What do the matching "L" shapes (near
20) mean in this forumla?
The document where I found this formula can be found here: http://www.cs.cmu.edu/~hcwong/Pdfs/icip02.ps
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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.