How do I prove this equality holds? If A is similar to a diagonal matrix with diagonal $a_1,…,a_n$ then...












0












$begingroup$


I'm not quite sure how to prove whats written in the title. We have a field $F$ and a matrix $A$ that is similar to the diagonal matrix $D=operatorname{diag}(a_1,...,a_n)=begin{bmatrix} a_1& 0 &0&. . .& 0 \ 0& a_2 &0&...&0 \ . \. \.\0&0&0 &...&a_nend{bmatrix}$
I need to show that $(A-a_1I) cdot (A-a_2I) cdot ... cdot (A-a_nI)=0$.
I tried this for long time but I am not sure how to do this. I know there is a matrix $P$ such that $A=P^{-1}DP$, but I'm not sure how that helps. It was written that it is enough to prove this for n=2 (n is the number of rows and columns of A) and I was able to do this for n=2 but I don't know how that helps me to prove this generally. If anyone can provide any help (I'd love to get some hints first, and only then the full solution if I'm still stuck), It will help me a lot. Thanks!

**All I know about similar matrices is that there exists $P$ such that $A=P^{-1}BP$, so the solution should be quite basic, using that fact only.










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  • 1




    $begingroup$
    Hint: Prove that similar matrices has the same characteristic polynomial and the fact a matrix satisfies its own characteristic polynomial.
    $endgroup$
    – dezdichado
    Dec 28 '18 at 21:01










  • $begingroup$
    @dezdichado Hi, I haven't learnt about characterisitc polynomial yet, all I know about similar matrices is that there exists P such that A=P^-1*B*P, and that's it. They said in the question that it is enough to show this for n=2 and than explain how that also implies it for all values of n. sorry for not mentioning that, I'll edit my post now.
    $endgroup$
    – Omer
    Dec 28 '18 at 21:07
















0












$begingroup$


I'm not quite sure how to prove whats written in the title. We have a field $F$ and a matrix $A$ that is similar to the diagonal matrix $D=operatorname{diag}(a_1,...,a_n)=begin{bmatrix} a_1& 0 &0&. . .& 0 \ 0& a_2 &0&...&0 \ . \. \.\0&0&0 &...&a_nend{bmatrix}$
I need to show that $(A-a_1I) cdot (A-a_2I) cdot ... cdot (A-a_nI)=0$.
I tried this for long time but I am not sure how to do this. I know there is a matrix $P$ such that $A=P^{-1}DP$, but I'm not sure how that helps. It was written that it is enough to prove this for n=2 (n is the number of rows and columns of A) and I was able to do this for n=2 but I don't know how that helps me to prove this generally. If anyone can provide any help (I'd love to get some hints first, and only then the full solution if I'm still stuck), It will help me a lot. Thanks!

**All I know about similar matrices is that there exists $P$ such that $A=P^{-1}BP$, so the solution should be quite basic, using that fact only.










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    Hint: Prove that similar matrices has the same characteristic polynomial and the fact a matrix satisfies its own characteristic polynomial.
    $endgroup$
    – dezdichado
    Dec 28 '18 at 21:01










  • $begingroup$
    @dezdichado Hi, I haven't learnt about characterisitc polynomial yet, all I know about similar matrices is that there exists P such that A=P^-1*B*P, and that's it. They said in the question that it is enough to show this for n=2 and than explain how that also implies it for all values of n. sorry for not mentioning that, I'll edit my post now.
    $endgroup$
    – Omer
    Dec 28 '18 at 21:07














0












0








0





$begingroup$


I'm not quite sure how to prove whats written in the title. We have a field $F$ and a matrix $A$ that is similar to the diagonal matrix $D=operatorname{diag}(a_1,...,a_n)=begin{bmatrix} a_1& 0 &0&. . .& 0 \ 0& a_2 &0&...&0 \ . \. \.\0&0&0 &...&a_nend{bmatrix}$
I need to show that $(A-a_1I) cdot (A-a_2I) cdot ... cdot (A-a_nI)=0$.
I tried this for long time but I am not sure how to do this. I know there is a matrix $P$ such that $A=P^{-1}DP$, but I'm not sure how that helps. It was written that it is enough to prove this for n=2 (n is the number of rows and columns of A) and I was able to do this for n=2 but I don't know how that helps me to prove this generally. If anyone can provide any help (I'd love to get some hints first, and only then the full solution if I'm still stuck), It will help me a lot. Thanks!

**All I know about similar matrices is that there exists $P$ such that $A=P^{-1}BP$, so the solution should be quite basic, using that fact only.










share|cite|improve this question











$endgroup$




I'm not quite sure how to prove whats written in the title. We have a field $F$ and a matrix $A$ that is similar to the diagonal matrix $D=operatorname{diag}(a_1,...,a_n)=begin{bmatrix} a_1& 0 &0&. . .& 0 \ 0& a_2 &0&...&0 \ . \. \.\0&0&0 &...&a_nend{bmatrix}$
I need to show that $(A-a_1I) cdot (A-a_2I) cdot ... cdot (A-a_nI)=0$.
I tried this for long time but I am not sure how to do this. I know there is a matrix $P$ such that $A=P^{-1}DP$, but I'm not sure how that helps. It was written that it is enough to prove this for n=2 (n is the number of rows and columns of A) and I was able to do this for n=2 but I don't know how that helps me to prove this generally. If anyone can provide any help (I'd love to get some hints first, and only then the full solution if I'm still stuck), It will help me a lot. Thanks!

**All I know about similar matrices is that there exists $P$ such that $A=P^{-1}BP$, so the solution should be quite basic, using that fact only.







linear-algebra matrices






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edited Dec 28 '18 at 21:09







Omer

















asked Dec 28 '18 at 20:56









OmerOmer

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535110








  • 1




    $begingroup$
    Hint: Prove that similar matrices has the same characteristic polynomial and the fact a matrix satisfies its own characteristic polynomial.
    $endgroup$
    – dezdichado
    Dec 28 '18 at 21:01










  • $begingroup$
    @dezdichado Hi, I haven't learnt about characterisitc polynomial yet, all I know about similar matrices is that there exists P such that A=P^-1*B*P, and that's it. They said in the question that it is enough to show this for n=2 and than explain how that also implies it for all values of n. sorry for not mentioning that, I'll edit my post now.
    $endgroup$
    – Omer
    Dec 28 '18 at 21:07














  • 1




    $begingroup$
    Hint: Prove that similar matrices has the same characteristic polynomial and the fact a matrix satisfies its own characteristic polynomial.
    $endgroup$
    – dezdichado
    Dec 28 '18 at 21:01










  • $begingroup$
    @dezdichado Hi, I haven't learnt about characterisitc polynomial yet, all I know about similar matrices is that there exists P such that A=P^-1*B*P, and that's it. They said in the question that it is enough to show this for n=2 and than explain how that also implies it for all values of n. sorry for not mentioning that, I'll edit my post now.
    $endgroup$
    – Omer
    Dec 28 '18 at 21:07








1




1




$begingroup$
Hint: Prove that similar matrices has the same characteristic polynomial and the fact a matrix satisfies its own characteristic polynomial.
$endgroup$
– dezdichado
Dec 28 '18 at 21:01




$begingroup$
Hint: Prove that similar matrices has the same characteristic polynomial and the fact a matrix satisfies its own characteristic polynomial.
$endgroup$
– dezdichado
Dec 28 '18 at 21:01












$begingroup$
@dezdichado Hi, I haven't learnt about characterisitc polynomial yet, all I know about similar matrices is that there exists P such that A=P^-1*B*P, and that's it. They said in the question that it is enough to show this for n=2 and than explain how that also implies it for all values of n. sorry for not mentioning that, I'll edit my post now.
$endgroup$
– Omer
Dec 28 '18 at 21:07




$begingroup$
@dezdichado Hi, I haven't learnt about characterisitc polynomial yet, all I know about similar matrices is that there exists P such that A=P^-1*B*P, and that's it. They said in the question that it is enough to show this for n=2 and than explain how that also implies it for all values of n. sorry for not mentioning that, I'll edit my post now.
$endgroup$
– Omer
Dec 28 '18 at 21:07










4 Answers
4






active

oldest

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0












$begingroup$

Hint:



First check that $;A-alpha I=P^{-1}DP-alpha I=P^{-1}(D-alpha I)P$.



Second, check that $;(D-alpha_1I)(D-alpha_2I)dots(D-alpha_nI)=0$






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$endgroup$





















    1












    $begingroup$

    Prove that if $p(x)=c_0+c_1x+dots+c^kx^k$ is a polynomial and
    $$
    p(B)=c_0I_n+c_1B+dots+c_kB^k=0
    $$

    (briefly, $B$ satisfies $p$) then also $p(A)=0$, whenever $A$ is similar to $B$.



    Next prove that $D=operatorname{diag}(a_1,dots,a_n)$ (the diagonal matrix with the given elements on the diagonal) satisfies $p(x)=(x-a_1)(x-a_2)dots(x-a_n)$.






    share|cite|improve this answer









    $endgroup$





















      0












      $begingroup$

      Hint: PT
      $$
      A= P^{-1}begin{bmatrix}
      a_1 & & \
      &a_2 & \
      & & ddots \
      end{bmatrix}Pimplies
      A-a_{1}I= P^{-1}begin{bmatrix}
      0 & & \
      &a_{2}-a_{1} & \
      & & ddots
      end{bmatrix}P
      $$



      then multiply all the $A-a_{i}I$.






      share|cite|improve this answer









      $endgroup$





















        0












        $begingroup$

        Let $P$ be as in the OP and let $e_k$; $k=1,2,ldots, n$ denote the vector such that the $k$-th component is 1 and every other component is 0.




        Fact 1: Then on the one hand, the $P^{-1}e_k$s span $mathbb{F}^n$, so if there is a matrix $B$ such that $B(P^{-1}e_k)$ is 0 for all $k$, then $B$ must be the 0 matrix.




        [Make sure you see why Fact 1 is true.]



        On the other hand, $A(P^{-1} e_k) = P^{-1}DP (P^{-1}e_k) = P^{-1}D(PP^{-1})e_k$ $ = P^{-1}De_k = P^{-1} a_ke_k = a_kP^{-1}e_k$. Thus $(A-a_kI)(P^{-1}e_k) = 0$ for each such $k$.



        This implies $[prod_{l=1}^n (A-a_lI)](P^{-1}e_k) = 0$ for each such $k$. This and Fact 1 implies $[prod_l (A-a_lI)]$ is the 0 matrix.






        share|cite|improve this answer











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          4 Answers
          4






          active

          oldest

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          4 Answers
          4






          active

          oldest

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          active

          oldest

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          active

          oldest

          votes









          0












          $begingroup$

          Hint:



          First check that $;A-alpha I=P^{-1}DP-alpha I=P^{-1}(D-alpha I)P$.



          Second, check that $;(D-alpha_1I)(D-alpha_2I)dots(D-alpha_nI)=0$






          share|cite|improve this answer









          $endgroup$


















            0












            $begingroup$

            Hint:



            First check that $;A-alpha I=P^{-1}DP-alpha I=P^{-1}(D-alpha I)P$.



            Second, check that $;(D-alpha_1I)(D-alpha_2I)dots(D-alpha_nI)=0$






            share|cite|improve this answer









            $endgroup$
















              0












              0








              0





              $begingroup$

              Hint:



              First check that $;A-alpha I=P^{-1}DP-alpha I=P^{-1}(D-alpha I)P$.



              Second, check that $;(D-alpha_1I)(D-alpha_2I)dots(D-alpha_nI)=0$






              share|cite|improve this answer









              $endgroup$



              Hint:



              First check that $;A-alpha I=P^{-1}DP-alpha I=P^{-1}(D-alpha I)P$.



              Second, check that $;(D-alpha_1I)(D-alpha_2I)dots(D-alpha_nI)=0$







              share|cite|improve this answer












              share|cite|improve this answer



              share|cite|improve this answer










              answered Dec 28 '18 at 21:22









              BernardBernard

              124k741117




              124k741117























                  1












                  $begingroup$

                  Prove that if $p(x)=c_0+c_1x+dots+c^kx^k$ is a polynomial and
                  $$
                  p(B)=c_0I_n+c_1B+dots+c_kB^k=0
                  $$

                  (briefly, $B$ satisfies $p$) then also $p(A)=0$, whenever $A$ is similar to $B$.



                  Next prove that $D=operatorname{diag}(a_1,dots,a_n)$ (the diagonal matrix with the given elements on the diagonal) satisfies $p(x)=(x-a_1)(x-a_2)dots(x-a_n)$.






                  share|cite|improve this answer









                  $endgroup$


















                    1












                    $begingroup$

                    Prove that if $p(x)=c_0+c_1x+dots+c^kx^k$ is a polynomial and
                    $$
                    p(B)=c_0I_n+c_1B+dots+c_kB^k=0
                    $$

                    (briefly, $B$ satisfies $p$) then also $p(A)=0$, whenever $A$ is similar to $B$.



                    Next prove that $D=operatorname{diag}(a_1,dots,a_n)$ (the diagonal matrix with the given elements on the diagonal) satisfies $p(x)=(x-a_1)(x-a_2)dots(x-a_n)$.






                    share|cite|improve this answer









                    $endgroup$
















                      1












                      1








                      1





                      $begingroup$

                      Prove that if $p(x)=c_0+c_1x+dots+c^kx^k$ is a polynomial and
                      $$
                      p(B)=c_0I_n+c_1B+dots+c_kB^k=0
                      $$

                      (briefly, $B$ satisfies $p$) then also $p(A)=0$, whenever $A$ is similar to $B$.



                      Next prove that $D=operatorname{diag}(a_1,dots,a_n)$ (the diagonal matrix with the given elements on the diagonal) satisfies $p(x)=(x-a_1)(x-a_2)dots(x-a_n)$.






                      share|cite|improve this answer









                      $endgroup$



                      Prove that if $p(x)=c_0+c_1x+dots+c^kx^k$ is a polynomial and
                      $$
                      p(B)=c_0I_n+c_1B+dots+c_kB^k=0
                      $$

                      (briefly, $B$ satisfies $p$) then also $p(A)=0$, whenever $A$ is similar to $B$.



                      Next prove that $D=operatorname{diag}(a_1,dots,a_n)$ (the diagonal matrix with the given elements on the diagonal) satisfies $p(x)=(x-a_1)(x-a_2)dots(x-a_n)$.







                      share|cite|improve this answer












                      share|cite|improve this answer



                      share|cite|improve this answer










                      answered Dec 28 '18 at 21:11









                      egregegreg

                      186k1486208




                      186k1486208























                          0












                          $begingroup$

                          Hint: PT
                          $$
                          A= P^{-1}begin{bmatrix}
                          a_1 & & \
                          &a_2 & \
                          & & ddots \
                          end{bmatrix}Pimplies
                          A-a_{1}I= P^{-1}begin{bmatrix}
                          0 & & \
                          &a_{2}-a_{1} & \
                          & & ddots
                          end{bmatrix}P
                          $$



                          then multiply all the $A-a_{i}I$.






                          share|cite|improve this answer









                          $endgroup$


















                            0












                            $begingroup$

                            Hint: PT
                            $$
                            A= P^{-1}begin{bmatrix}
                            a_1 & & \
                            &a_2 & \
                            & & ddots \
                            end{bmatrix}Pimplies
                            A-a_{1}I= P^{-1}begin{bmatrix}
                            0 & & \
                            &a_{2}-a_{1} & \
                            & & ddots
                            end{bmatrix}P
                            $$



                            then multiply all the $A-a_{i}I$.






                            share|cite|improve this answer









                            $endgroup$
















                              0












                              0








                              0





                              $begingroup$

                              Hint: PT
                              $$
                              A= P^{-1}begin{bmatrix}
                              a_1 & & \
                              &a_2 & \
                              & & ddots \
                              end{bmatrix}Pimplies
                              A-a_{1}I= P^{-1}begin{bmatrix}
                              0 & & \
                              &a_{2}-a_{1} & \
                              & & ddots
                              end{bmatrix}P
                              $$



                              then multiply all the $A-a_{i}I$.






                              share|cite|improve this answer









                              $endgroup$



                              Hint: PT
                              $$
                              A= P^{-1}begin{bmatrix}
                              a_1 & & \
                              &a_2 & \
                              & & ddots \
                              end{bmatrix}Pimplies
                              A-a_{1}I= P^{-1}begin{bmatrix}
                              0 & & \
                              &a_{2}-a_{1} & \
                              & & ddots
                              end{bmatrix}P
                              $$



                              then multiply all the $A-a_{i}I$.







                              share|cite|improve this answer












                              share|cite|improve this answer



                              share|cite|improve this answer










                              answered Dec 28 '18 at 21:14









                              zimbra314zimbra314

                              640312




                              640312























                                  0












                                  $begingroup$

                                  Let $P$ be as in the OP and let $e_k$; $k=1,2,ldots, n$ denote the vector such that the $k$-th component is 1 and every other component is 0.




                                  Fact 1: Then on the one hand, the $P^{-1}e_k$s span $mathbb{F}^n$, so if there is a matrix $B$ such that $B(P^{-1}e_k)$ is 0 for all $k$, then $B$ must be the 0 matrix.




                                  [Make sure you see why Fact 1 is true.]



                                  On the other hand, $A(P^{-1} e_k) = P^{-1}DP (P^{-1}e_k) = P^{-1}D(PP^{-1})e_k$ $ = P^{-1}De_k = P^{-1} a_ke_k = a_kP^{-1}e_k$. Thus $(A-a_kI)(P^{-1}e_k) = 0$ for each such $k$.



                                  This implies $[prod_{l=1}^n (A-a_lI)](P^{-1}e_k) = 0$ for each such $k$. This and Fact 1 implies $[prod_l (A-a_lI)]$ is the 0 matrix.






                                  share|cite|improve this answer











                                  $endgroup$


















                                    0












                                    $begingroup$

                                    Let $P$ be as in the OP and let $e_k$; $k=1,2,ldots, n$ denote the vector such that the $k$-th component is 1 and every other component is 0.




                                    Fact 1: Then on the one hand, the $P^{-1}e_k$s span $mathbb{F}^n$, so if there is a matrix $B$ such that $B(P^{-1}e_k)$ is 0 for all $k$, then $B$ must be the 0 matrix.




                                    [Make sure you see why Fact 1 is true.]



                                    On the other hand, $A(P^{-1} e_k) = P^{-1}DP (P^{-1}e_k) = P^{-1}D(PP^{-1})e_k$ $ = P^{-1}De_k = P^{-1} a_ke_k = a_kP^{-1}e_k$. Thus $(A-a_kI)(P^{-1}e_k) = 0$ for each such $k$.



                                    This implies $[prod_{l=1}^n (A-a_lI)](P^{-1}e_k) = 0$ for each such $k$. This and Fact 1 implies $[prod_l (A-a_lI)]$ is the 0 matrix.






                                    share|cite|improve this answer











                                    $endgroup$
















                                      0












                                      0








                                      0





                                      $begingroup$

                                      Let $P$ be as in the OP and let $e_k$; $k=1,2,ldots, n$ denote the vector such that the $k$-th component is 1 and every other component is 0.




                                      Fact 1: Then on the one hand, the $P^{-1}e_k$s span $mathbb{F}^n$, so if there is a matrix $B$ such that $B(P^{-1}e_k)$ is 0 for all $k$, then $B$ must be the 0 matrix.




                                      [Make sure you see why Fact 1 is true.]



                                      On the other hand, $A(P^{-1} e_k) = P^{-1}DP (P^{-1}e_k) = P^{-1}D(PP^{-1})e_k$ $ = P^{-1}De_k = P^{-1} a_ke_k = a_kP^{-1}e_k$. Thus $(A-a_kI)(P^{-1}e_k) = 0$ for each such $k$.



                                      This implies $[prod_{l=1}^n (A-a_lI)](P^{-1}e_k) = 0$ for each such $k$. This and Fact 1 implies $[prod_l (A-a_lI)]$ is the 0 matrix.






                                      share|cite|improve this answer











                                      $endgroup$



                                      Let $P$ be as in the OP and let $e_k$; $k=1,2,ldots, n$ denote the vector such that the $k$-th component is 1 and every other component is 0.




                                      Fact 1: Then on the one hand, the $P^{-1}e_k$s span $mathbb{F}^n$, so if there is a matrix $B$ such that $B(P^{-1}e_k)$ is 0 for all $k$, then $B$ must be the 0 matrix.




                                      [Make sure you see why Fact 1 is true.]



                                      On the other hand, $A(P^{-1} e_k) = P^{-1}DP (P^{-1}e_k) = P^{-1}D(PP^{-1})e_k$ $ = P^{-1}De_k = P^{-1} a_ke_k = a_kP^{-1}e_k$. Thus $(A-a_kI)(P^{-1}e_k) = 0$ for each such $k$.



                                      This implies $[prod_{l=1}^n (A-a_lI)](P^{-1}e_k) = 0$ for each such $k$. This and Fact 1 implies $[prod_l (A-a_lI)]$ is the 0 matrix.







                                      share|cite|improve this answer














                                      share|cite|improve this answer



                                      share|cite|improve this answer








                                      edited Dec 28 '18 at 21:21

























                                      answered Dec 28 '18 at 21:16









                                      MikeMike

                                      4,641512




                                      4,641512






























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