Definition:Unsigned Stirling Numbers of the First Kind/Technical Note

Technical Note for Unsigned Stirling Numbers of the First 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.