---
documentclass: book
output: bookdown::html_document2
---

# Examples

```{definition}
The characteristic function of a random variable $X$ is defined by
$$\varphi _{X}(t)=\operatorname {E} \left[e^{itX}\right], \; t\in\mathcal{R}$$
```

```{r}
a = 1
```