Number Game - Pythagorean Triples - Britannica

Perfect numbers and Mersenne numbers

Most numbers are either “abundant” or “deficient.” In an abundant number, the sum of its proper divisors (i.e., including 1 but excluding the number itself) is greater than the number; in a deficient number, the sum of its proper divisors is less than the number. A perfect number is an integer that equals the sum of its proper divisors. For example, 24 is abundant, its divisors giving a sum of 36; 32 is deficient, giving a sum of 31. The number 6 is a perfect number, since 1 + 2 + 3 = 6; so is 28, since 1 + 2 + 4 + 7 + 14 = 28. The next two perfect numbers are 496 and 8,128. The first four perfect numbers were known to the ancients. Indeed, Euclid suggested that any number of the form 2n − 1(2n − 1) is a perfect number whenever 2n − 1 is prime, but it was not until the 18th century that the Swiss mathematician Leonhard Euler proved that every even perfect number must be of the form 2n − 1(2n − 1), where 2n − 1 is a prime.

A number of the form 2n − 1 is called a Mersenne number after the French mathematician Marin Mersenne; it may be prime (i.e., having no factor except itself or 1) or composite (composed of two or more prime factors). A necessary though not sufficient condition that 2n − 1 be a prime is that n be a prime. Thus, all even perfect numbers have the form 2n − 1(2n − 1) where both n and 2n − 1 are prime numbers. Until comparatively recently, only 12 perfect numbers were known. In 1876 the French mathematician Édouard Lucas found a way to test the primality of Mersenne numbers. By 1952 the U.S. mathematician Raphael M. Robinson had applied Lucas’ test and, by means of electronic digital computers, had found the Mersenne primes for n = 521; 607; 1,279; 2,203; and 2,281, thus adding five more perfect numbers to the list. By the 21st century, more than 40 Mersenne primes had been found.

It is known that to every Mersenne prime there corresponds an even perfect number and vice versa. But two questions are still unanswered: the first is whether there are any odd perfect numbers, and the second is whether there are infinitely many perfect numbers.

Many remarkable properties are revealed by perfect numbers. All perfect numbers, for example, are triangular. Also, the sum of the reciprocals of the divisors of a perfect number (including the reciprocal of the number itself) is always equal to 2. Thus