Intereting Posts

Subrings of polynomial rings over the complex plane
Sum of first n natural numbers proof
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How to get to the formula for the sum of squares of first n numbers?
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integral of complex conjugate times the differential is purely imaginary
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Let $p > 3$ be a prime number. Show that $x^2 \equiv −3\mod p$ is solvable iff $p\equiv 1\mod 6$.

Find the relationship between an elliptic element of $SL(2,\mathbb{R})$ and rotation..

An element $A$ of $SL(2,\mathbb{R})$ is called an elliptic element if $|\text{tr}(A)|<2$

As $|\text{tr}(A)|<2$ so the characteristics equation of A does not have any real roots…so it has no real eigen value..but I am unable to go ahead…

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- Proof - Inverse of linear function is linear

Let $g$ be your elliptic element. Since the eigenvalues are complex conjugate, $g$ will have an eigenvector $\binom z1\in\Bbb C^2$ with the imaginary part of $z$ positive.

It can be shown that you can always find some $h\in{\rm SL}_2(\Bbb R)$ such that $h\binom i1=c\binom z1$ for some $c\in\Bbb R$.

Thus $h^{-1}gh$ will have $\binom i1$ as eigenvector.

The final step is to show that the elements of ${\rm SL}_2(\Bbb R)$ having $\binom i1$ as an eigenvector are precisely the elements in the subgroup ${\rm SO}_2(\Bbb R)$, i.e. the rotations.

I leave the details as an exercise.

Here is a second proof. Let $\theta\in\Bbb R\setminus\pi\Bbb Z$ be such that $\mathrm{Tr}(A)=2\cos(\theta)$. The characteristic polynomial of $A$ equals $\chi_A(X)=X^2-\mathrm{Tr}(A)X+\det(A)=(X-e^{i\theta})(X-e^{-i\theta})=\chi_{R_{\theta}}(X)$ where

$$

R_{\theta}=\begin{pmatrix}

\cos(\theta)&-\sin(\theta)\\\sin(\theta)&\cos(\theta)

\end{pmatrix}

$$

Since $\theta\notin\pi\Bbb Z$, $e^{i\theta}\neq e^{-i\theta}$ and $A$ and $R_{\theta}$ are diagonalzable (over $\Bbb C$) and conjugate (as complex matrices) to

$$\begin{pmatrix}

e^{i\theta}&0\\0&e^{-i\theta}

\end{pmatrix}$$

We conclude by invoking the general fact that two real matrices that are conjugate as complex matrices are conjugate as real matrices.

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