# Set notation, what does it mean

Can sonbody explain to me what does set notation of

$$C_NM\\$$

means.

The C in given notation is not letter C, it is some kind of very narrow C. I Could not find the alternative, to write.

Thanks

You probably mean $\complement_N M$, which I first saw in Bourbaki.
$\complement_N M$ is the complement of $M$ with respect to $N$, also known as $N \setminus M$.
$\complement_N M$ is written as \complement_N M in $\TeX$.