The answers to exercises marked IBB] can be found in the Back of the Book.
Let A be the set of all citizens of New York City. For a, b ∈ A, define a ~ b if and only if
· neither a nor b have a cell phone, or
· both a and b have cell phones in the same exchange (that is, the first three digits of each phone number are the same).
Show that ~ defines an equivalence relation on A and find the corresponding equivalence classes.
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