Question: What do the matching "L" shapes (near `.5` and `20`) mean in this forumla?

![fomula with unknown symbols](https://i.stack.imgur.com/MoxWg.png)

The document where I found this formula can be found here: <http://www.cs.cmu.edu/~hcwong/Pdfs/icip02.ps>

Answer: 
The bracketing symbol means Floor (see <http://en.wikipedia.org/wiki/Floor_and_ceiling_functions>)

The definition of Floor is $\lfloor x \rfloor$ = Largest integer less than x. This is very similar to rounding down as $\lfloor 2.3 \rfloor = \lfloor 2.999 \rfloor = 2$. However, the subtlety is that for negative numbers it acts slightly differently, as $\lfloor -1.5 \rfloor = -2$ which might not be what you expect at first.
Floor function: Round down to the nearest integer.