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What is the Name of Function for Probability of a Certain Sum on Random Die Rolls?

Hi.

I'm writing a book about using statistics for roleplaying game design and am using this equation for calculating the probability of rolling a particular sum "n" on "z" throws of an "a"-sided die

$$ {{1} \over {a^{z}}} \displaystyle \sum_{k=0}^{\lfloor (n - z) / a \rfloor } (-1)^{k} (_zC_k) (_{(n-a)(k-1)}C_{(z-1)})$$

What is the name for this function? I'm trying to search for calculators to recommend that support it, and I can't think of the words to search for.