MERSENNE NUMBERS AND PRIMITIVE PRIME DIVISORS.A notorious problem from elementary number theory is the "Mersenne Prime Conjecture." This asserts that the Mersenne sequence M = (M n ) defined by M n = 2 n − 1 (n = 1, 2, . . . ) contains infinitely many prime terms, which are known as Mersenne primes.The Mersenne prime conjecture is related to a classical problem in number theory concerning perfect numbers. A whole number is said to be perfect if, like 6 = 1 + 2 + 3 and 28 = 1 + 2 + 4 + 7 + 14, it is equal to the sum of all its proper divisors. Euclid pointed out that 2 k−1 (2 k − 1) is perfect whenever 2 k − 1 is prime. A much less obvious result, due to Euler, is a partial converse: if n is an even perfect number, then it must have the form 2 k−1 (2 k − 1) for some k with the property that 2 k − 1 is a prime. Whether there are any odd perfect numbers remains an open question. Thus finding Mersenne primes amounts to finding (even) perfect numbers.The sequence M certainly produces some primes initially, for example,However, the appearance of Mersenne primes quickly thins out: only forty-three are known, the largest of which, M 30,402,457 , has over nine million decimal digits. This was discovered by a team at Central Missouri State University as part of the GIMPS project [23], which harnesses idle time on thousands of computers all over the world to run a distributed version of the Lucas-Lehmer test. A paltry forty-three primes might seem rather a small return for such a huge effort. Anybody looking for gold or gems with the same level of success would surely abandon the search. It seems fair to ask why we should expect there to be infinitely many Mersenne primes. In the absence of a rigorous proof, our expectations may be informed by heuristic arguments. In section 3 we discuss heuristic arguments for this and other more or less tractable problems in number theory.Primitive prime divisors. In 1892, Zsigmondy [24] discovered a beautiful argument that shows that the sequence M does yield infinitely many prime numbers-but in a less restrictive sense. Given any integer sequence S = 1