Question

Let g' be the group of real matricies of the form [1 x 0 1]. Is the map that sends x to this matrix an isomorphism?

Answer 1

Yes. Conceptually, all the matrices in the group have the same structure, except for the variable component $x$. So, each matrix is identified by its top-right coefficient, since the other three entries remain constant.

However, let's prove in a more formal way that

$\phi:\ \mathbb{R} \to G,\quad \phi(x) = \left[\begin{array}{cc}1&x\\0&1\end{array}\right]$

is an isomorphism.

First of all, it is injective: suppose $x \neq y$. Then, you trivially have $\phi(x) \neq \phi(y)$, because they are two different matrices:

$\phi(x) = \left[\begin{array}{cc}1&x\\0&1\end{array}\right],\quad \phi(y) = \left[\begin{array}{cc}1&y\\0&1\end{array}\right]$

Secondly, it is trivially surjective: the matrix

$\phi(x) = \left[\begin{array}{cc}1&x\\0&1\end{array}\right]$

is clearly the image of the real number x.

Finally, $\phi$ and its inverse are both homomorphisms: if we consider the usual product between matrices to be the operation for the group G and the real numbers to be an additive group, we have

$\phi (x+y) = \left[\begin{array}{cc}1&x+y\\0&1\end{array}\right] = \left[\begin{array}{cc}1&x\\0&1\end{array}\right] \cdot \left[\begin{array}{cc}1&y\\0&1\end{array}\right] = \phi(x) \cdot \phi(y)$