Lower bound placement in summation sigma sign

Question

I have this equation and I would like to have lower bound (v \in N_i(u)) exactly under the summation symbol and not to the right side of it. How to accomplish it?

This is my LaTex notation:

Answer 1

\limits can be used after \sum to put the bounds below and above instead to the side:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
  \begin{gather}
    \hat{r}_{ui} = \bar r + \sigma_u \cdot
    \frac{
      \sum_{v \in N_i(u)} w_{uv} (r_{vi} - \bar{r}_v) / \sigma_v
    }{
      \sum_{v \in N_i(u)} |w_{uv}|
    }
    \\
    \hat{r}_{ui} = \bar r + \sigma_u \cdot
    \frac{
      \sum\limits_{v \in N_i(u)} w_{uv} (r_{vi} - \bar{r}_v) / \sigma_v
    }{
      \sum\limits_{v \in N_i(u)} |w_{uv}|
    }
  \end{gather}
\end{document}