LCM and GCD polynomial relationship












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I need some help with constructing a proof for the following statement,$ frac{P_1 P_2}{hcf(m,n)} = lcm(P_1,P_2)$ where $P_1$ and $P_2$ are polynomials with real coefficients.



I know how to do the same for integers using prime factors and their exponents but not sure where to go with polynomials.










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    $begingroup$


    I need some help with constructing a proof for the following statement,$ frac{P_1 P_2}{hcf(m,n)} = lcm(P_1,P_2)$ where $P_1$ and $P_2$ are polynomials with real coefficients.



    I know how to do the same for integers using prime factors and their exponents but not sure where to go with polynomials.










    share|cite|improve this question









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      0





      $begingroup$


      I need some help with constructing a proof for the following statement,$ frac{P_1 P_2}{hcf(m,n)} = lcm(P_1,P_2)$ where $P_1$ and $P_2$ are polynomials with real coefficients.



      I know how to do the same for integers using prime factors and their exponents but not sure where to go with polynomials.










      share|cite|improve this question









      $endgroup$




      I need some help with constructing a proof for the following statement,$ frac{P_1 P_2}{hcf(m,n)} = lcm(P_1,P_2)$ where $P_1$ and $P_2$ are polynomials with real coefficients.



      I know how to do the same for integers using prime factors and their exponents but not sure where to go with polynomials.







      polynomials proof-explanation greatest-common-divisor least-common-multiple






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      asked Dec 28 '18 at 21:51









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          $begingroup$

          Do it the exact same way. Suppose that the hcf/gcd of $P_1$ and $P_2$ is $G$. Because $G$ is a factor of $P_1$, there exists an $R_1$ such that $P_1$ is equal to $R_1G$, and likewise $P_2$ equals some $R_2G$.



          $P_1P_2 = R_1R_2GG$



          ${P_1P_2 over G} = R_1R_2G$



          $R_1$ and $R_2$ can have no factors in common as any factor $H$ could be multiplied by $G$ to obtain a new GCD.



          Because $R_1R_2G$ is a multiple of $R_1G$, it is a multiple of $P_1$, and likewise for $P_2$. It is a multiple of both, and no factor can be removed which would preserve its multiplicity. Therefore, it is the Least Common Multiple.



          Q.E.D.






          share|cite|improve this answer









          $endgroup$





















            1












            $begingroup$

            This proof works in any gcd domain. We use the $,overbrace{{rm involution}, x' :=, ab/x}^{rmlarge cofactor duality } $ on the divisors of $rm:ab,,$ which exposes $ $ cofactor reflection $,rm xmid ycolor{#c00}iff y'mid x', $ by ${, rmdfrac{y}x = dfrac{x'}{y'} }$ by $rm, yy'! = ab = xx'., $ Thus



            $$begin{align}rm cmidgcd(a,b)!iff&rm cmid a,b\[3px]
            color{#c00}iff& rm b',a'mid c'\[3px]
            iff & rm lcm(b',a')mid c'\[3px]
            color{#c00}iff & rm cmid lcm(b',a')' \
            {rm Thus}rmquad gcd(a,b), cong &rm , lcm(b',a')'= dfrac{ab}{lcm(a,b)}
            end{align}quad $$



            Above the black arrows are the definition of gcd and lcm, and the red arrows are cofactor reflections, and $,acong b,$ means $,a,b,$ are associates, i.e. $,amid bmid a$.






            share|cite|improve this answer











            $endgroup$





















              0












              $begingroup$

              It works pretty much the same for integers if you modify the argument a little. Let $L = lcm(P_1, P_2)$ and $G=gcd(P_1, P_2)$. Then
              $$P_1 = Gh_1, P_2 = Gh_2,$$
              with $gcd(h_1, h_2) = 1$. It's easy to see that $P_1$ and $P_2$ both divides $Gh_1h_2$ so $L$ also divides $Gh_1h_2$. Assume that
              $$ Gh_1h_2 = Lh,$$
              then $P_1 h_2 = L h$, or $h_2 = frac{L}{P_1} h$. That is, $h$ divides $h_2$. Similarly $h$ divides $h_1$. Since $gcd(h_1,h_2)=1$, $h$ must be as scalar as well. In other words
              $$L= Gh_1h_2 = frac{P_1P_2}{G}.$$






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                $begingroup$

                Think of the irreducible factors of P$_1$ and P$_2$ as your prime factors. Suppose P$_1$ = gcd(P$_1$,P$_2$)($q_1q_2ldots q_n$) and P$_2$=gcd(P$_1$,P$_2$)($r_1r_2ldots r_m$).
                Thus
                $$frac{P_1P_2}{gcd(P_1,P_2)}=gcd(P_1,P_2)(q_1ldots q_n)(r_1 ldots r_m).$$
                Note that the numerator has [gcd(P$_1$,P$_2$)]$^2$ as a factor.



                So the RHS is a common multiple of P$_1$ and P$_2$. You should be able to show that if there is a "smaller" lcm, then we can get a "larger" gcd.






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                  4 Answers
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                  4 Answers
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                  1












                  $begingroup$

                  Do it the exact same way. Suppose that the hcf/gcd of $P_1$ and $P_2$ is $G$. Because $G$ is a factor of $P_1$, there exists an $R_1$ such that $P_1$ is equal to $R_1G$, and likewise $P_2$ equals some $R_2G$.



                  $P_1P_2 = R_1R_2GG$



                  ${P_1P_2 over G} = R_1R_2G$



                  $R_1$ and $R_2$ can have no factors in common as any factor $H$ could be multiplied by $G$ to obtain a new GCD.



                  Because $R_1R_2G$ is a multiple of $R_1G$, it is a multiple of $P_1$, and likewise for $P_2$. It is a multiple of both, and no factor can be removed which would preserve its multiplicity. Therefore, it is the Least Common Multiple.



                  Q.E.D.






                  share|cite|improve this answer









                  $endgroup$


















                    1












                    $begingroup$

                    Do it the exact same way. Suppose that the hcf/gcd of $P_1$ and $P_2$ is $G$. Because $G$ is a factor of $P_1$, there exists an $R_1$ such that $P_1$ is equal to $R_1G$, and likewise $P_2$ equals some $R_2G$.



                    $P_1P_2 = R_1R_2GG$



                    ${P_1P_2 over G} = R_1R_2G$



                    $R_1$ and $R_2$ can have no factors in common as any factor $H$ could be multiplied by $G$ to obtain a new GCD.



                    Because $R_1R_2G$ is a multiple of $R_1G$, it is a multiple of $P_1$, and likewise for $P_2$. It is a multiple of both, and no factor can be removed which would preserve its multiplicity. Therefore, it is the Least Common Multiple.



                    Q.E.D.






                    share|cite|improve this answer









                    $endgroup$
















                      1












                      1








                      1





                      $begingroup$

                      Do it the exact same way. Suppose that the hcf/gcd of $P_1$ and $P_2$ is $G$. Because $G$ is a factor of $P_1$, there exists an $R_1$ such that $P_1$ is equal to $R_1G$, and likewise $P_2$ equals some $R_2G$.



                      $P_1P_2 = R_1R_2GG$



                      ${P_1P_2 over G} = R_1R_2G$



                      $R_1$ and $R_2$ can have no factors in common as any factor $H$ could be multiplied by $G$ to obtain a new GCD.



                      Because $R_1R_2G$ is a multiple of $R_1G$, it is a multiple of $P_1$, and likewise for $P_2$. It is a multiple of both, and no factor can be removed which would preserve its multiplicity. Therefore, it is the Least Common Multiple.



                      Q.E.D.






                      share|cite|improve this answer









                      $endgroup$



                      Do it the exact same way. Suppose that the hcf/gcd of $P_1$ and $P_2$ is $G$. Because $G$ is a factor of $P_1$, there exists an $R_1$ such that $P_1$ is equal to $R_1G$, and likewise $P_2$ equals some $R_2G$.



                      $P_1P_2 = R_1R_2GG$



                      ${P_1P_2 over G} = R_1R_2G$



                      $R_1$ and $R_2$ can have no factors in common as any factor $H$ could be multiplied by $G$ to obtain a new GCD.



                      Because $R_1R_2G$ is a multiple of $R_1G$, it is a multiple of $P_1$, and likewise for $P_2$. It is a multiple of both, and no factor can be removed which would preserve its multiplicity. Therefore, it is the Least Common Multiple.



                      Q.E.D.







                      share|cite|improve this answer












                      share|cite|improve this answer



                      share|cite|improve this answer










                      answered Dec 28 '18 at 22:25









                      William GrannisWilliam Grannis

                      998521




                      998521























                          1












                          $begingroup$

                          This proof works in any gcd domain. We use the $,overbrace{{rm involution}, x' :=, ab/x}^{rmlarge cofactor duality } $ on the divisors of $rm:ab,,$ which exposes $ $ cofactor reflection $,rm xmid ycolor{#c00}iff y'mid x', $ by ${, rmdfrac{y}x = dfrac{x'}{y'} }$ by $rm, yy'! = ab = xx'., $ Thus



                          $$begin{align}rm cmidgcd(a,b)!iff&rm cmid a,b\[3px]
                          color{#c00}iff& rm b',a'mid c'\[3px]
                          iff & rm lcm(b',a')mid c'\[3px]
                          color{#c00}iff & rm cmid lcm(b',a')' \
                          {rm Thus}rmquad gcd(a,b), cong &rm , lcm(b',a')'= dfrac{ab}{lcm(a,b)}
                          end{align}quad $$



                          Above the black arrows are the definition of gcd and lcm, and the red arrows are cofactor reflections, and $,acong b,$ means $,a,b,$ are associates, i.e. $,amid bmid a$.






                          share|cite|improve this answer











                          $endgroup$


















                            1












                            $begingroup$

                            This proof works in any gcd domain. We use the $,overbrace{{rm involution}, x' :=, ab/x}^{rmlarge cofactor duality } $ on the divisors of $rm:ab,,$ which exposes $ $ cofactor reflection $,rm xmid ycolor{#c00}iff y'mid x', $ by ${, rmdfrac{y}x = dfrac{x'}{y'} }$ by $rm, yy'! = ab = xx'., $ Thus



                            $$begin{align}rm cmidgcd(a,b)!iff&rm cmid a,b\[3px]
                            color{#c00}iff& rm b',a'mid c'\[3px]
                            iff & rm lcm(b',a')mid c'\[3px]
                            color{#c00}iff & rm cmid lcm(b',a')' \
                            {rm Thus}rmquad gcd(a,b), cong &rm , lcm(b',a')'= dfrac{ab}{lcm(a,b)}
                            end{align}quad $$



                            Above the black arrows are the definition of gcd and lcm, and the red arrows are cofactor reflections, and $,acong b,$ means $,a,b,$ are associates, i.e. $,amid bmid a$.






                            share|cite|improve this answer











                            $endgroup$
















                              1












                              1








                              1





                              $begingroup$

                              This proof works in any gcd domain. We use the $,overbrace{{rm involution}, x' :=, ab/x}^{rmlarge cofactor duality } $ on the divisors of $rm:ab,,$ which exposes $ $ cofactor reflection $,rm xmid ycolor{#c00}iff y'mid x', $ by ${, rmdfrac{y}x = dfrac{x'}{y'} }$ by $rm, yy'! = ab = xx'., $ Thus



                              $$begin{align}rm cmidgcd(a,b)!iff&rm cmid a,b\[3px]
                              color{#c00}iff& rm b',a'mid c'\[3px]
                              iff & rm lcm(b',a')mid c'\[3px]
                              color{#c00}iff & rm cmid lcm(b',a')' \
                              {rm Thus}rmquad gcd(a,b), cong &rm , lcm(b',a')'= dfrac{ab}{lcm(a,b)}
                              end{align}quad $$



                              Above the black arrows are the definition of gcd and lcm, and the red arrows are cofactor reflections, and $,acong b,$ means $,a,b,$ are associates, i.e. $,amid bmid a$.






                              share|cite|improve this answer











                              $endgroup$



                              This proof works in any gcd domain. We use the $,overbrace{{rm involution}, x' :=, ab/x}^{rmlarge cofactor duality } $ on the divisors of $rm:ab,,$ which exposes $ $ cofactor reflection $,rm xmid ycolor{#c00}iff y'mid x', $ by ${, rmdfrac{y}x = dfrac{x'}{y'} }$ by $rm, yy'! = ab = xx'., $ Thus



                              $$begin{align}rm cmidgcd(a,b)!iff&rm cmid a,b\[3px]
                              color{#c00}iff& rm b',a'mid c'\[3px]
                              iff & rm lcm(b',a')mid c'\[3px]
                              color{#c00}iff & rm cmid lcm(b',a')' \
                              {rm Thus}rmquad gcd(a,b), cong &rm , lcm(b',a')'= dfrac{ab}{lcm(a,b)}
                              end{align}quad $$



                              Above the black arrows are the definition of gcd and lcm, and the red arrows are cofactor reflections, and $,acong b,$ means $,a,b,$ are associates, i.e. $,amid bmid a$.







                              share|cite|improve this answer














                              share|cite|improve this answer



                              share|cite|improve this answer








                              edited Dec 28 '18 at 23:08

























                              answered Dec 28 '18 at 22:56









                              Bill DubuqueBill Dubuque

                              211k29194648




                              211k29194648























                                  0












                                  $begingroup$

                                  It works pretty much the same for integers if you modify the argument a little. Let $L = lcm(P_1, P_2)$ and $G=gcd(P_1, P_2)$. Then
                                  $$P_1 = Gh_1, P_2 = Gh_2,$$
                                  with $gcd(h_1, h_2) = 1$. It's easy to see that $P_1$ and $P_2$ both divides $Gh_1h_2$ so $L$ also divides $Gh_1h_2$. Assume that
                                  $$ Gh_1h_2 = Lh,$$
                                  then $P_1 h_2 = L h$, or $h_2 = frac{L}{P_1} h$. That is, $h$ divides $h_2$. Similarly $h$ divides $h_1$. Since $gcd(h_1,h_2)=1$, $h$ must be as scalar as well. In other words
                                  $$L= Gh_1h_2 = frac{P_1P_2}{G}.$$






                                  share|cite|improve this answer









                                  $endgroup$


















                                    0












                                    $begingroup$

                                    It works pretty much the same for integers if you modify the argument a little. Let $L = lcm(P_1, P_2)$ and $G=gcd(P_1, P_2)$. Then
                                    $$P_1 = Gh_1, P_2 = Gh_2,$$
                                    with $gcd(h_1, h_2) = 1$. It's easy to see that $P_1$ and $P_2$ both divides $Gh_1h_2$ so $L$ also divides $Gh_1h_2$. Assume that
                                    $$ Gh_1h_2 = Lh,$$
                                    then $P_1 h_2 = L h$, or $h_2 = frac{L}{P_1} h$. That is, $h$ divides $h_2$. Similarly $h$ divides $h_1$. Since $gcd(h_1,h_2)=1$, $h$ must be as scalar as well. In other words
                                    $$L= Gh_1h_2 = frac{P_1P_2}{G}.$$






                                    share|cite|improve this answer









                                    $endgroup$
















                                      0












                                      0








                                      0





                                      $begingroup$

                                      It works pretty much the same for integers if you modify the argument a little. Let $L = lcm(P_1, P_2)$ and $G=gcd(P_1, P_2)$. Then
                                      $$P_1 = Gh_1, P_2 = Gh_2,$$
                                      with $gcd(h_1, h_2) = 1$. It's easy to see that $P_1$ and $P_2$ both divides $Gh_1h_2$ so $L$ also divides $Gh_1h_2$. Assume that
                                      $$ Gh_1h_2 = Lh,$$
                                      then $P_1 h_2 = L h$, or $h_2 = frac{L}{P_1} h$. That is, $h$ divides $h_2$. Similarly $h$ divides $h_1$. Since $gcd(h_1,h_2)=1$, $h$ must be as scalar as well. In other words
                                      $$L= Gh_1h_2 = frac{P_1P_2}{G}.$$






                                      share|cite|improve this answer









                                      $endgroup$



                                      It works pretty much the same for integers if you modify the argument a little. Let $L = lcm(P_1, P_2)$ and $G=gcd(P_1, P_2)$. Then
                                      $$P_1 = Gh_1, P_2 = Gh_2,$$
                                      with $gcd(h_1, h_2) = 1$. It's easy to see that $P_1$ and $P_2$ both divides $Gh_1h_2$ so $L$ also divides $Gh_1h_2$. Assume that
                                      $$ Gh_1h_2 = Lh,$$
                                      then $P_1 h_2 = L h$, or $h_2 = frac{L}{P_1} h$. That is, $h$ divides $h_2$. Similarly $h$ divides $h_1$. Since $gcd(h_1,h_2)=1$, $h$ must be as scalar as well. In other words
                                      $$L= Gh_1h_2 = frac{P_1P_2}{G}.$$







                                      share|cite|improve this answer












                                      share|cite|improve this answer



                                      share|cite|improve this answer










                                      answered Dec 28 '18 at 22:17









                                      Quang HoangQuang Hoang

                                      13.2k1233




                                      13.2k1233























                                          0












                                          $begingroup$

                                          Think of the irreducible factors of P$_1$ and P$_2$ as your prime factors. Suppose P$_1$ = gcd(P$_1$,P$_2$)($q_1q_2ldots q_n$) and P$_2$=gcd(P$_1$,P$_2$)($r_1r_2ldots r_m$).
                                          Thus
                                          $$frac{P_1P_2}{gcd(P_1,P_2)}=gcd(P_1,P_2)(q_1ldots q_n)(r_1 ldots r_m).$$
                                          Note that the numerator has [gcd(P$_1$,P$_2$)]$^2$ as a factor.



                                          So the RHS is a common multiple of P$_1$ and P$_2$. You should be able to show that if there is a "smaller" lcm, then we can get a "larger" gcd.






                                          share|cite|improve this answer









                                          $endgroup$


















                                            0












                                            $begingroup$

                                            Think of the irreducible factors of P$_1$ and P$_2$ as your prime factors. Suppose P$_1$ = gcd(P$_1$,P$_2$)($q_1q_2ldots q_n$) and P$_2$=gcd(P$_1$,P$_2$)($r_1r_2ldots r_m$).
                                            Thus
                                            $$frac{P_1P_2}{gcd(P_1,P_2)}=gcd(P_1,P_2)(q_1ldots q_n)(r_1 ldots r_m).$$
                                            Note that the numerator has [gcd(P$_1$,P$_2$)]$^2$ as a factor.



                                            So the RHS is a common multiple of P$_1$ and P$_2$. You should be able to show that if there is a "smaller" lcm, then we can get a "larger" gcd.






                                            share|cite|improve this answer









                                            $endgroup$
















                                              0












                                              0








                                              0





                                              $begingroup$

                                              Think of the irreducible factors of P$_1$ and P$_2$ as your prime factors. Suppose P$_1$ = gcd(P$_1$,P$_2$)($q_1q_2ldots q_n$) and P$_2$=gcd(P$_1$,P$_2$)($r_1r_2ldots r_m$).
                                              Thus
                                              $$frac{P_1P_2}{gcd(P_1,P_2)}=gcd(P_1,P_2)(q_1ldots q_n)(r_1 ldots r_m).$$
                                              Note that the numerator has [gcd(P$_1$,P$_2$)]$^2$ as a factor.



                                              So the RHS is a common multiple of P$_1$ and P$_2$. You should be able to show that if there is a "smaller" lcm, then we can get a "larger" gcd.






                                              share|cite|improve this answer









                                              $endgroup$



                                              Think of the irreducible factors of P$_1$ and P$_2$ as your prime factors. Suppose P$_1$ = gcd(P$_1$,P$_2$)($q_1q_2ldots q_n$) and P$_2$=gcd(P$_1$,P$_2$)($r_1r_2ldots r_m$).
                                              Thus
                                              $$frac{P_1P_2}{gcd(P_1,P_2)}=gcd(P_1,P_2)(q_1ldots q_n)(r_1 ldots r_m).$$
                                              Note that the numerator has [gcd(P$_1$,P$_2$)]$^2$ as a factor.



                                              So the RHS is a common multiple of P$_1$ and P$_2$. You should be able to show that if there is a "smaller" lcm, then we can get a "larger" gcd.







                                              share|cite|improve this answer












                                              share|cite|improve this answer



                                              share|cite|improve this answer










                                              answered Dec 28 '18 at 22:19









                                              Joel PereiraJoel Pereira

                                              75919




                                              75919






























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