3: Counting
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- 3.3: Factorials and Permutations
- In working examples from the previous two sections you may have noticed that we often need to count the number of non-repetitive lists of length n that are made from n symbols. This kind of problem occurs so often that a special idea, called a factorial, is used to handle it.
- 3.4: Counting Subsets
- The previous section dealt with counting lists made by selecting k entries from a set of n elements. We turn now to a related question: How many subsets can be made by selecting k elements from a set with n elements?