Let’s take a breather. The previous chapters have seen some heavy going, with sums involving floor, ceiling, mod, phi, and mu functions. Now we’re going to study binomial coefficients, which turn out to be (a) more important in applications, and (b) easier to manipulate, than all those other quantities.

*Lucky us!*

The symbol is a binomial coefficient, so called because of an important property we look at later this section, the binomial theorem. But we read the symbol “n choose k.” This incantation arises from its combinatorial interpretation—it is the number of ways to choose a k-element subset from ...

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