Complex sentence: Suppose that, as seems likely, the probability of a relapse strictly decreases with length of abstinence; say P (n) is the probability of never smoking again after not smoking for "n" days.
Split into simpler sentences: Suppose that, as seems likely, the probability of a relapse strictly decreases with length of abstinence. Say P (n) is the probability of never smoking again after not smoking for "n" days.