Let denote the set of all triples of positive integers where . Compute
Solution
We view choosing five objects from a row of 19 objects in an unusual way. First, remove two of the chosen objects, the second and fourth, which are not adjacent nor at either end, forming three nonempty groups of consecutive objects. We then have , and choices for the first, third, and fifth objects. Because this is a reversible process taking a triple to choices, the answer is . A simple generating functions argument is also possible. Let . Then and so , yielding .
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