I have a page called Linear Transformations. When I go to it for the first time in a webrick session, it redirects to
the edit page, as follows:

127.0.0.1 - - [10/May/2008:10:27:27 PDT] "GET /wiki/show/Linear+Algebra HTTP/1.1" 200 5459
http://localhost:2500/wiki/show/HomePage -> /wiki/show/Linear+Algebra
127.0.0.1 - - [10/May/2008:10:27:29 PDT] "GET /wiki/show/Linear+Transformations HTTP/1.1" 302 120
http://localhost:2500/wiki/show/Linear+Algebra -> /wiki/show/Linear+Transformations
127.0.0.1 - - [10/May/2008:10:27:29 PDT] "GET /wiki/edit/Linear+Transformations HTTP/1.1" 200 7862
http://localhost:2500/wiki/show/Linear+Algebra -> /wiki/edit/Linear+Transformations

That edit page also has a "can't dup NilClass" error. If I edit that page, it goes back to normal afterward.
The page contents are as follows (I've integrated jsMath into it, but don't think that's the problem):

Each $(m \times n)$ matrix $A$ determines a function from $\mathbb{R}^n$ to $\mathbb{R}^m$ by $x \to Ax$. We will denote
this map by $T$. That is, $T(x)=Ax$.

For all $v,x \in \mathbb{R}^n,a \in \mathbb{R}$,
* $T(v+w)=T(v)+T(w)$
* $T(av)=a\,T(v)$

**Definition**. Given two vector spaces $V, W$, a function $T : V \to W$ is a _linear transformation_ if for all vectors
$u, v \in V$ and scalars $a \in \mathbb{R}$,
* $T(u+v)=T(u)+T(v)$, and
* $T(av)=a\,T(v)$.

**Example**. Every $(m \times n)$ matrix $A$ is a map from $\mathbb{R}^n$ to $\mathbb{R}^m$.

**Example**. On $C[a,b]$ define $T : C[a,b] \to C[a,b]$ by $T(f)(x)=\int_a^x f(t) \, dt$.

**Example**. Let $C^\infty [a,b]$ be functions on $[a,b]$ that have derivatives of all orders. In this case, the derivation
function $\frac{d}{dx}$ is a linear transformation.

**Example**. Vector addition and scalar-vector multiplication in $\mathbb{R}^2$.

**Non-Example**. Define a map $T : \mathbb{R}^n \to \mathbb{R}^n$ by $T(x)=x+b$, where $b$ is some fixed vector. Note
that $T(2b) \ne 2T(b)$.

**Facts**.
* $T(0)=0$
* $T(-x)=-T(x)$.

**Question**. Let $V$ be a vector space with with bases $\\{v\_1,\ldots,v\_n\\}$, $T:V \to W$. Suppose we know
$Tv\_1,\ldots,Tv\_n$. Can we calculate $T\_x$ if $x\in V$?
* Write $x=a\_1 v\_1 + \ldots + a\_n v\_n$.
* So $T(x)=a\_1 Tv\_1 + \ldots + a\_n Tv\_n$.
* So yes.

**Theorem**. Every linear transformation $T:\mathbb{R}^n \to \mathbb{R}^m$ is given by multiplication by some $(m \times
n)$ matrix $[Te\_1, \ldots, Te\_n]$.

**Example**. In $\mathbb{R}^2$, rotation by $\theta$ is given by multiplication by $\left[\begin{matrix}
\cos\theta & -\sin\theta\\\\
\sin\theta & \cos\theta
\end{matrix}\right]$.

The range of a linear transformation $T:V \to W$ is all vectors $w \in W$ such that there is some vector $v \in V$ such
that $Tv=w$. The null space is defined as expected.