I have the following `.Rmd`

file, which includes a cross reference to an equation, but seems not to work.

I am currently using the Rstudio cloud application, but I think it doesn't make any difference whether it is locally or online.

```
---
title: "Nice Title"
author: "Me"
output: html_document
---
Lorem Ipsum
\begin{equation} \label{eq:1}
\sum_{i=0}^{\infty} a_i x^i
\end{equation}
The equation \ref{eq:1} is a typical power series.
```

When the output format is pdf, the equation is cross referenced fine.

Is the syntax different when working with different output formats?

Thanks a lot!