Finding degree of a finite field extension












5












$begingroup$


Let $x=sqrt{2}+sqrt{3}+ldots+sqrt{n}, ngeq 2$. I want to show that $[mathbb{Q}(x):mathbb{Q}]=2^{phi(n)}$, where $phi$ is Euler's totient function.



I know that if $p_1,ldots,p_n$ are pairwise relatively prime then $[mathbb{Q}(sqrt{p_1}+ldots+sqrt{p_n}):mathbb{Q}]=2^n$. But how to proceed in the above case? I could not apply induction also. Any help is appreciated.




The assertion is false. Actually $[mathbb{Q}(x):mathbb{Q}]=2^{pi(n)}$, where $pi(n)$ is the number of prime numbers less than or equal to $n$.











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  • $begingroup$
    Is this a conjecture or a thm?
    $endgroup$
    – Wuestenfux
    Apr 13 at 15:18










  • $begingroup$
    This is a theorem.
    $endgroup$
    – Anupam
    Apr 13 at 15:23






  • 1




    $begingroup$
    Why not $2^{pi(n)}$?
    $endgroup$
    – Lord Shark the Unknown
    Apr 13 at 15:40
















5












$begingroup$


Let $x=sqrt{2}+sqrt{3}+ldots+sqrt{n}, ngeq 2$. I want to show that $[mathbb{Q}(x):mathbb{Q}]=2^{phi(n)}$, where $phi$ is Euler's totient function.



I know that if $p_1,ldots,p_n$ are pairwise relatively prime then $[mathbb{Q}(sqrt{p_1}+ldots+sqrt{p_n}):mathbb{Q}]=2^n$. But how to proceed in the above case? I could not apply induction also. Any help is appreciated.




The assertion is false. Actually $[mathbb{Q}(x):mathbb{Q}]=2^{pi(n)}$, where $pi(n)$ is the number of prime numbers less than or equal to $n$.











share|cite|improve this question











$endgroup$












  • $begingroup$
    Is this a conjecture or a thm?
    $endgroup$
    – Wuestenfux
    Apr 13 at 15:18










  • $begingroup$
    This is a theorem.
    $endgroup$
    – Anupam
    Apr 13 at 15:23






  • 1




    $begingroup$
    Why not $2^{pi(n)}$?
    $endgroup$
    – Lord Shark the Unknown
    Apr 13 at 15:40














5












5








5


3



$begingroup$


Let $x=sqrt{2}+sqrt{3}+ldots+sqrt{n}, ngeq 2$. I want to show that $[mathbb{Q}(x):mathbb{Q}]=2^{phi(n)}$, where $phi$ is Euler's totient function.



I know that if $p_1,ldots,p_n$ are pairwise relatively prime then $[mathbb{Q}(sqrt{p_1}+ldots+sqrt{p_n}):mathbb{Q}]=2^n$. But how to proceed in the above case? I could not apply induction also. Any help is appreciated.




The assertion is false. Actually $[mathbb{Q}(x):mathbb{Q}]=2^{pi(n)}$, where $pi(n)$ is the number of prime numbers less than or equal to $n$.











share|cite|improve this question











$endgroup$




Let $x=sqrt{2}+sqrt{3}+ldots+sqrt{n}, ngeq 2$. I want to show that $[mathbb{Q}(x):mathbb{Q}]=2^{phi(n)}$, where $phi$ is Euler's totient function.



I know that if $p_1,ldots,p_n$ are pairwise relatively prime then $[mathbb{Q}(sqrt{p_1}+ldots+sqrt{p_n}):mathbb{Q}]=2^n$. But how to proceed in the above case? I could not apply induction also. Any help is appreciated.




The assertion is false. Actually $[mathbb{Q}(x):mathbb{Q}]=2^{pi(n)}$, where $pi(n)$ is the number of prime numbers less than or equal to $n$.








abstract-algebra galois-theory extension-field






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share|cite|improve this question













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share|cite|improve this question








edited Apr 13 at 16:41







Anupam

















asked Apr 13 at 15:05









AnupamAnupam

2,5791925




2,5791925












  • $begingroup$
    Is this a conjecture or a thm?
    $endgroup$
    – Wuestenfux
    Apr 13 at 15:18










  • $begingroup$
    This is a theorem.
    $endgroup$
    – Anupam
    Apr 13 at 15:23






  • 1




    $begingroup$
    Why not $2^{pi(n)}$?
    $endgroup$
    – Lord Shark the Unknown
    Apr 13 at 15:40


















  • $begingroup$
    Is this a conjecture or a thm?
    $endgroup$
    – Wuestenfux
    Apr 13 at 15:18










  • $begingroup$
    This is a theorem.
    $endgroup$
    – Anupam
    Apr 13 at 15:23






  • 1




    $begingroup$
    Why not $2^{pi(n)}$?
    $endgroup$
    – Lord Shark the Unknown
    Apr 13 at 15:40
















$begingroup$
Is this a conjecture or a thm?
$endgroup$
– Wuestenfux
Apr 13 at 15:18




$begingroup$
Is this a conjecture or a thm?
$endgroup$
– Wuestenfux
Apr 13 at 15:18












$begingroup$
This is a theorem.
$endgroup$
– Anupam
Apr 13 at 15:23




$begingroup$
This is a theorem.
$endgroup$
– Anupam
Apr 13 at 15:23




1




1




$begingroup$
Why not $2^{pi(n)}$?
$endgroup$
– Lord Shark the Unknown
Apr 13 at 15:40




$begingroup$
Why not $2^{pi(n)}$?
$endgroup$
– Lord Shark the Unknown
Apr 13 at 15:40










3 Answers
3






active

oldest

votes


















4












$begingroup$

Let $L= mathbb Q ( sum _{j=1} ^n sqrt j )$ , $k= mathbb Q$ and $ N = mathbb Q ( sqrt 2, sqrt 3 ,... , sqrt n ) $ .



Clearly $ N|_k $ is Galois and the Galois group is of the form $ mathbb Z_2 ^m$ for some $m$ since every $k$ automorphism of $N$ has order at most $2$. Note that each element of $Gal (N|_k)$ is completely specified by it's action on $ { sqrt p : p prime p leq n } $ by the fundamental theorem of arithmetic. So this gives $$ m leq pi (n)$$



Now if the Galois group is $ mathbb Z_2 ^m $ then it will have $2^m -1$ subgroups of index $2$ and hence there exist $2^m -1 $ subfields $F$ of $N $ containing $k$ such that $F:k=2$ . But we already have $ 2^ {pi (n)} -1$ many such subfields by taking product of a nonempty subset of $ { sqrt p : p prime p leq n } $ and hence we get $$ 2^ {pi (n)} -1 leq 2^ m -1 $$
$$ implies pi (n) leq m $$



And hence $$Gal ( N|_k) = mathbb Z_2 ^ {pi(n)} $$



Now we just observe that the orbit of $ sum _{j=1} ^n sqrt j $ under the action of $Gal(N|_k) $ contains $2^ {pi (n)} $ distinct elements by linear independence of $ { sqrt {p_i }, sqrt {p_ip_j},... } $ and hence $N= L$



So $$Gal left ( mathbb Q ( sum _{j=1} ^n sqrt j ) |_ {mathbb Q} right ) cong mathbb Z _2 ^ {pi (n)} $$






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Yeah I am wrong. Thus I am changing the question.
    $endgroup$
    – Anupam
    Apr 13 at 16:45










  • $begingroup$
    @Soumik Ghosh You should have insisted on the "linear independence of {$sqrt {p_i},sqrt {p_ip_j},...$}", which I think is the cumbersome point.
    $endgroup$
    – nguyen quang do
    Apr 14 at 7:28












  • $begingroup$
    OP already knows $mathbb Q(sqrt { p_i } : 1=1,...,n) :mathbb Q$ is of degree $2^n$ . Then linear independence is a triviality.
    $endgroup$
    – Ignorant Mathematician
    Apr 14 at 7:37





















1












$begingroup$

This is false...take n=5 for instance






share|cite|improve this answer









$endgroup$





















    1












    $begingroup$

    I think that Kummer gives the neatest proof, using only the multiplicative structure of $mathbf Q^*$. For a fixed $nge 2$, let $K=mathbf Q (sqrt 2, sqrt 3,...,sqrt n)$ and $mu_2=(pm 1)$. Kummer theory tells us that $K/mathbf Q$ is an abelian extension, with Galois group $Gcong Hom (V,mu_2)$, where $V$ is the subgroup of $mathbf Q^*/{mathbf Q^*}^2$ generated by the classes $bar 2,...,bar n$ mod ${mathbf Q^*}^2$. Although $V$ is a multiplicative group, it will be convenient to view it as a vector space over $mathbf F_2$, a linear combination of $bar 2,...,bar n$ mod ${mathbf Q^*}^2$ being just a product ${bar 2}^{epsilon_2}...{bar n}^{epsilon_n}$, with $epsilon_i=0$ or $1$. We aim to show that the $mathbf F_2$-dimension of $V$ is $pi (n)$, the number of rational primes $le n$. Let $W$ be the $mathbf F_2$-subspace generated by the classes of these primes. For for any $mle n$, the prime factorization of $m$ in $mathbf Z$ immediately shows that $bar m$ is a linear combination of the classes of the primes $le m$, which implies that $V=W$. It remains only to show that $W$ has $mathbf F_2$-dimension $pi (n)$, e.g. that the classes $bar p_i$ of the primes $le n$ form a basis. But a relation of linear dependence among them would mean that some finite product $prod p_i$ is a rational square, which contradicts the fact that $mathbf Z$ is a UFD.






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      3 Answers
      3






      active

      oldest

      votes








      3 Answers
      3






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      4












      $begingroup$

      Let $L= mathbb Q ( sum _{j=1} ^n sqrt j )$ , $k= mathbb Q$ and $ N = mathbb Q ( sqrt 2, sqrt 3 ,... , sqrt n ) $ .



      Clearly $ N|_k $ is Galois and the Galois group is of the form $ mathbb Z_2 ^m$ for some $m$ since every $k$ automorphism of $N$ has order at most $2$. Note that each element of $Gal (N|_k)$ is completely specified by it's action on $ { sqrt p : p prime p leq n } $ by the fundamental theorem of arithmetic. So this gives $$ m leq pi (n)$$



      Now if the Galois group is $ mathbb Z_2 ^m $ then it will have $2^m -1$ subgroups of index $2$ and hence there exist $2^m -1 $ subfields $F$ of $N $ containing $k$ such that $F:k=2$ . But we already have $ 2^ {pi (n)} -1$ many such subfields by taking product of a nonempty subset of $ { sqrt p : p prime p leq n } $ and hence we get $$ 2^ {pi (n)} -1 leq 2^ m -1 $$
      $$ implies pi (n) leq m $$



      And hence $$Gal ( N|_k) = mathbb Z_2 ^ {pi(n)} $$



      Now we just observe that the orbit of $ sum _{j=1} ^n sqrt j $ under the action of $Gal(N|_k) $ contains $2^ {pi (n)} $ distinct elements by linear independence of $ { sqrt {p_i }, sqrt {p_ip_j},... } $ and hence $N= L$



      So $$Gal left ( mathbb Q ( sum _{j=1} ^n sqrt j ) |_ {mathbb Q} right ) cong mathbb Z _2 ^ {pi (n)} $$






      share|cite|improve this answer











      $endgroup$













      • $begingroup$
        Yeah I am wrong. Thus I am changing the question.
        $endgroup$
        – Anupam
        Apr 13 at 16:45










      • $begingroup$
        @Soumik Ghosh You should have insisted on the "linear independence of {$sqrt {p_i},sqrt {p_ip_j},...$}", which I think is the cumbersome point.
        $endgroup$
        – nguyen quang do
        Apr 14 at 7:28












      • $begingroup$
        OP already knows $mathbb Q(sqrt { p_i } : 1=1,...,n) :mathbb Q$ is of degree $2^n$ . Then linear independence is a triviality.
        $endgroup$
        – Ignorant Mathematician
        Apr 14 at 7:37


















      4












      $begingroup$

      Let $L= mathbb Q ( sum _{j=1} ^n sqrt j )$ , $k= mathbb Q$ and $ N = mathbb Q ( sqrt 2, sqrt 3 ,... , sqrt n ) $ .



      Clearly $ N|_k $ is Galois and the Galois group is of the form $ mathbb Z_2 ^m$ for some $m$ since every $k$ automorphism of $N$ has order at most $2$. Note that each element of $Gal (N|_k)$ is completely specified by it's action on $ { sqrt p : p prime p leq n } $ by the fundamental theorem of arithmetic. So this gives $$ m leq pi (n)$$



      Now if the Galois group is $ mathbb Z_2 ^m $ then it will have $2^m -1$ subgroups of index $2$ and hence there exist $2^m -1 $ subfields $F$ of $N $ containing $k$ such that $F:k=2$ . But we already have $ 2^ {pi (n)} -1$ many such subfields by taking product of a nonempty subset of $ { sqrt p : p prime p leq n } $ and hence we get $$ 2^ {pi (n)} -1 leq 2^ m -1 $$
      $$ implies pi (n) leq m $$



      And hence $$Gal ( N|_k) = mathbb Z_2 ^ {pi(n)} $$



      Now we just observe that the orbit of $ sum _{j=1} ^n sqrt j $ under the action of $Gal(N|_k) $ contains $2^ {pi (n)} $ distinct elements by linear independence of $ { sqrt {p_i }, sqrt {p_ip_j},... } $ and hence $N= L$



      So $$Gal left ( mathbb Q ( sum _{j=1} ^n sqrt j ) |_ {mathbb Q} right ) cong mathbb Z _2 ^ {pi (n)} $$






      share|cite|improve this answer











      $endgroup$













      • $begingroup$
        Yeah I am wrong. Thus I am changing the question.
        $endgroup$
        – Anupam
        Apr 13 at 16:45










      • $begingroup$
        @Soumik Ghosh You should have insisted on the "linear independence of {$sqrt {p_i},sqrt {p_ip_j},...$}", which I think is the cumbersome point.
        $endgroup$
        – nguyen quang do
        Apr 14 at 7:28












      • $begingroup$
        OP already knows $mathbb Q(sqrt { p_i } : 1=1,...,n) :mathbb Q$ is of degree $2^n$ . Then linear independence is a triviality.
        $endgroup$
        – Ignorant Mathematician
        Apr 14 at 7:37
















      4












      4








      4





      $begingroup$

      Let $L= mathbb Q ( sum _{j=1} ^n sqrt j )$ , $k= mathbb Q$ and $ N = mathbb Q ( sqrt 2, sqrt 3 ,... , sqrt n ) $ .



      Clearly $ N|_k $ is Galois and the Galois group is of the form $ mathbb Z_2 ^m$ for some $m$ since every $k$ automorphism of $N$ has order at most $2$. Note that each element of $Gal (N|_k)$ is completely specified by it's action on $ { sqrt p : p prime p leq n } $ by the fundamental theorem of arithmetic. So this gives $$ m leq pi (n)$$



      Now if the Galois group is $ mathbb Z_2 ^m $ then it will have $2^m -1$ subgroups of index $2$ and hence there exist $2^m -1 $ subfields $F$ of $N $ containing $k$ such that $F:k=2$ . But we already have $ 2^ {pi (n)} -1$ many such subfields by taking product of a nonempty subset of $ { sqrt p : p prime p leq n } $ and hence we get $$ 2^ {pi (n)} -1 leq 2^ m -1 $$
      $$ implies pi (n) leq m $$



      And hence $$Gal ( N|_k) = mathbb Z_2 ^ {pi(n)} $$



      Now we just observe that the orbit of $ sum _{j=1} ^n sqrt j $ under the action of $Gal(N|_k) $ contains $2^ {pi (n)} $ distinct elements by linear independence of $ { sqrt {p_i }, sqrt {p_ip_j},... } $ and hence $N= L$



      So $$Gal left ( mathbb Q ( sum _{j=1} ^n sqrt j ) |_ {mathbb Q} right ) cong mathbb Z _2 ^ {pi (n)} $$






      share|cite|improve this answer











      $endgroup$



      Let $L= mathbb Q ( sum _{j=1} ^n sqrt j )$ , $k= mathbb Q$ and $ N = mathbb Q ( sqrt 2, sqrt 3 ,... , sqrt n ) $ .



      Clearly $ N|_k $ is Galois and the Galois group is of the form $ mathbb Z_2 ^m$ for some $m$ since every $k$ automorphism of $N$ has order at most $2$. Note that each element of $Gal (N|_k)$ is completely specified by it's action on $ { sqrt p : p prime p leq n } $ by the fundamental theorem of arithmetic. So this gives $$ m leq pi (n)$$



      Now if the Galois group is $ mathbb Z_2 ^m $ then it will have $2^m -1$ subgroups of index $2$ and hence there exist $2^m -1 $ subfields $F$ of $N $ containing $k$ such that $F:k=2$ . But we already have $ 2^ {pi (n)} -1$ many such subfields by taking product of a nonempty subset of $ { sqrt p : p prime p leq n } $ and hence we get $$ 2^ {pi (n)} -1 leq 2^ m -1 $$
      $$ implies pi (n) leq m $$



      And hence $$Gal ( N|_k) = mathbb Z_2 ^ {pi(n)} $$



      Now we just observe that the orbit of $ sum _{j=1} ^n sqrt j $ under the action of $Gal(N|_k) $ contains $2^ {pi (n)} $ distinct elements by linear independence of $ { sqrt {p_i }, sqrt {p_ip_j},... } $ and hence $N= L$



      So $$Gal left ( mathbb Q ( sum _{j=1} ^n sqrt j ) |_ {mathbb Q} right ) cong mathbb Z _2 ^ {pi (n)} $$







      share|cite|improve this answer














      share|cite|improve this answer



      share|cite|improve this answer








      edited Apr 13 at 19:04

























      answered Apr 13 at 15:36









      Ignorant MathematicianIgnorant Mathematician

      1,850114




      1,850114












      • $begingroup$
        Yeah I am wrong. Thus I am changing the question.
        $endgroup$
        – Anupam
        Apr 13 at 16:45










      • $begingroup$
        @Soumik Ghosh You should have insisted on the "linear independence of {$sqrt {p_i},sqrt {p_ip_j},...$}", which I think is the cumbersome point.
        $endgroup$
        – nguyen quang do
        Apr 14 at 7:28












      • $begingroup$
        OP already knows $mathbb Q(sqrt { p_i } : 1=1,...,n) :mathbb Q$ is of degree $2^n$ . Then linear independence is a triviality.
        $endgroup$
        – Ignorant Mathematician
        Apr 14 at 7:37




















      • $begingroup$
        Yeah I am wrong. Thus I am changing the question.
        $endgroup$
        – Anupam
        Apr 13 at 16:45










      • $begingroup$
        @Soumik Ghosh You should have insisted on the "linear independence of {$sqrt {p_i},sqrt {p_ip_j},...$}", which I think is the cumbersome point.
        $endgroup$
        – nguyen quang do
        Apr 14 at 7:28












      • $begingroup$
        OP already knows $mathbb Q(sqrt { p_i } : 1=1,...,n) :mathbb Q$ is of degree $2^n$ . Then linear independence is a triviality.
        $endgroup$
        – Ignorant Mathematician
        Apr 14 at 7:37


















      $begingroup$
      Yeah I am wrong. Thus I am changing the question.
      $endgroup$
      – Anupam
      Apr 13 at 16:45




      $begingroup$
      Yeah I am wrong. Thus I am changing the question.
      $endgroup$
      – Anupam
      Apr 13 at 16:45












      $begingroup$
      @Soumik Ghosh You should have insisted on the "linear independence of {$sqrt {p_i},sqrt {p_ip_j},...$}", which I think is the cumbersome point.
      $endgroup$
      – nguyen quang do
      Apr 14 at 7:28






      $begingroup$
      @Soumik Ghosh You should have insisted on the "linear independence of {$sqrt {p_i},sqrt {p_ip_j},...$}", which I think is the cumbersome point.
      $endgroup$
      – nguyen quang do
      Apr 14 at 7:28














      $begingroup$
      OP already knows $mathbb Q(sqrt { p_i } : 1=1,...,n) :mathbb Q$ is of degree $2^n$ . Then linear independence is a triviality.
      $endgroup$
      – Ignorant Mathematician
      Apr 14 at 7:37






      $begingroup$
      OP already knows $mathbb Q(sqrt { p_i } : 1=1,...,n) :mathbb Q$ is of degree $2^n$ . Then linear independence is a triviality.
      $endgroup$
      – Ignorant Mathematician
      Apr 14 at 7:37













      1












      $begingroup$

      This is false...take n=5 for instance






      share|cite|improve this answer









      $endgroup$


















        1












        $begingroup$

        This is false...take n=5 for instance






        share|cite|improve this answer









        $endgroup$
















          1












          1








          1





          $begingroup$

          This is false...take n=5 for instance






          share|cite|improve this answer









          $endgroup$



          This is false...take n=5 for instance







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Apr 13 at 15:49









          Sagnik DuttaSagnik Dutta

          542




          542























              1












              $begingroup$

              I think that Kummer gives the neatest proof, using only the multiplicative structure of $mathbf Q^*$. For a fixed $nge 2$, let $K=mathbf Q (sqrt 2, sqrt 3,...,sqrt n)$ and $mu_2=(pm 1)$. Kummer theory tells us that $K/mathbf Q$ is an abelian extension, with Galois group $Gcong Hom (V,mu_2)$, where $V$ is the subgroup of $mathbf Q^*/{mathbf Q^*}^2$ generated by the classes $bar 2,...,bar n$ mod ${mathbf Q^*}^2$. Although $V$ is a multiplicative group, it will be convenient to view it as a vector space over $mathbf F_2$, a linear combination of $bar 2,...,bar n$ mod ${mathbf Q^*}^2$ being just a product ${bar 2}^{epsilon_2}...{bar n}^{epsilon_n}$, with $epsilon_i=0$ or $1$. We aim to show that the $mathbf F_2$-dimension of $V$ is $pi (n)$, the number of rational primes $le n$. Let $W$ be the $mathbf F_2$-subspace generated by the classes of these primes. For for any $mle n$, the prime factorization of $m$ in $mathbf Z$ immediately shows that $bar m$ is a linear combination of the classes of the primes $le m$, which implies that $V=W$. It remains only to show that $W$ has $mathbf F_2$-dimension $pi (n)$, e.g. that the classes $bar p_i$ of the primes $le n$ form a basis. But a relation of linear dependence among them would mean that some finite product $prod p_i$ is a rational square, which contradicts the fact that $mathbf Z$ is a UFD.






              share|cite|improve this answer









              $endgroup$


















                1












                $begingroup$

                I think that Kummer gives the neatest proof, using only the multiplicative structure of $mathbf Q^*$. For a fixed $nge 2$, let $K=mathbf Q (sqrt 2, sqrt 3,...,sqrt n)$ and $mu_2=(pm 1)$. Kummer theory tells us that $K/mathbf Q$ is an abelian extension, with Galois group $Gcong Hom (V,mu_2)$, where $V$ is the subgroup of $mathbf Q^*/{mathbf Q^*}^2$ generated by the classes $bar 2,...,bar n$ mod ${mathbf Q^*}^2$. Although $V$ is a multiplicative group, it will be convenient to view it as a vector space over $mathbf F_2$, a linear combination of $bar 2,...,bar n$ mod ${mathbf Q^*}^2$ being just a product ${bar 2}^{epsilon_2}...{bar n}^{epsilon_n}$, with $epsilon_i=0$ or $1$. We aim to show that the $mathbf F_2$-dimension of $V$ is $pi (n)$, the number of rational primes $le n$. Let $W$ be the $mathbf F_2$-subspace generated by the classes of these primes. For for any $mle n$, the prime factorization of $m$ in $mathbf Z$ immediately shows that $bar m$ is a linear combination of the classes of the primes $le m$, which implies that $V=W$. It remains only to show that $W$ has $mathbf F_2$-dimension $pi (n)$, e.g. that the classes $bar p_i$ of the primes $le n$ form a basis. But a relation of linear dependence among them would mean that some finite product $prod p_i$ is a rational square, which contradicts the fact that $mathbf Z$ is a UFD.






                share|cite|improve this answer









                $endgroup$
















                  1












                  1








                  1





                  $begingroup$

                  I think that Kummer gives the neatest proof, using only the multiplicative structure of $mathbf Q^*$. For a fixed $nge 2$, let $K=mathbf Q (sqrt 2, sqrt 3,...,sqrt n)$ and $mu_2=(pm 1)$. Kummer theory tells us that $K/mathbf Q$ is an abelian extension, with Galois group $Gcong Hom (V,mu_2)$, where $V$ is the subgroup of $mathbf Q^*/{mathbf Q^*}^2$ generated by the classes $bar 2,...,bar n$ mod ${mathbf Q^*}^2$. Although $V$ is a multiplicative group, it will be convenient to view it as a vector space over $mathbf F_2$, a linear combination of $bar 2,...,bar n$ mod ${mathbf Q^*}^2$ being just a product ${bar 2}^{epsilon_2}...{bar n}^{epsilon_n}$, with $epsilon_i=0$ or $1$. We aim to show that the $mathbf F_2$-dimension of $V$ is $pi (n)$, the number of rational primes $le n$. Let $W$ be the $mathbf F_2$-subspace generated by the classes of these primes. For for any $mle n$, the prime factorization of $m$ in $mathbf Z$ immediately shows that $bar m$ is a linear combination of the classes of the primes $le m$, which implies that $V=W$. It remains only to show that $W$ has $mathbf F_2$-dimension $pi (n)$, e.g. that the classes $bar p_i$ of the primes $le n$ form a basis. But a relation of linear dependence among them would mean that some finite product $prod p_i$ is a rational square, which contradicts the fact that $mathbf Z$ is a UFD.






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                  $endgroup$



                  I think that Kummer gives the neatest proof, using only the multiplicative structure of $mathbf Q^*$. For a fixed $nge 2$, let $K=mathbf Q (sqrt 2, sqrt 3,...,sqrt n)$ and $mu_2=(pm 1)$. Kummer theory tells us that $K/mathbf Q$ is an abelian extension, with Galois group $Gcong Hom (V,mu_2)$, where $V$ is the subgroup of $mathbf Q^*/{mathbf Q^*}^2$ generated by the classes $bar 2,...,bar n$ mod ${mathbf Q^*}^2$. Although $V$ is a multiplicative group, it will be convenient to view it as a vector space over $mathbf F_2$, a linear combination of $bar 2,...,bar n$ mod ${mathbf Q^*}^2$ being just a product ${bar 2}^{epsilon_2}...{bar n}^{epsilon_n}$, with $epsilon_i=0$ or $1$. We aim to show that the $mathbf F_2$-dimension of $V$ is $pi (n)$, the number of rational primes $le n$. Let $W$ be the $mathbf F_2$-subspace generated by the classes of these primes. For for any $mle n$, the prime factorization of $m$ in $mathbf Z$ immediately shows that $bar m$ is a linear combination of the classes of the primes $le m$, which implies that $V=W$. It remains only to show that $W$ has $mathbf F_2$-dimension $pi (n)$, e.g. that the classes $bar p_i$ of the primes $le n$ form a basis. But a relation of linear dependence among them would mean that some finite product $prod p_i$ is a rational square, which contradicts the fact that $mathbf Z$ is a UFD.







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                  answered Apr 14 at 7:33









                  nguyen quang donguyen quang do

                  9,2241724




                  9,2241724






























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