Prove $x equiv a pmod{p}$ and $x equiv a pmod{q}$ then $x equiv apmod{pq}$












3












$begingroup$


$p$ and $q$ are distinct primes.



Where can I start with this proof?
It looks similar to the Chinese Remainder Theorem, but that deals with two different a values.










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$endgroup$








  • 1




    $begingroup$
    but that deals with two different $a$ values - What makes you think that?
    $endgroup$
    – anon
    Sep 3 '12 at 15:59










  • $begingroup$
    Because for the explanations of Chinese Remainder Theorem I've read, they use something like a = x (mod p) and a = y (mod p)
    $endgroup$
    – Takkun
    Sep 3 '12 at 16:03






  • 3




    $begingroup$
    A couple of variables denoted with two different letters may take on two different values, but not necessarily unless they are explicitly stated to have distinct values.
    $endgroup$
    – anon
    Sep 3 '12 at 16:04


















3












$begingroup$


$p$ and $q$ are distinct primes.



Where can I start with this proof?
It looks similar to the Chinese Remainder Theorem, but that deals with two different a values.










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    but that deals with two different $a$ values - What makes you think that?
    $endgroup$
    – anon
    Sep 3 '12 at 15:59










  • $begingroup$
    Because for the explanations of Chinese Remainder Theorem I've read, they use something like a = x (mod p) and a = y (mod p)
    $endgroup$
    – Takkun
    Sep 3 '12 at 16:03






  • 3




    $begingroup$
    A couple of variables denoted with two different letters may take on two different values, but not necessarily unless they are explicitly stated to have distinct values.
    $endgroup$
    – anon
    Sep 3 '12 at 16:04
















3












3








3





$begingroup$


$p$ and $q$ are distinct primes.



Where can I start with this proof?
It looks similar to the Chinese Remainder Theorem, but that deals with two different a values.










share|cite|improve this question











$endgroup$




$p$ and $q$ are distinct primes.



Where can I start with this proof?
It looks similar to the Chinese Remainder Theorem, but that deals with two different a values.







number-theory






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Sep 3 '12 at 16:08









Thomas Russell

7,89632552




7,89632552










asked Sep 3 '12 at 15:57









TakkunTakkun

268311




268311








  • 1




    $begingroup$
    but that deals with two different $a$ values - What makes you think that?
    $endgroup$
    – anon
    Sep 3 '12 at 15:59










  • $begingroup$
    Because for the explanations of Chinese Remainder Theorem I've read, they use something like a = x (mod p) and a = y (mod p)
    $endgroup$
    – Takkun
    Sep 3 '12 at 16:03






  • 3




    $begingroup$
    A couple of variables denoted with two different letters may take on two different values, but not necessarily unless they are explicitly stated to have distinct values.
    $endgroup$
    – anon
    Sep 3 '12 at 16:04
















  • 1




    $begingroup$
    but that deals with two different $a$ values - What makes you think that?
    $endgroup$
    – anon
    Sep 3 '12 at 15:59










  • $begingroup$
    Because for the explanations of Chinese Remainder Theorem I've read, they use something like a = x (mod p) and a = y (mod p)
    $endgroup$
    – Takkun
    Sep 3 '12 at 16:03






  • 3




    $begingroup$
    A couple of variables denoted with two different letters may take on two different values, but not necessarily unless they are explicitly stated to have distinct values.
    $endgroup$
    – anon
    Sep 3 '12 at 16:04










1




1




$begingroup$
but that deals with two different $a$ values - What makes you think that?
$endgroup$
– anon
Sep 3 '12 at 15:59




$begingroup$
but that deals with two different $a$ values - What makes you think that?
$endgroup$
– anon
Sep 3 '12 at 15:59












$begingroup$
Because for the explanations of Chinese Remainder Theorem I've read, they use something like a = x (mod p) and a = y (mod p)
$endgroup$
– Takkun
Sep 3 '12 at 16:03




$begingroup$
Because for the explanations of Chinese Remainder Theorem I've read, they use something like a = x (mod p) and a = y (mod p)
$endgroup$
– Takkun
Sep 3 '12 at 16:03




3




3




$begingroup$
A couple of variables denoted with two different letters may take on two different values, but not necessarily unless they are explicitly stated to have distinct values.
$endgroup$
– anon
Sep 3 '12 at 16:04






$begingroup$
A couple of variables denoted with two different letters may take on two different values, but not necessarily unless they are explicitly stated to have distinct values.
$endgroup$
– anon
Sep 3 '12 at 16:04












3 Answers
3






active

oldest

votes


















6












$begingroup$

Hint $ $ Below are few proofs of this constant-case CCRT of Chinese Remainder Theorem (CRT). The first three proofs work for for arbitrary coprime naturals $rm,p,q.$



$rm(1) x equiv apmod {pq}:$ is clearly a solution, and the solution is $color{#C00}{unique}$ mod $rm,pq,$ by CRT.



$rm(2) p,q:|:x!-!aiff lcm(p,q):|:x!-!a:$ by the Universal Property of $rm lcm$.
$qquad! $ Further $rm:(p,q)=1iff:lcm(p,q) = pq,,$ by this answer.



$(3) , $ By Euclid's Lemma: $rm:(p,q)=1, p:|:qn =:x!-!a:Rightarrow:p:|:n:Rightarrow:pq:|:nq = x!-!a.$



$rm(4) , $ Since the prime factorization of $rm,x!-!a,$ is $color{#C00}{unique}$, and the prime $rm,p,$ occurs in one factorization, and the distinct prime $rm,q,$ occurs in another, both factorizations are the same up to order, hence contain both $rm,p,$ and $rm,q,:$ hence have $rm,pq,$ as a divisor.



Remark $ $ This constant-case optimization of CRT arises frequently in practice so is well-worth memorizing, e.g. see some prior posts for many examples.



Quite frequently, $color{#C00}{uniqueness} theorems$ provide powerful tools for proving equalities.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    in (2) do you mean to say $gcd(p, q) = 1 iff lcm(p, q) = pq$
    $endgroup$
    – CodeKingPlusPlus
    Sep 23 '12 at 23:17










  • $begingroup$
    @Code Yes, $rm:(x,y):$ means $rm:gcd(x,y):$ in number theory (common notation).
    $endgroup$
    – Bill Dubuque
    Sep 23 '12 at 23:37



















4












$begingroup$

Let $[A,B]=lcm(A,B)$ and $(A,B)=gcd(A,B)$



If $p,q$ are different integers, $pmid(x-a)$ and $qmid(x-a)implies [p,q]mid(x-a)$



We know $[p,q]cdot (p,q)=pcdot q$



If $(p,q)=1, [p,q]=pcdot q$



If $p,q$ are distinct primes, $(p,q)=1$






share|cite|improve this answer











$endgroup$





















    3












    $begingroup$

    Let $y=x-a$. We want to show that if $p$ divides $y$ and $q$ divides $y$ then $pq$ divides $y$.



    Since $p$ divides $y$, we have $y=pz$ for some $z$. Thus $q$ divides $pz$. Since $q$ is prime, this implies $q$ divides $p$ or $q$ divides $z$. But $q$ cannot divide $p$, so $q$ divides $z$. Suppose that $z=qw$. Then $y=pqw$.






    share|cite|improve this answer











    $endgroup$













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      3 Answers
      3






      active

      oldest

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      3 Answers
      3






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      6












      $begingroup$

      Hint $ $ Below are few proofs of this constant-case CCRT of Chinese Remainder Theorem (CRT). The first three proofs work for for arbitrary coprime naturals $rm,p,q.$



      $rm(1) x equiv apmod {pq}:$ is clearly a solution, and the solution is $color{#C00}{unique}$ mod $rm,pq,$ by CRT.



      $rm(2) p,q:|:x!-!aiff lcm(p,q):|:x!-!a:$ by the Universal Property of $rm lcm$.
      $qquad! $ Further $rm:(p,q)=1iff:lcm(p,q) = pq,,$ by this answer.



      $(3) , $ By Euclid's Lemma: $rm:(p,q)=1, p:|:qn =:x!-!a:Rightarrow:p:|:n:Rightarrow:pq:|:nq = x!-!a.$



      $rm(4) , $ Since the prime factorization of $rm,x!-!a,$ is $color{#C00}{unique}$, and the prime $rm,p,$ occurs in one factorization, and the distinct prime $rm,q,$ occurs in another, both factorizations are the same up to order, hence contain both $rm,p,$ and $rm,q,:$ hence have $rm,pq,$ as a divisor.



      Remark $ $ This constant-case optimization of CRT arises frequently in practice so is well-worth memorizing, e.g. see some prior posts for many examples.



      Quite frequently, $color{#C00}{uniqueness} theorems$ provide powerful tools for proving equalities.






      share|cite|improve this answer











      $endgroup$













      • $begingroup$
        in (2) do you mean to say $gcd(p, q) = 1 iff lcm(p, q) = pq$
        $endgroup$
        – CodeKingPlusPlus
        Sep 23 '12 at 23:17










      • $begingroup$
        @Code Yes, $rm:(x,y):$ means $rm:gcd(x,y):$ in number theory (common notation).
        $endgroup$
        – Bill Dubuque
        Sep 23 '12 at 23:37
















      6












      $begingroup$

      Hint $ $ Below are few proofs of this constant-case CCRT of Chinese Remainder Theorem (CRT). The first three proofs work for for arbitrary coprime naturals $rm,p,q.$



      $rm(1) x equiv apmod {pq}:$ is clearly a solution, and the solution is $color{#C00}{unique}$ mod $rm,pq,$ by CRT.



      $rm(2) p,q:|:x!-!aiff lcm(p,q):|:x!-!a:$ by the Universal Property of $rm lcm$.
      $qquad! $ Further $rm:(p,q)=1iff:lcm(p,q) = pq,,$ by this answer.



      $(3) , $ By Euclid's Lemma: $rm:(p,q)=1, p:|:qn =:x!-!a:Rightarrow:p:|:n:Rightarrow:pq:|:nq = x!-!a.$



      $rm(4) , $ Since the prime factorization of $rm,x!-!a,$ is $color{#C00}{unique}$, and the prime $rm,p,$ occurs in one factorization, and the distinct prime $rm,q,$ occurs in another, both factorizations are the same up to order, hence contain both $rm,p,$ and $rm,q,:$ hence have $rm,pq,$ as a divisor.



      Remark $ $ This constant-case optimization of CRT arises frequently in practice so is well-worth memorizing, e.g. see some prior posts for many examples.



      Quite frequently, $color{#C00}{uniqueness} theorems$ provide powerful tools for proving equalities.






      share|cite|improve this answer











      $endgroup$













      • $begingroup$
        in (2) do you mean to say $gcd(p, q) = 1 iff lcm(p, q) = pq$
        $endgroup$
        – CodeKingPlusPlus
        Sep 23 '12 at 23:17










      • $begingroup$
        @Code Yes, $rm:(x,y):$ means $rm:gcd(x,y):$ in number theory (common notation).
        $endgroup$
        – Bill Dubuque
        Sep 23 '12 at 23:37














      6












      6








      6





      $begingroup$

      Hint $ $ Below are few proofs of this constant-case CCRT of Chinese Remainder Theorem (CRT). The first three proofs work for for arbitrary coprime naturals $rm,p,q.$



      $rm(1) x equiv apmod {pq}:$ is clearly a solution, and the solution is $color{#C00}{unique}$ mod $rm,pq,$ by CRT.



      $rm(2) p,q:|:x!-!aiff lcm(p,q):|:x!-!a:$ by the Universal Property of $rm lcm$.
      $qquad! $ Further $rm:(p,q)=1iff:lcm(p,q) = pq,,$ by this answer.



      $(3) , $ By Euclid's Lemma: $rm:(p,q)=1, p:|:qn =:x!-!a:Rightarrow:p:|:n:Rightarrow:pq:|:nq = x!-!a.$



      $rm(4) , $ Since the prime factorization of $rm,x!-!a,$ is $color{#C00}{unique}$, and the prime $rm,p,$ occurs in one factorization, and the distinct prime $rm,q,$ occurs in another, both factorizations are the same up to order, hence contain both $rm,p,$ and $rm,q,:$ hence have $rm,pq,$ as a divisor.



      Remark $ $ This constant-case optimization of CRT arises frequently in practice so is well-worth memorizing, e.g. see some prior posts for many examples.



      Quite frequently, $color{#C00}{uniqueness} theorems$ provide powerful tools for proving equalities.






      share|cite|improve this answer











      $endgroup$



      Hint $ $ Below are few proofs of this constant-case CCRT of Chinese Remainder Theorem (CRT). The first three proofs work for for arbitrary coprime naturals $rm,p,q.$



      $rm(1) x equiv apmod {pq}:$ is clearly a solution, and the solution is $color{#C00}{unique}$ mod $rm,pq,$ by CRT.



      $rm(2) p,q:|:x!-!aiff lcm(p,q):|:x!-!a:$ by the Universal Property of $rm lcm$.
      $qquad! $ Further $rm:(p,q)=1iff:lcm(p,q) = pq,,$ by this answer.



      $(3) , $ By Euclid's Lemma: $rm:(p,q)=1, p:|:qn =:x!-!a:Rightarrow:p:|:n:Rightarrow:pq:|:nq = x!-!a.$



      $rm(4) , $ Since the prime factorization of $rm,x!-!a,$ is $color{#C00}{unique}$, and the prime $rm,p,$ occurs in one factorization, and the distinct prime $rm,q,$ occurs in another, both factorizations are the same up to order, hence contain both $rm,p,$ and $rm,q,:$ hence have $rm,pq,$ as a divisor.



      Remark $ $ This constant-case optimization of CRT arises frequently in practice so is well-worth memorizing, e.g. see some prior posts for many examples.



      Quite frequently, $color{#C00}{uniqueness} theorems$ provide powerful tools for proving equalities.







      share|cite|improve this answer














      share|cite|improve this answer



      share|cite|improve this answer








      edited Feb 7 at 16:34

























      answered Sep 3 '12 at 16:18









      Bill DubuqueBill Dubuque

      213k29195654




      213k29195654












      • $begingroup$
        in (2) do you mean to say $gcd(p, q) = 1 iff lcm(p, q) = pq$
        $endgroup$
        – CodeKingPlusPlus
        Sep 23 '12 at 23:17










      • $begingroup$
        @Code Yes, $rm:(x,y):$ means $rm:gcd(x,y):$ in number theory (common notation).
        $endgroup$
        – Bill Dubuque
        Sep 23 '12 at 23:37


















      • $begingroup$
        in (2) do you mean to say $gcd(p, q) = 1 iff lcm(p, q) = pq$
        $endgroup$
        – CodeKingPlusPlus
        Sep 23 '12 at 23:17










      • $begingroup$
        @Code Yes, $rm:(x,y):$ means $rm:gcd(x,y):$ in number theory (common notation).
        $endgroup$
        – Bill Dubuque
        Sep 23 '12 at 23:37
















      $begingroup$
      in (2) do you mean to say $gcd(p, q) = 1 iff lcm(p, q) = pq$
      $endgroup$
      – CodeKingPlusPlus
      Sep 23 '12 at 23:17




      $begingroup$
      in (2) do you mean to say $gcd(p, q) = 1 iff lcm(p, q) = pq$
      $endgroup$
      – CodeKingPlusPlus
      Sep 23 '12 at 23:17












      $begingroup$
      @Code Yes, $rm:(x,y):$ means $rm:gcd(x,y):$ in number theory (common notation).
      $endgroup$
      – Bill Dubuque
      Sep 23 '12 at 23:37




      $begingroup$
      @Code Yes, $rm:(x,y):$ means $rm:gcd(x,y):$ in number theory (common notation).
      $endgroup$
      – Bill Dubuque
      Sep 23 '12 at 23:37











      4












      $begingroup$

      Let $[A,B]=lcm(A,B)$ and $(A,B)=gcd(A,B)$



      If $p,q$ are different integers, $pmid(x-a)$ and $qmid(x-a)implies [p,q]mid(x-a)$



      We know $[p,q]cdot (p,q)=pcdot q$



      If $(p,q)=1, [p,q]=pcdot q$



      If $p,q$ are distinct primes, $(p,q)=1$






      share|cite|improve this answer











      $endgroup$


















        4












        $begingroup$

        Let $[A,B]=lcm(A,B)$ and $(A,B)=gcd(A,B)$



        If $p,q$ are different integers, $pmid(x-a)$ and $qmid(x-a)implies [p,q]mid(x-a)$



        We know $[p,q]cdot (p,q)=pcdot q$



        If $(p,q)=1, [p,q]=pcdot q$



        If $p,q$ are distinct primes, $(p,q)=1$






        share|cite|improve this answer











        $endgroup$
















          4












          4








          4





          $begingroup$

          Let $[A,B]=lcm(A,B)$ and $(A,B)=gcd(A,B)$



          If $p,q$ are different integers, $pmid(x-a)$ and $qmid(x-a)implies [p,q]mid(x-a)$



          We know $[p,q]cdot (p,q)=pcdot q$



          If $(p,q)=1, [p,q]=pcdot q$



          If $p,q$ are distinct primes, $(p,q)=1$






          share|cite|improve this answer











          $endgroup$



          Let $[A,B]=lcm(A,B)$ and $(A,B)=gcd(A,B)$



          If $p,q$ are different integers, $pmid(x-a)$ and $qmid(x-a)implies [p,q]mid(x-a)$



          We know $[p,q]cdot (p,q)=pcdot q$



          If $(p,q)=1, [p,q]=pcdot q$



          If $p,q$ are distinct primes, $(p,q)=1$







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Sep 3 '12 at 16:06

























          answered Sep 3 '12 at 16:01









          lab bhattacharjeelab bhattacharjee

          227k15158278




          227k15158278























              3












              $begingroup$

              Let $y=x-a$. We want to show that if $p$ divides $y$ and $q$ divides $y$ then $pq$ divides $y$.



              Since $p$ divides $y$, we have $y=pz$ for some $z$. Thus $q$ divides $pz$. Since $q$ is prime, this implies $q$ divides $p$ or $q$ divides $z$. But $q$ cannot divide $p$, so $q$ divides $z$. Suppose that $z=qw$. Then $y=pqw$.






              share|cite|improve this answer











              $endgroup$


















                3












                $begingroup$

                Let $y=x-a$. We want to show that if $p$ divides $y$ and $q$ divides $y$ then $pq$ divides $y$.



                Since $p$ divides $y$, we have $y=pz$ for some $z$. Thus $q$ divides $pz$. Since $q$ is prime, this implies $q$ divides $p$ or $q$ divides $z$. But $q$ cannot divide $p$, so $q$ divides $z$. Suppose that $z=qw$. Then $y=pqw$.






                share|cite|improve this answer











                $endgroup$
















                  3












                  3








                  3





                  $begingroup$

                  Let $y=x-a$. We want to show that if $p$ divides $y$ and $q$ divides $y$ then $pq$ divides $y$.



                  Since $p$ divides $y$, we have $y=pz$ for some $z$. Thus $q$ divides $pz$. Since $q$ is prime, this implies $q$ divides $p$ or $q$ divides $z$. But $q$ cannot divide $p$, so $q$ divides $z$. Suppose that $z=qw$. Then $y=pqw$.






                  share|cite|improve this answer











                  $endgroup$



                  Let $y=x-a$. We want to show that if $p$ divides $y$ and $q$ divides $y$ then $pq$ divides $y$.



                  Since $p$ divides $y$, we have $y=pz$ for some $z$. Thus $q$ divides $pz$. Since $q$ is prime, this implies $q$ divides $p$ or $q$ divides $z$. But $q$ cannot divide $p$, so $q$ divides $z$. Suppose that $z=qw$. Then $y=pqw$.







                  share|cite|improve this answer














                  share|cite|improve this answer



                  share|cite|improve this answer








                  edited Sep 3 '12 at 16:30

























                  answered Sep 3 '12 at 16:08









                  André NicolasAndré Nicolas

                  454k36432819




                  454k36432819






























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