Differential difference equation












1














$y'(t)+y(t-frac{1}{2}pi)-y(t-pi)=cos t$



is the equation I am trying to solve. I want to find all solutions with period $2pi$. Now I have tried with assuming that $y(t)sim sum c_ne^{int}=frac{a_0}{2}+sum (a_ncos nt +b_nsin nt)$ and then differentiating and putting it back into the equation, but I do not know how to handle the second term $y(t-frac{1}{2}pi)$.



Let me show you: $y(t-frac{1}{2}pi)sim sum c_ne^{int-infrac{pi}{2}}=sum c_ne^{int}e^{-infrac{pi}{2}}$ ... from here I don't know how to convert the $e^{-infrac{pi}{2}}$ into something "nicer", for example $e^{-inpi}=(-1)^n$. Is there an analog for this term?



I hope I made myself clear. If someone would show a complete solution that would help, too! Best regards //










share|cite|improve this question



























    1














    $y'(t)+y(t-frac{1}{2}pi)-y(t-pi)=cos t$



    is the equation I am trying to solve. I want to find all solutions with period $2pi$. Now I have tried with assuming that $y(t)sim sum c_ne^{int}=frac{a_0}{2}+sum (a_ncos nt +b_nsin nt)$ and then differentiating and putting it back into the equation, but I do not know how to handle the second term $y(t-frac{1}{2}pi)$.



    Let me show you: $y(t-frac{1}{2}pi)sim sum c_ne^{int-infrac{pi}{2}}=sum c_ne^{int}e^{-infrac{pi}{2}}$ ... from here I don't know how to convert the $e^{-infrac{pi}{2}}$ into something "nicer", for example $e^{-inpi}=(-1)^n$. Is there an analog for this term?



    I hope I made myself clear. If someone would show a complete solution that would help, too! Best regards //










    share|cite|improve this question

























      1












      1








      1


      2





      $y'(t)+y(t-frac{1}{2}pi)-y(t-pi)=cos t$



      is the equation I am trying to solve. I want to find all solutions with period $2pi$. Now I have tried with assuming that $y(t)sim sum c_ne^{int}=frac{a_0}{2}+sum (a_ncos nt +b_nsin nt)$ and then differentiating and putting it back into the equation, but I do not know how to handle the second term $y(t-frac{1}{2}pi)$.



      Let me show you: $y(t-frac{1}{2}pi)sim sum c_ne^{int-infrac{pi}{2}}=sum c_ne^{int}e^{-infrac{pi}{2}}$ ... from here I don't know how to convert the $e^{-infrac{pi}{2}}$ into something "nicer", for example $e^{-inpi}=(-1)^n$. Is there an analog for this term?



      I hope I made myself clear. If someone would show a complete solution that would help, too! Best regards //










      share|cite|improve this question













      $y'(t)+y(t-frac{1}{2}pi)-y(t-pi)=cos t$



      is the equation I am trying to solve. I want to find all solutions with period $2pi$. Now I have tried with assuming that $y(t)sim sum c_ne^{int}=frac{a_0}{2}+sum (a_ncos nt +b_nsin nt)$ and then differentiating and putting it back into the equation, but I do not know how to handle the second term $y(t-frac{1}{2}pi)$.



      Let me show you: $y(t-frac{1}{2}pi)sim sum c_ne^{int-infrac{pi}{2}}=sum c_ne^{int}e^{-infrac{pi}{2}}$ ... from here I don't know how to convert the $e^{-infrac{pi}{2}}$ into something "nicer", for example $e^{-inpi}=(-1)^n$. Is there an analog for this term?



      I hope I made myself clear. If someone would show a complete solution that would help, too! Best regards //







      differential-equations recurrence-relations fourier-series






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      asked Jan 5 at 22:27









      SimpleProgrammer SimpleProgrammer

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          To the complex numbers question: $e^{-ifracpi2}=-i$ so $e^{-ifracpi2}=(-i)^n=i^{-n}$.





          Consider first $y_k(t)=y(t-kfracpi2)$ as independent $2pi$ periodic functions. At the end, the integration constant will have to be adapted so that they all are the same function again, if at all possible.



          Then these $4$ functions satisfy the first order system
          $$
          pmatrix{y_0'\y_1'\y_2'\y_3'}
          +
          pmatrix{0&1&-1&0\0&0&1&-1\-1&0&0&1\1&-1&0&0}
          pmatrix{y_0\y_1\y_2\y_3}
          =
          pmatrix{cos(t)\sin(t)\-cos(t)\-sin(t)}
          $$

          The circulant matrix can be written as $A=Q-Q^2$ where $Q^4=I$, so $A$ has eigenvalues $lambda_k=q^k-(q^k)^2=0$, $1+i$, $-2$, $1-i$ for the eigenvalues $q^k=i^k$ and eigenvectors $v^k_j=i^{jk}$, $j=0,1,2,3$, $k=0,1,2,3$, of $Q$. Of these components of the homogeneous solution, only the first one as a constant is again $2pi$ periodic, the others have exponentially increasing or decreasing factors.



          So the qualitative conclusion is that a periodic solution for $y$ can only have the components $$y(t)=A+Bcos(t)+Csin(t).$$ Inserting this gives
          $$
          (-Bsin t+Ccos t)+(A+Bsin t-Ccos t)-(A-Bcos t-Csin t)=cos t
          \
          impliesleft{begin{aligned}
          B&=1\C&=0
          end{aligned}\right.
          $$





          One could also do the first part slightly different by considering linear combinations of the shifted functions,
          $$u_k(t)=sum_{k=0}^3i^ky(t-kfracpi2),~~k=0,1,2,3, ~~~~ (i^4=1, ~~ y(t-2pi)=y(t)). $$ Then
          begin{align}
          u_k'(t)+sum_{m=0}^3i^{km}y(t-(m+1)fracpi2)-sum_{m=0}^3i^{km}y(t-(m+2)fracpi2)
          &=sum_{k=0}^3i^{km}cos(t-mfracpi2)
          \~\
          u_k'(t)+(i^k-(-1)^k)u(t)&=(1-(-1)^k)cos(t)+i^k(1-(-1)^k)sin(t)
          end{align}

          This is a linear ODE with constant coefficients for each of the 4 values of $k$, and the solution can be reconstructed from them by solving the corresponding linear system or a là the inverse Fourier transform.






          share|cite|improve this answer























          • I appreciate your answer, although I am yet not familiar with the Fourier transform or its inverse. Is there another way to solve it? I don't understand what the q represents, and how do you factor the second term? I don't follow.
            – SimpleProgrammer
            2 days ago










          • This is just 4-dimensional linear algebra. Essentially it shows that the solution has to be a combination of $sin t$ and $cos t$ as you have to exclude the exponential terms for a periodic solution. Note that $sin(t+kfracpi2)$ is $sin t,cos t, -sin t, -cos t, sin t,...$ for $k=0,1,2,3,4,..$
            – LutzL
            2 days ago











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          1 Answer
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          1 Answer
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          active

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          1














          To the complex numbers question: $e^{-ifracpi2}=-i$ so $e^{-ifracpi2}=(-i)^n=i^{-n}$.





          Consider first $y_k(t)=y(t-kfracpi2)$ as independent $2pi$ periodic functions. At the end, the integration constant will have to be adapted so that they all are the same function again, if at all possible.



          Then these $4$ functions satisfy the first order system
          $$
          pmatrix{y_0'\y_1'\y_2'\y_3'}
          +
          pmatrix{0&1&-1&0\0&0&1&-1\-1&0&0&1\1&-1&0&0}
          pmatrix{y_0\y_1\y_2\y_3}
          =
          pmatrix{cos(t)\sin(t)\-cos(t)\-sin(t)}
          $$

          The circulant matrix can be written as $A=Q-Q^2$ where $Q^4=I$, so $A$ has eigenvalues $lambda_k=q^k-(q^k)^2=0$, $1+i$, $-2$, $1-i$ for the eigenvalues $q^k=i^k$ and eigenvectors $v^k_j=i^{jk}$, $j=0,1,2,3$, $k=0,1,2,3$, of $Q$. Of these components of the homogeneous solution, only the first one as a constant is again $2pi$ periodic, the others have exponentially increasing or decreasing factors.



          So the qualitative conclusion is that a periodic solution for $y$ can only have the components $$y(t)=A+Bcos(t)+Csin(t).$$ Inserting this gives
          $$
          (-Bsin t+Ccos t)+(A+Bsin t-Ccos t)-(A-Bcos t-Csin t)=cos t
          \
          impliesleft{begin{aligned}
          B&=1\C&=0
          end{aligned}\right.
          $$





          One could also do the first part slightly different by considering linear combinations of the shifted functions,
          $$u_k(t)=sum_{k=0}^3i^ky(t-kfracpi2),~~k=0,1,2,3, ~~~~ (i^4=1, ~~ y(t-2pi)=y(t)). $$ Then
          begin{align}
          u_k'(t)+sum_{m=0}^3i^{km}y(t-(m+1)fracpi2)-sum_{m=0}^3i^{km}y(t-(m+2)fracpi2)
          &=sum_{k=0}^3i^{km}cos(t-mfracpi2)
          \~\
          u_k'(t)+(i^k-(-1)^k)u(t)&=(1-(-1)^k)cos(t)+i^k(1-(-1)^k)sin(t)
          end{align}

          This is a linear ODE with constant coefficients for each of the 4 values of $k$, and the solution can be reconstructed from them by solving the corresponding linear system or a là the inverse Fourier transform.






          share|cite|improve this answer























          • I appreciate your answer, although I am yet not familiar with the Fourier transform or its inverse. Is there another way to solve it? I don't understand what the q represents, and how do you factor the second term? I don't follow.
            – SimpleProgrammer
            2 days ago










          • This is just 4-dimensional linear algebra. Essentially it shows that the solution has to be a combination of $sin t$ and $cos t$ as you have to exclude the exponential terms for a periodic solution. Note that $sin(t+kfracpi2)$ is $sin t,cos t, -sin t, -cos t, sin t,...$ for $k=0,1,2,3,4,..$
            – LutzL
            2 days ago
















          1














          To the complex numbers question: $e^{-ifracpi2}=-i$ so $e^{-ifracpi2}=(-i)^n=i^{-n}$.





          Consider first $y_k(t)=y(t-kfracpi2)$ as independent $2pi$ periodic functions. At the end, the integration constant will have to be adapted so that they all are the same function again, if at all possible.



          Then these $4$ functions satisfy the first order system
          $$
          pmatrix{y_0'\y_1'\y_2'\y_3'}
          +
          pmatrix{0&1&-1&0\0&0&1&-1\-1&0&0&1\1&-1&0&0}
          pmatrix{y_0\y_1\y_2\y_3}
          =
          pmatrix{cos(t)\sin(t)\-cos(t)\-sin(t)}
          $$

          The circulant matrix can be written as $A=Q-Q^2$ where $Q^4=I$, so $A$ has eigenvalues $lambda_k=q^k-(q^k)^2=0$, $1+i$, $-2$, $1-i$ for the eigenvalues $q^k=i^k$ and eigenvectors $v^k_j=i^{jk}$, $j=0,1,2,3$, $k=0,1,2,3$, of $Q$. Of these components of the homogeneous solution, only the first one as a constant is again $2pi$ periodic, the others have exponentially increasing or decreasing factors.



          So the qualitative conclusion is that a periodic solution for $y$ can only have the components $$y(t)=A+Bcos(t)+Csin(t).$$ Inserting this gives
          $$
          (-Bsin t+Ccos t)+(A+Bsin t-Ccos t)-(A-Bcos t-Csin t)=cos t
          \
          impliesleft{begin{aligned}
          B&=1\C&=0
          end{aligned}\right.
          $$





          One could also do the first part slightly different by considering linear combinations of the shifted functions,
          $$u_k(t)=sum_{k=0}^3i^ky(t-kfracpi2),~~k=0,1,2,3, ~~~~ (i^4=1, ~~ y(t-2pi)=y(t)). $$ Then
          begin{align}
          u_k'(t)+sum_{m=0}^3i^{km}y(t-(m+1)fracpi2)-sum_{m=0}^3i^{km}y(t-(m+2)fracpi2)
          &=sum_{k=0}^3i^{km}cos(t-mfracpi2)
          \~\
          u_k'(t)+(i^k-(-1)^k)u(t)&=(1-(-1)^k)cos(t)+i^k(1-(-1)^k)sin(t)
          end{align}

          This is a linear ODE with constant coefficients for each of the 4 values of $k$, and the solution can be reconstructed from them by solving the corresponding linear system or a là the inverse Fourier transform.






          share|cite|improve this answer























          • I appreciate your answer, although I am yet not familiar with the Fourier transform or its inverse. Is there another way to solve it? I don't understand what the q represents, and how do you factor the second term? I don't follow.
            – SimpleProgrammer
            2 days ago










          • This is just 4-dimensional linear algebra. Essentially it shows that the solution has to be a combination of $sin t$ and $cos t$ as you have to exclude the exponential terms for a periodic solution. Note that $sin(t+kfracpi2)$ is $sin t,cos t, -sin t, -cos t, sin t,...$ for $k=0,1,2,3,4,..$
            – LutzL
            2 days ago














          1












          1








          1






          To the complex numbers question: $e^{-ifracpi2}=-i$ so $e^{-ifracpi2}=(-i)^n=i^{-n}$.





          Consider first $y_k(t)=y(t-kfracpi2)$ as independent $2pi$ periodic functions. At the end, the integration constant will have to be adapted so that they all are the same function again, if at all possible.



          Then these $4$ functions satisfy the first order system
          $$
          pmatrix{y_0'\y_1'\y_2'\y_3'}
          +
          pmatrix{0&1&-1&0\0&0&1&-1\-1&0&0&1\1&-1&0&0}
          pmatrix{y_0\y_1\y_2\y_3}
          =
          pmatrix{cos(t)\sin(t)\-cos(t)\-sin(t)}
          $$

          The circulant matrix can be written as $A=Q-Q^2$ where $Q^4=I$, so $A$ has eigenvalues $lambda_k=q^k-(q^k)^2=0$, $1+i$, $-2$, $1-i$ for the eigenvalues $q^k=i^k$ and eigenvectors $v^k_j=i^{jk}$, $j=0,1,2,3$, $k=0,1,2,3$, of $Q$. Of these components of the homogeneous solution, only the first one as a constant is again $2pi$ periodic, the others have exponentially increasing or decreasing factors.



          So the qualitative conclusion is that a periodic solution for $y$ can only have the components $$y(t)=A+Bcos(t)+Csin(t).$$ Inserting this gives
          $$
          (-Bsin t+Ccos t)+(A+Bsin t-Ccos t)-(A-Bcos t-Csin t)=cos t
          \
          impliesleft{begin{aligned}
          B&=1\C&=0
          end{aligned}\right.
          $$





          One could also do the first part slightly different by considering linear combinations of the shifted functions,
          $$u_k(t)=sum_{k=0}^3i^ky(t-kfracpi2),~~k=0,1,2,3, ~~~~ (i^4=1, ~~ y(t-2pi)=y(t)). $$ Then
          begin{align}
          u_k'(t)+sum_{m=0}^3i^{km}y(t-(m+1)fracpi2)-sum_{m=0}^3i^{km}y(t-(m+2)fracpi2)
          &=sum_{k=0}^3i^{km}cos(t-mfracpi2)
          \~\
          u_k'(t)+(i^k-(-1)^k)u(t)&=(1-(-1)^k)cos(t)+i^k(1-(-1)^k)sin(t)
          end{align}

          This is a linear ODE with constant coefficients for each of the 4 values of $k$, and the solution can be reconstructed from them by solving the corresponding linear system or a là the inverse Fourier transform.






          share|cite|improve this answer














          To the complex numbers question: $e^{-ifracpi2}=-i$ so $e^{-ifracpi2}=(-i)^n=i^{-n}$.





          Consider first $y_k(t)=y(t-kfracpi2)$ as independent $2pi$ periodic functions. At the end, the integration constant will have to be adapted so that they all are the same function again, if at all possible.



          Then these $4$ functions satisfy the first order system
          $$
          pmatrix{y_0'\y_1'\y_2'\y_3'}
          +
          pmatrix{0&1&-1&0\0&0&1&-1\-1&0&0&1\1&-1&0&0}
          pmatrix{y_0\y_1\y_2\y_3}
          =
          pmatrix{cos(t)\sin(t)\-cos(t)\-sin(t)}
          $$

          The circulant matrix can be written as $A=Q-Q^2$ where $Q^4=I$, so $A$ has eigenvalues $lambda_k=q^k-(q^k)^2=0$, $1+i$, $-2$, $1-i$ for the eigenvalues $q^k=i^k$ and eigenvectors $v^k_j=i^{jk}$, $j=0,1,2,3$, $k=0,1,2,3$, of $Q$. Of these components of the homogeneous solution, only the first one as a constant is again $2pi$ periodic, the others have exponentially increasing or decreasing factors.



          So the qualitative conclusion is that a periodic solution for $y$ can only have the components $$y(t)=A+Bcos(t)+Csin(t).$$ Inserting this gives
          $$
          (-Bsin t+Ccos t)+(A+Bsin t-Ccos t)-(A-Bcos t-Csin t)=cos t
          \
          impliesleft{begin{aligned}
          B&=1\C&=0
          end{aligned}\right.
          $$





          One could also do the first part slightly different by considering linear combinations of the shifted functions,
          $$u_k(t)=sum_{k=0}^3i^ky(t-kfracpi2),~~k=0,1,2,3, ~~~~ (i^4=1, ~~ y(t-2pi)=y(t)). $$ Then
          begin{align}
          u_k'(t)+sum_{m=0}^3i^{km}y(t-(m+1)fracpi2)-sum_{m=0}^3i^{km}y(t-(m+2)fracpi2)
          &=sum_{k=0}^3i^{km}cos(t-mfracpi2)
          \~\
          u_k'(t)+(i^k-(-1)^k)u(t)&=(1-(-1)^k)cos(t)+i^k(1-(-1)^k)sin(t)
          end{align}

          This is a linear ODE with constant coefficients for each of the 4 values of $k$, and the solution can be reconstructed from them by solving the corresponding linear system or a là the inverse Fourier transform.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited 2 days ago

























          answered Jan 6 at 0:02









          LutzLLutzL

          56.5k42054




          56.5k42054












          • I appreciate your answer, although I am yet not familiar with the Fourier transform or its inverse. Is there another way to solve it? I don't understand what the q represents, and how do you factor the second term? I don't follow.
            – SimpleProgrammer
            2 days ago










          • This is just 4-dimensional linear algebra. Essentially it shows that the solution has to be a combination of $sin t$ and $cos t$ as you have to exclude the exponential terms for a periodic solution. Note that $sin(t+kfracpi2)$ is $sin t,cos t, -sin t, -cos t, sin t,...$ for $k=0,1,2,3,4,..$
            – LutzL
            2 days ago


















          • I appreciate your answer, although I am yet not familiar with the Fourier transform or its inverse. Is there another way to solve it? I don't understand what the q represents, and how do you factor the second term? I don't follow.
            – SimpleProgrammer
            2 days ago










          • This is just 4-dimensional linear algebra. Essentially it shows that the solution has to be a combination of $sin t$ and $cos t$ as you have to exclude the exponential terms for a periodic solution. Note that $sin(t+kfracpi2)$ is $sin t,cos t, -sin t, -cos t, sin t,...$ for $k=0,1,2,3,4,..$
            – LutzL
            2 days ago
















          I appreciate your answer, although I am yet not familiar with the Fourier transform or its inverse. Is there another way to solve it? I don't understand what the q represents, and how do you factor the second term? I don't follow.
          – SimpleProgrammer
          2 days ago




          I appreciate your answer, although I am yet not familiar with the Fourier transform or its inverse. Is there another way to solve it? I don't understand what the q represents, and how do you factor the second term? I don't follow.
          – SimpleProgrammer
          2 days ago












          This is just 4-dimensional linear algebra. Essentially it shows that the solution has to be a combination of $sin t$ and $cos t$ as you have to exclude the exponential terms for a periodic solution. Note that $sin(t+kfracpi2)$ is $sin t,cos t, -sin t, -cos t, sin t,...$ for $k=0,1,2,3,4,..$
          – LutzL
          2 days ago




          This is just 4-dimensional linear algebra. Essentially it shows that the solution has to be a combination of $sin t$ and $cos t$ as you have to exclude the exponential terms for a periodic solution. Note that $sin(t+kfracpi2)$ is $sin t,cos t, -sin t, -cos t, sin t,...$ for $k=0,1,2,3,4,..$
          – LutzL
          2 days ago


















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