions of the $n$ variable polynomial ring $F[x_1,x_2,ldots,x_n]$, $F$ some field, and let $k$ be the subfield of symmetric rational functions, then $f(T)=prod_i(T-x_i)$ has coefficients in $k$ and works for this purpose. IOW, the same old :-)
            $endgroup$
            – Jyrki Lahtonen
            Dec 26 '18 at 9:27











          Your Answer





          StackExchange.ifUsing("editor", function () {
          return StackExchange.using("mathjaxEditing", function () {
          StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
          StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
          });
          });
          }, "mathjax-editing");

          StackExchange.ready(function() {
          var channelOptions = {
          tags: "".split(" "),
          id: "69"
          };
          initTagRenderer("".split(" "), "".split(" "), channelOptions);

          StackExchange.using("externalEditor", function() {
          // Have to fire editor after snippets, if snippets enabled
          if (StackExchange.settings.snippets.snippetsEnabled) {
          StackExchange.using("snippets", function() {
          createEditor();
          });
          }
          else {
          createEditor();
          }
          });

          function createEditor() {
          StackExchange.prepareEditor({
          heartbeatType: 'answer',
          autoActivateHeartbeat: false,
          convertImagesToLinks: true,
          noModals: true,
          showLowRepImageUploadWarning: true,
          reputationToPostImages: 10,
          bindNavPrevention: true,
          postfix: "",
          imageUploader: {
          brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
          contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
          allowUrls: true
          },
          noCode: true, onDemand: true,
          discardSelector: ".discard-answer"
          ,immediatelyShowMarkdownHelp:true
          });


          }
          });














          draft saved

          draft discarded


















          StackExchange.ready(
          function () {
          StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3052379%2fdegree-of-ab-over-a-field-k-where-a-and-b-are-distinct-roots-of-the-sa%23new-answer', 'question_page');
          }
          );

          Post as a guest















          Required, but never shown

























          1 Answer
          1






          active

          oldest

          votes








          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          5












          $begingroup$

          If $f$ is inseparable, then $k$ has prime characteristic $p$ and $f(X) = X^{p^l} - t$ for some $tin k$ and $lgeq 0$. Then $a+b$ satisfies the polynomial $X^{p^l} - 2t$, which has degree $p^l=n$, which is $leq {n choose 2}$ when $ngeq 3$. $n=2$ is a special case, since $p=2$ and so $a+b=0$, while $n=1$ is impossible.



          On the other hand, suppose that $f$ is separable. Then every conjugate of $a+b$ is a sum of two distinct roots of $f$, and there are ${nchoose 2}$ of these. Since $prod_{gamma sim a+b} (X-gamma)$ is fixed by the Galois group, it has coefficients in $k$, and it follows that $a+b$ satisfies a polynomial of degree $leq {nchoose 2}$.



          To see that the inequality is tight, let $f$ be any polynomial with doubly-transitive Galois group, e.g. $S_n$, such that the pairwise sums of roots are distinct. (These should be common, but is there a good way to construct them?) Then $a+b$ has exactly ${nchoose 2}$ conjugates.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Very nice! I'm refraining from accepting for the moment just in case anyone is able to offer an explicit example
            $endgroup$
            – leibnewtz
            Dec 25 '18 at 20:58










          • $begingroup$
            @leibnewtz For starters, I'd guess that $x^n - sx - s$ is an example for most $sin mathbb{Q}$. See here for a discussion of why this has full Galois group.
            $endgroup$
            – Slade
            Dec 25 '18 at 21:52






          • 1




            $begingroup$
            Good job! Commenting on the last paragraph. If we let $K$ be the field of fractions of the $n$ variable polynomial ring $F[x_1,x_2,ldots,x_n]$, $F$ some field, and let $k$ be the subfield of symmetric rational functions, then $f(T)=prod_i(T-x_i)$ has coefficients in $k$ and works for this purpose. IOW, the same old :-)
            $endgroup$
            – Jyrki Lahtonen
            Dec 26 '18 at 9:27
















          5












          $begingroup$

          If $f$ is inseparable, then $k$ has prime characteristic $p$ and $f(X) = X^{p^l} - t$ for some $tin k$ and $lgeq 0$. Then $a+b$ satisfies the polynomial $X^{p^l} - 2t$, which has degree $p^l=n$, which is $leq {n choose 2}$ when $ngeq 3$. $n=2$ is a special case, since $p=2$ and so $a+b=0$, while $n=1$ is impossible.



          On the other hand, suppose that $f$ is separable. Then every conjugate of $a+b$ is a sum of two distinct roots of $f$, and there are ${nchoose 2}$ of these. Since $prod_{gamma sim a+b} (X-gamma)$ is fixed by the Galois group, it has coefficients in $k$, and it follows that $a+b$ satisfies a polynomial of degree $leq {nchoose 2}$.



          To see that the inequality is tight, let $f$ be any polynomial with doubly-transitive Galois group, e.g. $S_n$, such that the pairwise sums of roots are distinct. (These should be common, but is there a good way to construct them?) Then $a+b$ has exactly ${nchoose 2}$ conjugates.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Very nice! I'm refraining from accepting for the moment just in case anyone is able to offer an explicit example
            $endgroup$
            – leibnewtz
            Dec 25 '18 at 20:58










          • $begingroup$
            @leibnewtz For starters, I'd guess that $x^n - sx - s$ is an example for most $sin mathbb{Q}$. See here for a discussion of why this has full Galois group.
            $endgroup$
            – Slade
            Dec 25 '18 at 21:52






          • 1




            $begingroup$
            Good job! Commenting on the last paragraph. If we let $K$ be the field of fractions of the $n$ variable polynomial ring $F[x_1,x_2,ldots,x_n]$, $F$ some field, and let $k$ be the subfield of symmetric rational functions, then $f(T)=prod_i(T-x_i)$ has coefficients in $k$ and works for this purpose. IOW, the same old :-)
            $endgroup$
            – Jyrki Lahtonen
            Dec 26 '18 at 9:27














          5












          5








          5





          $begingroup$

          If $f$ is inseparable, then $k$ has prime characteristic $p$ and $f(X) = X^{p^l} - t$ for some $tin k$ and $lgeq 0$. Then $a+b$ satisfies the polynomial $X^{p^l} - 2t$, which has degree $p^l=n$, which is $leq {n choose 2}$ when $ngeq 3$. $n=2$ is a special case, since $p=2$ and so $a+b=0$, while $n=1$ is impossible.



          On the other hand, suppose that $f$ is separable. Then every conjugate of $a+b$ is a sum of two distinct roots of $f$, and there are ${nchoose 2}$ of these. Since $prod_{gamma sim a+b} (X-gamma)$ is fixed by the Galois group, it has coefficients in $k$, and it follows that $a+b$ satisfies a polynomial of degree $leq {nchoose 2}$.



          To see that the inequality is tight, let $f$ be any polynomial with doubly-transitive Galois group, e.g. $S_n$, such that the pairwise sums of roots are distinct. (These should be common, but is there a good way to construct them?) Then $a+b$ has exactly ${nchoose 2}$ conjugates.






          share|cite|improve this answer











          $endgroup$



          If $f$ is inseparable, then $k$ has prime characteristic $p$ and $f(X) = X^{p^l} - t$ for some $tin k$ and $lgeq 0$. Then $a+b$ satisfies the polynomial $X^{p^l} - 2t$, which has degree $p^l=n$, which is $leq {n choose 2}$ when $ngeq 3$. $n=2$ is a special case, since $p=2$ and so $a+b=0$, while $n=1$ is impossible.



          On the other hand, suppose that $f$ is separable. Then every conjugate of $a+b$ is a sum of two distinct roots of $f$, and there are ${nchoose 2}$ of these. Since $prod_{gamma sim a+b} (X-gamma)$ is fixed by the Galois group, it has coefficients in $k$, and it follows that $a+b$ satisfies a polynomial of degree $leq {nchoose 2}$.



          To see that the inequality is tight, let $f$ be any polynomial with doubly-transitive Galois group, e.g. $S_n$, such that the pairwise sums of roots are distinct. (These should be common, but is there a good way to construct them?) Then $a+b$ has exactly ${nchoose 2}$ conjugates.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Dec 25 '18 at 20:24

























          answered Dec 25 '18 at 20:13









          SladeSlade

          25.2k12665




          25.2k12665












          • $begingroup$
            Very nice! I'm refraining from accepting for the moment just in case anyone is able to offer an explicit example
            $endgroup$
            – leibnewtz
            Dec 25 '18 at 20:58










          • $begingroup$
            @leibnewtz For starters, I'd guess that $x^n - sx - s$ is an example for most $sin mathbb{Q}$. See here for a discussion of why this has full Galois group.
            $endgroup$
            – Slade
            Dec 25 '18 at 21:52






          • 1




            $begingroup$
            Good job! Commenting on the last paragraph. If we let $K$ be the field of fractions of the $n$ variable polynomial ring $F[x_1,x_2,ldots,x_n]$, $F$ some field, and let $k$ be the subfield of symmetric rational functions, then $f(T)=prod_i(T-x_i)$ has coefficients in $k$ and works for this purpose. IOW, the same old :-)
            $endgroup$
            – Jyrki Lahtonen
            Dec 26 '18 at 9:27


















          • $begingroup$
            Very nice! I'm refraining from accepting for the moment just in case anyone is able to offer an explicit example
            $endgroup$
            – leibnewtz
            Dec 25 '18 at 20:58










          • $begingroup$
            @leibnewtz For starters, I'd guess that $x^n - sx - s$ is an example for most $sin mathbb{Q}$. See here for a discussion of why this has full Galois group.
            $endgroup$
            – Slade
            Dec 25 '18 at 21:52






          • 1




            $begingroup$
            Good job! Commenting on the last paragraph. If we let $K$ be the field of fractions of the $n$ variable polynomial ring $F[x_1,x_2,ldots,x_n]$, $F$ some field, and let $k$ be the subfield of symmetric rational functions, then $f(T)=prod_i(T-x_i)$ has coefficients in $k$ and works for this purpose. IOW, the same old :-)
            $endgroup$
            – Jyrki Lahtonen
            Dec 26 '18 at 9:27
















          $begingroup$
          Very nice! I'm refraining from accepting for the moment just in case anyone is able to offer an explicit example
          $endgroup$
          – leibnewtz
          Dec 25 '18 at 20:58




          $begingroup$
          Very nice! I'm refraining from accepting for the moment just in case anyone is able to offer an explicit example
          $endgroup$
          – leibnewtz
          Dec 25 '18 at 20:58












          $begingroup$
          @leibnewtz For starters, I'd guess that $x^n - sx - s$ is an example for most $sin mathbb{Q}$. See here for a discussion of why this has full Galois group.
          $endgroup$
          – Slade
          Dec 25 '18 at 21:52




          $begingroup$
          @leibnewtz For starters, I'd guess that $x^n - sx - s$ is an example for most $sin mathbb{Q}$. See here for a discussion of why this has full Galois group.
          $endgroup$
          – Slade
          Dec 25 '18 at 21:52




          1




          1




          $begingroup$
          Good job! Commenting on the last paragraph. If we let $K$ be the field of fractions of the $n$ variable polynomial ring $F[x_1,x_2,ldots,x_n]$, $F$ some field, and let $k$ be the subfield of symmetric rational functions, then $f(T)=prod_i(T-x_i)$ has coefficients in $k$ and works for this purpose. IOW, the same old :-)
          $endgroup$
          – Jyrki Lahtonen
          Dec 26 '18 at 9:27




          $begingroup$
          Good job! Commenting on the last paragraph. If we let $K$ be the field of fractions of the $n$ variable polynomial ring $F[x_1,x_2,ldots,x_n]$, $F$ some field, and let $k$ be the subfield of symmetric rational functions, then $f(T)=prod_i(T-x_i)$ has coefficients in $k$ and works for this purpose. IOW, the same old :-)
          $endgroup$
          – Jyrki Lahtonen
          Dec 26 '18 at 9:27


















          draft saved

          draft discarded




















































          Thanks for contributing an answer to Mathematics Stack Exchange!


          • Please be sure to answer the question. Provide details and share your research!

          But avoid



          • Asking for help, clarification, or responding to other answers.

          • Making statements based on opinion; back them up with references or personal experience.


          Use MathJax to format equations. MathJax reference.


          To learn more, see our tips on writing great answers.




          draft saved


          draft discarded














          StackExchange.ready(
          function () {
          StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3052379%2fdegree-of-ab-over-a-field-k-where-a-and-b-are-distinct-roots-of-the-sa%23new-answer', 'question_page');
          }
          );

          Post as a guest















          Required, but never shown





















































          Required, but never shown














          Required, but never shown












          Required, but never shown







          Required, but never shown

































          Required, but never shown














          Required, but never shown












          Required, but never shown







          Required, but never shown







          Popular posts from this blog

          Probability when a professor distributes a quiz and homework assignment to a class of n students.

          Aardman Animations

          Are they similar matrix