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}
N
really supposed to represent the physical unit for force (in which case you should render it in an upright rather than italic style), or are you using the symbol to denote an integer?