Solving a non-homogenous system of differential equations












0












$begingroup$



If X' = AX + b then can I use the same principle as if this was an first order ODE?




What do I mean by that, to solve a first order ODE we multiply with the so-called integrating factor: $e^{-int a(t)dt}$ can I apply this here and solve it this way?



So what would I do:



$$X'-AX=b$$



multiply both sides with $e^{-At}$



$$(Xe^{-At})'=be^{-At}$$



integrate both sides



$$Xe^{-At}=int be^{-At}dt$$



Giving us:



$$X=e^{At}int be^{-At}dt$$



Where $A in mathcal M_n(mathbb{R})$



Is this proof correct? How do I integrate a Matrix or a vector? Do I integrate on every component? Also how do I raise $e$ to some matrix power? Do I use the fact that:



$$e^{At}=sum_{n=0}^{infty}frac{(At)^n}{n!}$$



And if $A$ is diagonalizable I can:



$$e^{At}=P(sum_{n=0}^{infty}frac{(Dt)^n}{n!})P^{-1}$$



Now:



$$e^{At}=Pe^{Dt}P^{-1}$$



where: $D$ is a diagonal matrix, but still how do I compute $e^{Dt}$?



And what if $A$ is not diagnolaziable and it's root are complex, conjugate?



How can I solve then?










share|cite|improve this question









$endgroup$












  • $begingroup$
    Did you review something like math.berkeley.edu/~conway/Teaching/old/summer2016-2552bc/… and math.okstate.edu/people/binegar/4233/4233-l03.pdf and ndsu.edu/pubweb/~novozhil/Teaching/266%20Data/lecture_24.pdf?
    $endgroup$
    – Moo
    Jan 15 at 13:53












  • $begingroup$
    No, but can you exaplin me in more details what really happens when you raise $e$ to a matrix?
    $endgroup$
    – C. Cristi
    Jan 15 at 14:31










  • $begingroup$
    Sure, read this - cs.cornell.edu/cv/researchpdf/19ways+.pdf
    $endgroup$
    – Moo
    Jan 15 at 16:07












  • $begingroup$
    Your proof is formally correct although it doesn't make much sense if you don't know about the matrix exponential. For the integral of a vector-valued function, you integrate each entry, yes. For the calculation of the matrix exponential see my answer below.
    $endgroup$
    – Christoph
    Jan 16 at 5:50
















0












$begingroup$



If X' = AX + b then can I use the same principle as if this was an first order ODE?




What do I mean by that, to solve a first order ODE we multiply with the so-called integrating factor: $e^{-int a(t)dt}$ can I apply this here and solve it this way?



So what would I do:



$$X'-AX=b$$



multiply both sides with $e^{-At}$



$$(Xe^{-At})'=be^{-At}$$



integrate both sides



$$Xe^{-At}=int be^{-At}dt$$



Giving us:



$$X=e^{At}int be^{-At}dt$$



Where $A in mathcal M_n(mathbb{R})$



Is this proof correct? How do I integrate a Matrix or a vector? Do I integrate on every component? Also how do I raise $e$ to some matrix power? Do I use the fact that:



$$e^{At}=sum_{n=0}^{infty}frac{(At)^n}{n!}$$



And if $A$ is diagonalizable I can:



$$e^{At}=P(sum_{n=0}^{infty}frac{(Dt)^n}{n!})P^{-1}$$



Now:



$$e^{At}=Pe^{Dt}P^{-1}$$



where: $D$ is a diagonal matrix, but still how do I compute $e^{Dt}$?



And what if $A$ is not diagnolaziable and it's root are complex, conjugate?



How can I solve then?










share|cite|improve this question









$endgroup$












  • $begingroup$
    Did you review something like math.berkeley.edu/~conway/Teaching/old/summer2016-2552bc/… and math.okstate.edu/people/binegar/4233/4233-l03.pdf and ndsu.edu/pubweb/~novozhil/Teaching/266%20Data/lecture_24.pdf?
    $endgroup$
    – Moo
    Jan 15 at 13:53












  • $begingroup$
    No, but can you exaplin me in more details what really happens when you raise $e$ to a matrix?
    $endgroup$
    – C. Cristi
    Jan 15 at 14:31










  • $begingroup$
    Sure, read this - cs.cornell.edu/cv/researchpdf/19ways+.pdf
    $endgroup$
    – Moo
    Jan 15 at 16:07












  • $begingroup$
    Your proof is formally correct although it doesn't make much sense if you don't know about the matrix exponential. For the integral of a vector-valued function, you integrate each entry, yes. For the calculation of the matrix exponential see my answer below.
    $endgroup$
    – Christoph
    Jan 16 at 5:50














0












0








0


1



$begingroup$



If X' = AX + b then can I use the same principle as if this was an first order ODE?




What do I mean by that, to solve a first order ODE we multiply with the so-called integrating factor: $e^{-int a(t)dt}$ can I apply this here and solve it this way?



So what would I do:



$$X'-AX=b$$



multiply both sides with $e^{-At}$



$$(Xe^{-At})'=be^{-At}$$



integrate both sides



$$Xe^{-At}=int be^{-At}dt$$



Giving us:



$$X=e^{At}int be^{-At}dt$$



Where $A in mathcal M_n(mathbb{R})$



Is this proof correct? How do I integrate a Matrix or a vector? Do I integrate on every component? Also how do I raise $e$ to some matrix power? Do I use the fact that:



$$e^{At}=sum_{n=0}^{infty}frac{(At)^n}{n!}$$



And if $A$ is diagonalizable I can:



$$e^{At}=P(sum_{n=0}^{infty}frac{(Dt)^n}{n!})P^{-1}$$



Now:



$$e^{At}=Pe^{Dt}P^{-1}$$



where: $D$ is a diagonal matrix, but still how do I compute $e^{Dt}$?



And what if $A$ is not diagnolaziable and it's root are complex, conjugate?



How can I solve then?










share|cite|improve this question









$endgroup$





If X' = AX + b then can I use the same principle as if this was an first order ODE?




What do I mean by that, to solve a first order ODE we multiply with the so-called integrating factor: $e^{-int a(t)dt}$ can I apply this here and solve it this way?



So what would I do:



$$X'-AX=b$$



multiply both sides with $e^{-At}$



$$(Xe^{-At})'=be^{-At}$$



integrate both sides



$$Xe^{-At}=int be^{-At}dt$$



Giving us:



$$X=e^{At}int be^{-At}dt$$



Where $A in mathcal M_n(mathbb{R})$



Is this proof correct? How do I integrate a Matrix or a vector? Do I integrate on every component? Also how do I raise $e$ to some matrix power? Do I use the fact that:



$$e^{At}=sum_{n=0}^{infty}frac{(At)^n}{n!}$$



And if $A$ is diagonalizable I can:



$$e^{At}=P(sum_{n=0}^{infty}frac{(Dt)^n}{n!})P^{-1}$$



Now:



$$e^{At}=Pe^{Dt}P^{-1}$$



where: $D$ is a diagonal matrix, but still how do I compute $e^{Dt}$?



And what if $A$ is not diagnolaziable and it's root are complex, conjugate?



How can I solve then?







ordinary-differential-equations systems-of-equations






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Jan 15 at 13:41









C. CristiC. Cristi

1,629218




1,629218












  • $begingroup$
    Did you review something like math.berkeley.edu/~conway/Teaching/old/summer2016-2552bc/… and math.okstate.edu/people/binegar/4233/4233-l03.pdf and ndsu.edu/pubweb/~novozhil/Teaching/266%20Data/lecture_24.pdf?
    $endgroup$
    – Moo
    Jan 15 at 13:53












  • $begingroup$
    No, but can you exaplin me in more details what really happens when you raise $e$ to a matrix?
    $endgroup$
    – C. Cristi
    Jan 15 at 14:31










  • $begingroup$
    Sure, read this - cs.cornell.edu/cv/researchpdf/19ways+.pdf
    $endgroup$
    – Moo
    Jan 15 at 16:07












  • $begingroup$
    Your proof is formally correct although it doesn't make much sense if you don't know about the matrix exponential. For the integral of a vector-valued function, you integrate each entry, yes. For the calculation of the matrix exponential see my answer below.
    $endgroup$
    – Christoph
    Jan 16 at 5:50


















  • $begingroup$
    Did you review something like math.berkeley.edu/~conway/Teaching/old/summer2016-2552bc/… and math.okstate.edu/people/binegar/4233/4233-l03.pdf and ndsu.edu/pubweb/~novozhil/Teaching/266%20Data/lecture_24.pdf?
    $endgroup$
    – Moo
    Jan 15 at 13:53












  • $begingroup$
    No, but can you exaplin me in more details what really happens when you raise $e$ to a matrix?
    $endgroup$
    – C. Cristi
    Jan 15 at 14:31










  • $begingroup$
    Sure, read this - cs.cornell.edu/cv/researchpdf/19ways+.pdf
    $endgroup$
    – Moo
    Jan 15 at 16:07












  • $begingroup$
    Your proof is formally correct although it doesn't make much sense if you don't know about the matrix exponential. For the integral of a vector-valued function, you integrate each entry, yes. For the calculation of the matrix exponential see my answer below.
    $endgroup$
    – Christoph
    Jan 16 at 5:50
















$begingroup$
Did you review something like math.berkeley.edu/~conway/Teaching/old/summer2016-2552bc/… and math.okstate.edu/people/binegar/4233/4233-l03.pdf and ndsu.edu/pubweb/~novozhil/Teaching/266%20Data/lecture_24.pdf?
$endgroup$
– Moo
Jan 15 at 13:53






$begingroup$
Did you review something like math.berkeley.edu/~conway/Teaching/old/summer2016-2552bc/… and math.okstate.edu/people/binegar/4233/4233-l03.pdf and ndsu.edu/pubweb/~novozhil/Teaching/266%20Data/lecture_24.pdf?
$endgroup$
– Moo
Jan 15 at 13:53














$begingroup$
No, but can you exaplin me in more details what really happens when you raise $e$ to a matrix?
$endgroup$
– C. Cristi
Jan 15 at 14:31




$begingroup$
No, but can you exaplin me in more details what really happens when you raise $e$ to a matrix?
$endgroup$
– C. Cristi
Jan 15 at 14:31












$begingroup$
Sure, read this - cs.cornell.edu/cv/researchpdf/19ways+.pdf
$endgroup$
– Moo
Jan 15 at 16:07






$begingroup$
Sure, read this - cs.cornell.edu/cv/researchpdf/19ways+.pdf
$endgroup$
– Moo
Jan 15 at 16:07














$begingroup$
Your proof is formally correct although it doesn't make much sense if you don't know about the matrix exponential. For the integral of a vector-valued function, you integrate each entry, yes. For the calculation of the matrix exponential see my answer below.
$endgroup$
– Christoph
Jan 16 at 5:50




$begingroup$
Your proof is formally correct although it doesn't make much sense if you don't know about the matrix exponential. For the integral of a vector-valued function, you integrate each entry, yes. For the calculation of the matrix exponential see my answer below.
$endgroup$
– Christoph
Jan 16 at 5:50










1 Answer
1






active

oldest

votes


















1












$begingroup$

In general you use the Jordan normal form $J$ of $A$, $A = P J P^{-1}$. The matrix $J$ can be written as $J = D + N$ with a diagonal matrix $D$ (containing the eigenvalues of $A$) and with a nilpotent matrix $N$ ($N=0$ if and only if $A$ is diagonalizable). From the power series definition of the exponential function which you already wrote down we obtain
begin{equation}
e^{tA} = P e^{tJ} P^{-1} = P e^{t(D+N)} P^{-1} = P e^{tD} e^{tN} P^{-1}.
end{equation}

Now the matrix exponentials $e^{tD}$ and $e^{tN}$ are easy to evaluate:



1.
begin{equation}
e^{tD} = sum_{k=0}^{infty} frac{(tD)^k}{k!} = sum_{k=0}^{infty} frac{t^k}{k!} D^k = sum_{k=0}^{infty} frac{t^k}{k!} left( begin{array}{cccc}
d_1^k\
& d_2^k\
& & ddots\
& & & d_n^k
end{array}
right) = left( begin{array}{cccc}
sum_{k=0}^{infty} frac{(td_1)^k}{k!}\
& sum_{k=0}^{infty} frac{(td_2)^k}{k!}\
& & ddots\
& & & sum_{k=0}^{infty} frac{(td_n)^k}{k!}
end{array}
right) = left( begin{array}{cccc}
e^{td_1}\
& e^{td_2}\
& & ddots\
& & & e^{td_n}
end{array}
right).
end{equation}

2. For the matrix exponential of the nilpotent matrix $N$, which by definition satisfies $N^k = 0$, $k geq m$, for some $m in mathbb{N}$, you simply obtain a finite sum of powers of $tN$,
begin{equation}
e^{tN} = sum_{k=0}^{m-1} frac{(tN)^k}{k!}.
end{equation}

Finally you compute the product $e^{tA} = P e^{tD} e^{tN} P^{-1}$.



By the way all of this also works for complex-valued entries, there is no difference.






share|cite|improve this answer











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    $begingroup$

    In general you use the Jordan normal form $J$ of $A$, $A = P J P^{-1}$. The matrix $J$ can be written as $J = D + N$ with a diagonal matrix $D$ (containing the eigenvalues of $A$) and with a nilpotent matrix $N$ ($N=0$ if and only if $A$ is diagonalizable). From the power series definition of the exponential function which you already wrote down we obtain
    begin{equation}
    e^{tA} = P e^{tJ} P^{-1} = P e^{t(D+N)} P^{-1} = P e^{tD} e^{tN} P^{-1}.
    end{equation}

    Now the matrix exponentials $e^{tD}$ and $e^{tN}$ are easy to evaluate:



    1.
    begin{equation}
    e^{tD} = sum_{k=0}^{infty} frac{(tD)^k}{k!} = sum_{k=0}^{infty} frac{t^k}{k!} D^k = sum_{k=0}^{infty} frac{t^k}{k!} left( begin{array}{cccc}
    d_1^k\
    & d_2^k\
    & & ddots\
    & & & d_n^k
    end{array}
    right) = left( begin{array}{cccc}
    sum_{k=0}^{infty} frac{(td_1)^k}{k!}\
    & sum_{k=0}^{infty} frac{(td_2)^k}{k!}\
    & & ddots\
    & & & sum_{k=0}^{infty} frac{(td_n)^k}{k!}
    end{array}
    right) = left( begin{array}{cccc}
    e^{td_1}\
    & e^{td_2}\
    & & ddots\
    & & & e^{td_n}
    end{array}
    right).
    end{equation}

    2. For the matrix exponential of the nilpotent matrix $N$, which by definition satisfies $N^k = 0$, $k geq m$, for some $m in mathbb{N}$, you simply obtain a finite sum of powers of $tN$,
    begin{equation}
    e^{tN} = sum_{k=0}^{m-1} frac{(tN)^k}{k!}.
    end{equation}

    Finally you compute the product $e^{tA} = P e^{tD} e^{tN} P^{-1}$.



    By the way all of this also works for complex-valued entries, there is no difference.






    share|cite|improve this answer











    $endgroup$


















      1












      $begingroup$

      In general you use the Jordan normal form $J$ of $A$, $A = P J P^{-1}$. The matrix $J$ can be written as $J = D + N$ with a diagonal matrix $D$ (containing the eigenvalues of $A$) and with a nilpotent matrix $N$ ($N=0$ if and only if $A$ is diagonalizable). From the power series definition of the exponential function which you already wrote down we obtain
      begin{equation}
      e^{tA} = P e^{tJ} P^{-1} = P e^{t(D+N)} P^{-1} = P e^{tD} e^{tN} P^{-1}.
      end{equation}

      Now the matrix exponentials $e^{tD}$ and $e^{tN}$ are easy to evaluate:



      1.
      begin{equation}
      e^{tD} = sum_{k=0}^{infty} frac{(tD)^k}{k!} = sum_{k=0}^{infty} frac{t^k}{k!} D^k = sum_{k=0}^{infty} frac{t^k}{k!} left( begin{array}{cccc}
      d_1^k\
      & d_2^k\
      & & ddots\
      & & & d_n^k
      end{array}
      right) = left( begin{array}{cccc}
      sum_{k=0}^{infty} frac{(td_1)^k}{k!}\
      & sum_{k=0}^{infty} frac{(td_2)^k}{k!}\
      & & ddots\
      & & & sum_{k=0}^{infty} frac{(td_n)^k}{k!}
      end{array}
      right) = left( begin{array}{cccc}
      e^{td_1}\
      & e^{td_2}\
      & & ddots\
      & & & e^{td_n}
      end{array}
      right).
      end{equation}

      2. For the matrix exponential of the nilpotent matrix $N$, which by definition satisfies $N^k = 0$, $k geq m$, for some $m in mathbb{N}$, you simply obtain a finite sum of powers of $tN$,
      begin{equation}
      e^{tN} = sum_{k=0}^{m-1} frac{(tN)^k}{k!}.
      end{equation}

      Finally you compute the product $e^{tA} = P e^{tD} e^{tN} P^{-1}$.



      By the way all of this also works for complex-valued entries, there is no difference.






      share|cite|improve this answer











      $endgroup$
















        1












        1








        1





        $begingroup$

        In general you use the Jordan normal form $J$ of $A$, $A = P J P^{-1}$. The matrix $J$ can be written as $J = D + N$ with a diagonal matrix $D$ (containing the eigenvalues of $A$) and with a nilpotent matrix $N$ ($N=0$ if and only if $A$ is diagonalizable). From the power series definition of the exponential function which you already wrote down we obtain
        begin{equation}
        e^{tA} = P e^{tJ} P^{-1} = P e^{t(D+N)} P^{-1} = P e^{tD} e^{tN} P^{-1}.
        end{equation}

        Now the matrix exponentials $e^{tD}$ and $e^{tN}$ are easy to evaluate:



        1.
        begin{equation}
        e^{tD} = sum_{k=0}^{infty} frac{(tD)^k}{k!} = sum_{k=0}^{infty} frac{t^k}{k!} D^k = sum_{k=0}^{infty} frac{t^k}{k!} left( begin{array}{cccc}
        d_1^k\
        & d_2^k\
        & & ddots\
        & & & d_n^k
        end{array}
        right) = left( begin{array}{cccc}
        sum_{k=0}^{infty} frac{(td_1)^k}{k!}\
        & sum_{k=0}^{infty} frac{(td_2)^k}{k!}\
        & & ddots\
        & & & sum_{k=0}^{infty} frac{(td_n)^k}{k!}
        end{array}
        right) = left( begin{array}{cccc}
        e^{td_1}\
        & e^{td_2}\
        & & ddots\
        & & & e^{td_n}
        end{array}
        right).
        end{equation}

        2. For the matrix exponential of the nilpotent matrix $N$, which by definition satisfies $N^k = 0$, $k geq m$, for some $m in mathbb{N}$, you simply obtain a finite sum of powers of $tN$,
        begin{equation}
        e^{tN} = sum_{k=0}^{m-1} frac{(tN)^k}{k!}.
        end{equation}

        Finally you compute the product $e^{tA} = P e^{tD} e^{tN} P^{-1}$.



        By the way all of this also works for complex-valued entries, there is no difference.






        share|cite|improve this answer











        $endgroup$



        In general you use the Jordan normal form $J$ of $A$, $A = P J P^{-1}$. The matrix $J$ can be written as $J = D + N$ with a diagonal matrix $D$ (containing the eigenvalues of $A$) and with a nilpotent matrix $N$ ($N=0$ if and only if $A$ is diagonalizable). From the power series definition of the exponential function which you already wrote down we obtain
        begin{equation}
        e^{tA} = P e^{tJ} P^{-1} = P e^{t(D+N)} P^{-1} = P e^{tD} e^{tN} P^{-1}.
        end{equation}

        Now the matrix exponentials $e^{tD}$ and $e^{tN}$ are easy to evaluate:



        1.
        begin{equation}
        e^{tD} = sum_{k=0}^{infty} frac{(tD)^k}{k!} = sum_{k=0}^{infty} frac{t^k}{k!} D^k = sum_{k=0}^{infty} frac{t^k}{k!} left( begin{array}{cccc}
        d_1^k\
        & d_2^k\
        & & ddots\
        & & & d_n^k
        end{array}
        right) = left( begin{array}{cccc}
        sum_{k=0}^{infty} frac{(td_1)^k}{k!}\
        & sum_{k=0}^{infty} frac{(td_2)^k}{k!}\
        & & ddots\
        & & & sum_{k=0}^{infty} frac{(td_n)^k}{k!}
        end{array}
        right) = left( begin{array}{cccc}
        e^{td_1}\
        & e^{td_2}\
        & & ddots\
        & & & e^{td_n}
        end{array}
        right).
        end{equation}

        2. For the matrix exponential of the nilpotent matrix $N$, which by definition satisfies $N^k = 0$, $k geq m$, for some $m in mathbb{N}$, you simply obtain a finite sum of powers of $tN$,
        begin{equation}
        e^{tN} = sum_{k=0}^{m-1} frac{(tN)^k}{k!}.
        end{equation}

        Finally you compute the product $e^{tA} = P e^{tD} e^{tN} P^{-1}$.



        By the way all of this also works for complex-valued entries, there is no difference.







        share|cite|improve this answer














        share|cite|improve this answer



        share|cite|improve this answer








        edited Jan 16 at 6:09

























        answered Jan 16 at 5:46









        ChristophChristoph

        58116




        58116






























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