Added simplification for bucket index equation

The problem with the "old" equation was, that you cannot implement it in
C as-is. You have to transform the equation into the form which is added
by this PR to get it compiling.

For referencing, this simplification should be added in the paper, too.
In the other equation I labeled, I put the label in a seperate line just before the `\end{equation}`. Please do the same here.