Firstly, I would like to say that I know that it was asked before here, but the question wasn't answered. I'm self studying PDE by Evans's book and I'm trying understand why the integral of a mollifier is $1$. I will put the definition of a mollifier below according Evans's book.
$\textbf{Definition:}$
Let $\eta: \mathbb{R}^n \longrightarrow \mathbb{R}$ defined by
$\eta(x) := \begin{cases} C \exp \left( \frac{1}{|x|^2 - 1} \right) \hspace{1.0cm} \text{if} \ |x| < 1\\ 0 \hspace{3.6cm} \text{if} \ |x| \geq 1, \end{cases}$
the constant $C > 0$ selected so that $\int_{\mathbb{R}^n} \eta \ dx = 1$.
The function $\eta$ is called $\textit{the standard mollifier}$.
I'll be gratefull if someone can tells me how can I compute $\int_{\mathbb{R}^n} \eta \ dx$.