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 Post subject: Finding eigenvectors of an unusual matrix
PostPosted: Sat, 15 Jan 2011 10:55:09 UTC 
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Hello, I'm trying to find the eigenvector(s) of the following matrix:
\[ \left( \begin{array}{cc} 2  & -4 \\ 0 & 2 \end{array} \right) \]

I already know that it's a doubled eigenvalue of \lambda_{1,2}=2, obvious given that it's a diagonal matrix. However, I'm having trouble finding the eigenvectors, because when solving:

\[ \left( \begin{array}{cc} 2-\lambda & -4 \\ 0 & 2-\lambda \end{array} \right) \rightarrow \left( \begin{array}{cc} 0 & 4 \\ 0 & 0 \end{array} \right)  \]

You can't really do the system of equations because you just end up with:
\[ \left( \begin{array}{cc} 0 & 4 \\ 0 & 0 \end{array} \right) \cdot \left( \begin{array}{c} \eta_{1} \\ \eta_{2} \end{array} \right) = \left( \begin{array}{cc} 0 \\ 0 \end{array} \right)\].

Which just gives you
\begin{array}{c} 4\eta_{2}=0 \\ 0=0 \end{array}
which isn't really anything you can use to solve for the eigenvector \vec{\eta}.

Could anyone tell me what the eigenvectors are, and more importantly, how they were found?


This is in the context of finding the general solution to the system
\[ X' = \left( \begin{array}{cc} 2 & -4 \\ 0 & 2 \end{array} \right) X \]
though I already know the answer is x_{1}=c_{1}e^{2t}+c_{2}te^{2t}


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 Post subject: Re: Finding eigenvectors of an unusual matrix
PostPosted: Sat, 15 Jan 2011 13:19:43 UTC 
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shallarock wrote:
Hello, I'm trying to find the eigenvector(s) of the following matrix:
\[ \left( \begin{array}{cc} 2  & -4 \\ 0 & 2 \end{array} \right) \]

I already know that it's a doubled eigenvalue of \lambda_{1,2}=2, obvious given that it's a diagonal matrix. However, I'm having trouble finding the eigenvectors, because when solving:

\[ \left( \begin{array}{cc} 2-\lambda & -4 \\ 0 & 2-\lambda \end{array} \right) \rightarrow \left( \begin{array}{cc} 0 & 4 \\ 0 & 0 \end{array} \right)  \]

You can't really do the system of equations because you just end up with:
\[ \left( \begin{array}{cc} 0 & 4 \\ 0 & 0 \end{array} \right) \cdot \left( \begin{array}{c} \eta_{1} \\ \eta_{2} \end{array} \right) = \left( \begin{array}{cc} 0 \\ 0 \end{array} \right)\].

Which just gives you
\begin{array}{c} 4\eta_{2}=0 \\ 0=0 \end{array}
which isn't really anything you can use to solve for the eigenvector \vec{\eta}.

Could anyone tell me what the eigenvectors are, and more importantly, how they were found?


This is in the context of finding the general solution to the system
\[ X' = \left( \begin{array}{cc} 2 & -4 \\ 0 & 2 \end{array} \right) X \]
though I already know the answer is x_{1}=c_{1}e^{2t}+c_{2}te^{2t}


It is not a diagonal matrix, and it is not diagonalisable either.

The dimension of the 2-eigenspace is only 1. You should see immediately from your system of equations that \mathcal{E}_2=\mathop{\mathrm{span}}\begin{pmatrix}1\\0\end{pmatrix}.

_________________
\begin{aligned}
Spin(1)&=O(1)=\mathbb{Z}/2&\quad&\text{and}\\
Spin(2)&=U(1)=SO(2)&&\text{are obvious}\\
Spin(3)&=Sp(1)=SU(2)&&\text{by }q\mapsto(\mathop{\mathrm{Im}}\mathbb{H}\ni p\mapsto qp\bar{q})\\
Spin(4)&=Sp(1)\times Sp(1)&&\text{by }(q_1,q_2)\mapsto(\mathbb{H}\ni p\mapsto q_1p\bar{q_2})\\
Spin(5)&=Sp(2)&&\text{by }\mathbb{HP}^1\cong S^4_{round}\hookrightarrow\mathbb{R}^5\\
Spin(6)&=SU(4)&&\text{by the irrep }\Lambda_+\mathbb{C}^4
\end{aligned}


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