Definition:Stirling Numbers of the Second Kind/Technical Note

Technical Note for Stirling Numbers of the Second Kind
The braces around the  are important.

The  is needed to create the symbol in its proper house display style.

Note that while it appears that  should do the same job, it is rendered with a subtle difference:
 * $\displaystyle \left\{ {n \atop k} \right\}$

and so is not recommended on.