Is the SL(2,R) action on the upper half-plane transitive?

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In summary, SL(2,R) is a mathematical notation for a group of 2x2 matrices with real number entries that preserve the determinant to be 1. The upper half-plane, denoted as H, is a set of complex numbers with positive imaginary part. An action is transitive if every element in the target space can be reached from any other element through the action. The SL(2,R) action on the upper half-plane is defined by a transformation formula. This action has significant applications in various areas of mathematics and has connections to physics and cryptography.
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Chris L T521
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Here's this week's problem.

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Problem: Let $G=\text{SL}_2(\mathbb{R})$ and let $\mathcal{H}=\{z\in\mathbb{C}:\text{Im}(z)>0\}$. Let $g=\begin{pmatrix}a & b\\ c & d\end{pmatrix}\in G$ and define an action $\cdot :G\times\mathcal{H}\rightarrow\mathcal{H}$ where $g\cdot z=\dfrac{az+b}{cz+d}$. Compute $\text{Orb}_G(i)$ and $\text{stab}_G(i)$. Is this action transitive?

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No one answered this question correctly. You can find my solution below.

Let us first compute $\text{stab}_G(i)=\{g\in G:g\cdot i=i\}$. Let $g=\begin{pmatrix}a & b\\ c & d\end{pmatrix}\in\text{SL}_2(\mathbb{R})$. Then $\begin{pmatrix}a & b\\ c & d\end{pmatrix}\cdot i=i\implies \dfrac{ai+b}{ci+d}=i\implies ai+b=di-c\implies a=d\text{ and }b=-c$.
It now follows that $g$ is of the form $g=\begin{pmatrix} a & -c \\ c & a\end{pmatrix}$. Futhermore, since $g\in \text{SL}_2(\mathbb{R})$, we must have $\det g=1\implies a^2+c^2=1$. Thus, our matrix $g$ must represent a rotation on the unit circle. Hence, $g\in SO(2)$, and thus $\text{stab}_G(i)\cong SO(2)$.

To compute $\text{Orb}_G(i)$, we first observe that any element of our orbit must lie in the upper half plane $\mathcal{H}$; i.e. $g\cdot i=\dfrac{ai+b}{ci+d}\in\mathcal{H}$. Now, note that for $y>0$ and for any $x\in\mathbb{R}$, we have that $g=\begin{pmatrix}y^{1/2} & xy^{-1/2}\\ 0 & y^{-1/2}\end{pmatrix}\in \text{SL}_2(\mathbb{R})$. It now follows that $g\cdot i = \dfrac{y^{1/2}i+xy^{-1/2}}{y^{-1/2}}=y^{1/2}(y^{1/2}i+xy^{-1/2})=x+iy\in\mathcal{H}$. From this, it now follows that $g\cdot i$ generates the entire upper half plane; thus, $\text{Orb}_G(i)=\mathcal{H}$. Furthermore, $\text{Orb}_G(\mathcal{H})=\mathcal{H}$, and thus it follows that the action is transitive.
 
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FAQ: Is the SL(2,R) action on the upper half-plane transitive?

What is SL(2,R)?

SL(2,R) is a mathematical notation for the special linear group of 2x2 matrices with real number entries. It is a group of linear transformations that preserve the determinant of the matrix to be 1.

What is the upper half-plane?

The upper half-plane, denoted as H, is a mathematical concept in complex analysis. It is the set of complex numbers with positive imaginary part, i.e. numbers of the form a + bi where b is greater than 0.

What does it mean for an action to be transitive?

An action is said to be transitive if every element in the target space can be reached from any other element by applying the action. In this case, the SL(2,R) action on the upper half-plane is transitive if any point in H can be transformed into any other point in H through a transformation in SL(2,R).

How is the SL(2,R) action defined on the upper half-plane?

The SL(2,R) action on the upper half-plane is defined as follows: given a point z in H and a matrix A in SL(2,R), the action is given by the transformation Az = (az + b)/(cz + d), where a, b, c, and d are the entries of the matrix A.

Why is the SL(2,R) action on the upper half-plane important?

The SL(2,R) action on the upper half-plane is important in several areas of mathematics, including complex analysis, number theory, and geometry. It has applications in studying modular forms, hyperbolic geometry, and automorphic forms, among others. It also has connections to other areas such as physics and cryptography.

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