Finding specific solutions of a system of differential equations without computations











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I have a reflection matrix $A=begin{pmatrix}-frac{1}{2} & frac{sqrt{3}}{2} \ frac{sqrt{3}}{2} & frac{1}{2} end{pmatrix}$ and a linear, autonomous, and homogeneous system $frac{doverrightarrow{w}}{dt} = Aoverrightarrow{w}$, $overrightarrow{w}(t) = begin{pmatrix} x(t) \ y(t) end{pmatrix}$.



I've noticed that $left{begin{pmatrix}frac{1}{2} \ frac{sqrt{3}}{2}end{pmatrix},begin{pmatrix} -frac{sqrt{3}}{2} \ frac{1}{2}end{pmatrix}right}$ is an eigenbasis with eigenvalues $lambda = 1$ and $lambda = -1$. Thus, I thought that a general solution would be somewhere along the lines of $begin{pmatrix} x'(t) \ y'(t) end{pmatrix} = c_1e^tbegin{pmatrix}frac{1}{2} \ frac{sqrt{3}}{2} end{pmatrix} + c_2e^{-t}begin{pmatrix} -frac{sqrt{3}}{2} \ frac{1}{2} end{pmatrix}$.



I am unsure about how to go from here to finding specific solutions. I'm asked to find two solutions to this system "without performing calculations", and to indicate where the solutions curves start in the plane. Furthermore, I'm asked to find a solution that starts at $begin{pmatrix} 0 \ 5 end{pmatrix}$.



What am I missing here?










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    I have a reflection matrix $A=begin{pmatrix}-frac{1}{2} & frac{sqrt{3}}{2} \ frac{sqrt{3}}{2} & frac{1}{2} end{pmatrix}$ and a linear, autonomous, and homogeneous system $frac{doverrightarrow{w}}{dt} = Aoverrightarrow{w}$, $overrightarrow{w}(t) = begin{pmatrix} x(t) \ y(t) end{pmatrix}$.



    I've noticed that $left{begin{pmatrix}frac{1}{2} \ frac{sqrt{3}}{2}end{pmatrix},begin{pmatrix} -frac{sqrt{3}}{2} \ frac{1}{2}end{pmatrix}right}$ is an eigenbasis with eigenvalues $lambda = 1$ and $lambda = -1$. Thus, I thought that a general solution would be somewhere along the lines of $begin{pmatrix} x'(t) \ y'(t) end{pmatrix} = c_1e^tbegin{pmatrix}frac{1}{2} \ frac{sqrt{3}}{2} end{pmatrix} + c_2e^{-t}begin{pmatrix} -frac{sqrt{3}}{2} \ frac{1}{2} end{pmatrix}$.



    I am unsure about how to go from here to finding specific solutions. I'm asked to find two solutions to this system "without performing calculations", and to indicate where the solutions curves start in the plane. Furthermore, I'm asked to find a solution that starts at $begin{pmatrix} 0 \ 5 end{pmatrix}$.



    What am I missing here?










    share|cite|improve this question


























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      I have a reflection matrix $A=begin{pmatrix}-frac{1}{2} & frac{sqrt{3}}{2} \ frac{sqrt{3}}{2} & frac{1}{2} end{pmatrix}$ and a linear, autonomous, and homogeneous system $frac{doverrightarrow{w}}{dt} = Aoverrightarrow{w}$, $overrightarrow{w}(t) = begin{pmatrix} x(t) \ y(t) end{pmatrix}$.



      I've noticed that $left{begin{pmatrix}frac{1}{2} \ frac{sqrt{3}}{2}end{pmatrix},begin{pmatrix} -frac{sqrt{3}}{2} \ frac{1}{2}end{pmatrix}right}$ is an eigenbasis with eigenvalues $lambda = 1$ and $lambda = -1$. Thus, I thought that a general solution would be somewhere along the lines of $begin{pmatrix} x'(t) \ y'(t) end{pmatrix} = c_1e^tbegin{pmatrix}frac{1}{2} \ frac{sqrt{3}}{2} end{pmatrix} + c_2e^{-t}begin{pmatrix} -frac{sqrt{3}}{2} \ frac{1}{2} end{pmatrix}$.



      I am unsure about how to go from here to finding specific solutions. I'm asked to find two solutions to this system "without performing calculations", and to indicate where the solutions curves start in the plane. Furthermore, I'm asked to find a solution that starts at $begin{pmatrix} 0 \ 5 end{pmatrix}$.



      What am I missing here?










      share|cite|improve this question















      I have a reflection matrix $A=begin{pmatrix}-frac{1}{2} & frac{sqrt{3}}{2} \ frac{sqrt{3}}{2} & frac{1}{2} end{pmatrix}$ and a linear, autonomous, and homogeneous system $frac{doverrightarrow{w}}{dt} = Aoverrightarrow{w}$, $overrightarrow{w}(t) = begin{pmatrix} x(t) \ y(t) end{pmatrix}$.



      I've noticed that $left{begin{pmatrix}frac{1}{2} \ frac{sqrt{3}}{2}end{pmatrix},begin{pmatrix} -frac{sqrt{3}}{2} \ frac{1}{2}end{pmatrix}right}$ is an eigenbasis with eigenvalues $lambda = 1$ and $lambda = -1$. Thus, I thought that a general solution would be somewhere along the lines of $begin{pmatrix} x'(t) \ y'(t) end{pmatrix} = c_1e^tbegin{pmatrix}frac{1}{2} \ frac{sqrt{3}}{2} end{pmatrix} + c_2e^{-t}begin{pmatrix} -frac{sqrt{3}}{2} \ frac{1}{2} end{pmatrix}$.



      I am unsure about how to go from here to finding specific solutions. I'm asked to find two solutions to this system "without performing calculations", and to indicate where the solutions curves start in the plane. Furthermore, I'm asked to find a solution that starts at $begin{pmatrix} 0 \ 5 end{pmatrix}$.



      What am I missing here?







      differential-equations






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      edited Nov 13 at 7:35

























      asked Nov 13 at 6:35









      Dominic Hicks

      826




      826






















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          $$A^{-1}=A,$$
          $$e^{At}=A.
          begin{pmatrix}{e^t} & 0\
          0 & {e^{-t}}end{pmatrix}
          .A^{-1}$$

          Final solution is
          $$overrightarrow{w}(t) =e^{At}.begin{pmatrix} 0 \ 5 end{pmatrix}
          =begin{pmatrix}frac{5 sqrt{3}, {{e}^{t}}}{4}-frac{5 sqrt{3}, {{e}^{-t}}}{4}\
          frac{15 {{e}^{t}}}{4}+frac{5 {{e}^{-t}}}{4}end{pmatrix} $$






          share|cite|improve this answer

















          • 1




            Or alternatively, because of $A^2=I$, sorting the power series for even and odd powers gives $e^{tA}=cosh(t)I+sinh(t)A$.
            – LutzL
            Nov 13 at 9:35











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          $$A^{-1}=A,$$
          $$e^{At}=A.
          begin{pmatrix}{e^t} & 0\
          0 & {e^{-t}}end{pmatrix}
          .A^{-1}$$

          Final solution is
          $$overrightarrow{w}(t) =e^{At}.begin{pmatrix} 0 \ 5 end{pmatrix}
          =begin{pmatrix}frac{5 sqrt{3}, {{e}^{t}}}{4}-frac{5 sqrt{3}, {{e}^{-t}}}{4}\
          frac{15 {{e}^{t}}}{4}+frac{5 {{e}^{-t}}}{4}end{pmatrix} $$






          share|cite|improve this answer

















          • 1




            Or alternatively, because of $A^2=I$, sorting the power series for even and odd powers gives $e^{tA}=cosh(t)I+sinh(t)A$.
            – LutzL
            Nov 13 at 9:35















          up vote
          1
          down vote













          $$A^{-1}=A,$$
          $$e^{At}=A.
          begin{pmatrix}{e^t} & 0\
          0 & {e^{-t}}end{pmatrix}
          .A^{-1}$$

          Final solution is
          $$overrightarrow{w}(t) =e^{At}.begin{pmatrix} 0 \ 5 end{pmatrix}
          =begin{pmatrix}frac{5 sqrt{3}, {{e}^{t}}}{4}-frac{5 sqrt{3}, {{e}^{-t}}}{4}\
          frac{15 {{e}^{t}}}{4}+frac{5 {{e}^{-t}}}{4}end{pmatrix} $$






          share|cite|improve this answer

















          • 1




            Or alternatively, because of $A^2=I$, sorting the power series for even and odd powers gives $e^{tA}=cosh(t)I+sinh(t)A$.
            – LutzL
            Nov 13 at 9:35













          up vote
          1
          down vote










          up vote
          1
          down vote









          $$A^{-1}=A,$$
          $$e^{At}=A.
          begin{pmatrix}{e^t} & 0\
          0 & {e^{-t}}end{pmatrix}
          .A^{-1}$$

          Final solution is
          $$overrightarrow{w}(t) =e^{At}.begin{pmatrix} 0 \ 5 end{pmatrix}
          =begin{pmatrix}frac{5 sqrt{3}, {{e}^{t}}}{4}-frac{5 sqrt{3}, {{e}^{-t}}}{4}\
          frac{15 {{e}^{t}}}{4}+frac{5 {{e}^{-t}}}{4}end{pmatrix} $$






          share|cite|improve this answer












          $$A^{-1}=A,$$
          $$e^{At}=A.
          begin{pmatrix}{e^t} & 0\
          0 & {e^{-t}}end{pmatrix}
          .A^{-1}$$

          Final solution is
          $$overrightarrow{w}(t) =e^{At}.begin{pmatrix} 0 \ 5 end{pmatrix}
          =begin{pmatrix}frac{5 sqrt{3}, {{e}^{t}}}{4}-frac{5 sqrt{3}, {{e}^{-t}}}{4}\
          frac{15 {{e}^{t}}}{4}+frac{5 {{e}^{-t}}}{4}end{pmatrix} $$







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Nov 13 at 9:17









          Aleksas Domarkas

          7385




          7385








          • 1




            Or alternatively, because of $A^2=I$, sorting the power series for even and odd powers gives $e^{tA}=cosh(t)I+sinh(t)A$.
            – LutzL
            Nov 13 at 9:35














          • 1




            Or alternatively, because of $A^2=I$, sorting the power series for even and odd powers gives $e^{tA}=cosh(t)I+sinh(t)A$.
            – LutzL
            Nov 13 at 9:35








          1




          1




          Or alternatively, because of $A^2=I$, sorting the power series for even and odd powers gives $e^{tA}=cosh(t)I+sinh(t)A$.
          – LutzL
          Nov 13 at 9:35




          Or alternatively, because of $A^2=I$, sorting the power series for even and odd powers gives $e^{tA}=cosh(t)I+sinh(t)A$.
          – LutzL
          Nov 13 at 9:35


















           

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