Combinatorial mathematics is often described as “the study of permutations, combinations, etc.,” so we turn our attention now to combinations. A *combination of n things, taken t at a time*, often called simply a *t*-combination of *n* things, is a way to select a subset of size *t* from a given set of size *n*. We know from Eq. 1.2.6–(2) that there are exactly ways to do this; and we learned in Section 3.4.2 how to choose *t*-combinations at random.

Selecting *t* of *n* objects is equivalent to choosing the *n* – *t* elements not selected. We will emphasize this symmetry by letting

throughout our discussion, and we will often ...

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