! Undefined control sequence. \[ p(u)= \sum_{v\epsilonN

Question

I used the following statement:

\[ p(u)= \sum_{v\epsilonN(u)} W(u,v) \]

But it shows the error:

! Undefined control sequence. \[ p(u)= \sum_{v\epsilonN

Answer 1

Okay your main problem is, as the error message says, \epsilonN. LaTeX doesn't understand that \epsilonN is intended to end at the first n with N being separate. It can't think for itself, you have to tell it what goes with what. Where you have v\epsilon the \ tells LaTeX that a new command starts now, which is how it knows v and \epsilon are separate. But it doesn't know where the new command ends. You can solve this really simply with \epsilon N(u). Just a space. Spaces are ignored within math mode as far as the output goes, so you can put in as many spaces as you like and it won't make any difference.

$a    bcd    e           f g$

Looks just like

$abcdefg$

However, I've got a feeling that you're using \epsilon, the Greek letter, where what you really want is the set inclusion/'is an element of' symbol

It's important to use the right symbol, not just so that you get the right shape, but also so that LaTeX applies the correct spacing. Also, just in terms of your code input, you don't want the Greek letter epsilon, you want the symbol for some object being "in" some set. So logically, it makes sense to use a command name that corresponds to the symbol you actually want.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\[
  p(u) = \sum_{v \in N(u)} W(u,v)
\]

\end{document}

Finally, as an extra, you may like to try this:

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}

\begin{document}

\[
  p(u) = \smashoperator{\sum_{v \in N(u)}} W(u,v)
\]

\end{document}

To get rid of the pretty cavernous spaces around the summation sign, as Manuel recommends in the comments. Please note, this requires the mathtools package.