Can You Solve This Symplectic Vector Space Problem?

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In summary, Symplectic Vector Spaces are mathematical structures used in symplectic geometry, equipped with a bilinear form called a symplectic form. They are important in physics and engineering, and can be used to solve problems involving dynamical systems, conserving quantities, and symplectic reduction. To solve a problem, one must understand the properties of symplectic forms and use techniques such as symplectic reduction and canonical transformations. Some common challenges in solving these problems include understanding the abstract concepts and finding the right approach to simplify the problem.
  • #1
Chris L T521
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Here's this week's problem.

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Background Info: Let $\mathbb{V}$ be an $m$-dimensional vector space over the field $\mathbb{F}$ (where $\mathbb{F}$ can be either $\mathbb{R}$ or $\mathbb{C}$). If $\omega$ is a non-degenerate alternating bilinear form, then the pairing $(\mathbb{V},\omega)$ is called a symplectic vector space. A symplectic basis for $\mathbb{V}$ is a basis $v_1,\ldots,v_{2n}$ such that $\omega(v_i,v_j)=J_{i,j}$, which is the $(i,j)$-th entry of the $2n\times 2n$ matrix
\[J=\begin{pmatrix} \mathbf{0}_n & I_n\\ -I_n & \mathbf{0}_n\end{pmatrix}.\]A Lagrangian space $\mathbb{U}$ is a subspace of $\mathbb{V}$ of dimension $n$ such that $\omega$ is zero on $\mathbb{U}$; i.e. $\omega(u,w)=0$ for all $u,w\in\mathbb{U}$. A direct sum decomposition $\mathbb{V}=\mathbb{U}\oplus\mathbb{W}$ where $\mathbb{U}$ and $\mathbb{W}$ are Lagrangian subspaces is called a Lagrangian splitting, and $\mathbb{W}$ is called the Lagrangian complement of $\mathbb{U}$.

Problem:
  1. Let $\mathbb{U}$ be a Lagrangian subspace of $\mathbb{V}$. Show that there exists a Lagrangian complement of $\mathbb{U}$
  2. Let $\mathbb{V}=\mathbb{U}\oplus\mathbb{W}$ be a Lagrangian splitting and $x_1,\ldots,x_n$ any basis form $\mathbb{U}$. Show that there exists a unique basis $y_1,\ldots y_n$ of $\mathbb{W}$ such that $x_1,\ldots,x_n,y_1,\ldots,y_n$ is a symplectic basis for $\mathbb{V}$.

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Hints:
  1. Take $\mathbb{W}=J\mathbb{U}$ and show that it's a Lagrangian complement to $\mathbb{U}$.
  2. Define the annihilator of $\mathbb{W}$ by $\mathbb{W}^0 = \{f\in\mathbb{V}^{\ast}: f(e)=0\text{ for all $e\in\mathbb{W}$}\}$. Define $\phi_i\in\mathbb{W}^0$ by $\phi_i(w)=\omega(x_i,w)$ for $w\in\mathbb{W}$. Show that $\phi_1,\ldots,\phi_n$ forms a basis for $\mathbb{W}^0$.

 
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  • #2
No one answered this week's question. You can find my solution below.

Proof:
  1. The following shows that the complement is not unique. Let $\mathbb{V}=\mathbb{F}^{2n}$ and let $\mathbb{U}\subset\mathbb{F}^{2n}$.
    Claim: $\mathbb{W}=J\mathbb{U}$ is a Lagrangian complement to $\mathbb{U}$.
    Proof of Claim: If $x,y\in\mathbb{W}$, then $x=Ju$, $y=Jv$ where $u,v\in\mathbb{U}$ or $\{u,v\}=0$ (here, $\{\cdot,\cdot\}$ denotes the Poisson bracket). But $\{x,y\}=\{Ju,Jv\}=\{u,v\}=0$, so $\mathbb{W}$ is Lagrangian. If $x\in\mathbb{U}\cap J\mathbb{U}$, the $x=Jy$ with $y\in\mathbb{U}$. So $x,Jx\in\mathbb{U}$ and so $\{x,Jx\}=-\|x\|^2=0$ or $x=0$. Thus, $\mathbb{U}\cap\mathbb{W}=\emptyset$. $\hspace{.25in}\blacksquare$
  2. Define $\phi_i\in\mathbb{W}^0$ by $\phi_i(w)=\omega(x_i,w)$ for $w\in\mathbb{W}$. If $\sum\alpha_i\phi_i=0$, then $\omega\left(\sum\alpha_ix_i,w\right)=0$ for all $w\in\mathbb{W}$ or $\omega\left(\sum\alpha_ix_i,\mathbb{W}\right)=0$. But because $\mathbb{V}=\mathbb{U}\oplus\mathbb{W}$ and $\omega(\mathbb{U},\mathbb{U})=0$, it follows that $\omega\left(\sum\alpha_ix_i,\mathbb{V}\right)=0$. This implies $\sum\alpha_ix_i=0$, because $\omega$ is nondegenerate, and this implies that $\alpha_i=0$ because the $x_i$'s are independent. Thus, $\phi_1,\ldots,\phi_n$ are independent, and so they form a basis for $\mathbb{W}^0$. Let $y_1,\ldots,y_n$ be the dual basis in $\mathbb{W}$; so $\omega(x_i,y_j)=\phi_i(y_j)=\delta_{ij}$. $\hspace{.25in}\blacksquare$
 

FAQ: Can You Solve This Symplectic Vector Space Problem?

What is a Symplectic Vector Space?

A Symplectic Vector Space is a mathematical structure used in the field of symplectic geometry. It is a vector space equipped with a special type of bilinear form called a symplectic form, which satisfies certain properties such as non-degeneracy and skew-symmetry.

What is the importance of Symplectic Vector Spaces?

Symplectic Vector Spaces are important in physics and engineering, particularly in the study of classical mechanics and Hamiltonian systems. They also have applications in other areas of mathematics, such as algebraic geometry and topology.

What types of problems can be solved using Symplectic Vector Spaces?

Symplectic Vector Spaces can be used to solve problems involving dynamical systems, conserving quantities, and symplectic reduction. They are also used in the study of symplectic manifolds and symplectic geometry.

How do you solve a Symplectic Vector Space problem?

To solve a Symplectic Vector Space problem, you first need to understand the properties of symplectic forms and how they interact with vector spaces. Then, you can use various techniques such as symplectic reduction and canonical transformations to simplify the problem and find a solution.

What are some common challenges in solving Symplectic Vector Space problems?

One common challenge in solving Symplectic Vector Space problems is understanding the underlying concepts and properties, as they can be quite abstract and require a strong mathematical background. Another challenge is finding the right approach or technique to simplify the problem and find a solution.

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