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This question already has an answer here:

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?

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This is my LaTex notation:

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marked as duplicate by Mico, Sean Allred, user31729, Jesse, Heiko Oberdiek Apr 12 '15 at 8:42

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    Can't you just copy/paste the code into your question? That would make it a lot easier for us to help. Making it compilable wouldn't be bad, either :) (just a short document with \documentclass{…}\begin{document}…\end{document}. See also tex.stackexchange.com/q/503/17423. – Sean Allred Apr 12 '15 at 6:16
  • @Mico While the answer there would work here, note that this isn't inline math :) – Sean Allred Apr 12 '15 at 6:19
  • @SeanAllred -- That posting's title is a bit of a mis-nomer: it's about text-style ("inline math mode") vs display-style ("displayed equation mode"). Anyway, since the OP hasn't indicated whether he/she is loading the amsmath package, it's not quite appropriate to suggest using dfrac instead of \frac... – Mico Apr 12 '15 at 6:22
  • @Mico well yes, I know that, but others may not see this connection :) it's pretty obscure unless you know how TeX works with math. – Sean Allred Apr 12 '15 at 6:33
  • Try \limits and \nolimits right after the \sum (\prod, \int, ...) symbol. – Malipivo Apr 12 '15 at 6:34
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\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}

Result

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