Question: I need to put a letter under the min symbol in the equation below but i couldn't do it,It now looks like this:

![enter image description here](https://i.stack.imgur.com/D0PIC.png)

and I need it to look like this:
![enter image description here](https://i.stack.imgur.com/gC0tg.png)

```
\begin{document}
\begin{equation}
mina\sum^{N}_{i=1}\sum^{N}_{j=1}|y_i - y_j|^2 W_{ij}
\label{eqn4}
\end{equation}
\end{document}

```

Answer: 
```
\begin{document}
\begin{equation}
\min_A\sum^{N}_{i=1}\sum^{N}_{j=1}|y_i - y_j|^2 W_{ij}
\label{eqn4}
\end{equation}
\end{document}

```
To add to Przemysław Scherwent's answer, in the case you want to use an operator that is not already defined, you can use `\operatorname{}` or `\operatorname*{}` from the `amsmath` package.  

The former displays its argument like an operator -- like `sin`, for example --, and the latter does the same, but the following superscripts and subscripts will be typeset above and below -- juste like `min`.  

If you want to use a custom operator often, you might also want to use `\DeclareMathOperator` or `\DeclareMathOperator*`.

For instance, say you want to use the operator `foo` with subscripts typeset under it. The following code produces the same equation twice :

```
\documentclass{article}
\usepackage{amsmath}

\DeclareMathOperator*{\foo}{foo}
\begin{document}
    \[
        \foo_A
    \]
    \[
        \operatorname*{foo}_A
    \]
\end{document}

```

The nice thing about these is that it handles the spacing around the operators pretty well.