Displaying the binomial coefficient symbol in math mode

Question

The problem is caused by the symbol of binomial coefficient (symbol of Newton), often used in math:

{N}\choose{k}

In my document I have formula:

$$ P(A) = \sum P(\{ (e_1,...,e_N) \})  =  {N}\choose{k} \cdot p^kq^{N-k}$$

which is rendered as:

but should be:

Answer 1

To fix this, simply add a pair of braces around the whole binomial coefficient, i.e.

{N\choose k}

(The braces around N and k are not needed.)

However, as you're using LaTeX, it is better to use \binom from amsmath, i.e.

\binom{N}{k}

Further, it is not recommended to use $$ ... $$, see Why is \[ … \] preferable to $$? Last, I'll note that amsmath provides different commands for 'continuation dots', including \dotsc for triple dots between commas.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
 P(A) = \sum P(\{ (e_1,\dotsc,e_N) \})  =  \binom{N}{k} \cdot p^kq^{N-k}
\]
\end{document}

Answer 2

Just add curly braces around it to separate it:

$$ P(A) = \sum P(\{ (e_1,...,e_N) \})  =  {{N}\choose{k}} \cdot p^kq^{N-k}$$

That should fix it.