These friendly sounding numbers are defined by their divisibility and sums. A perfect number is a number whose positive divisors (except for itself) sum to itself; an amicable number is a pair of numbers each of which equals the sum of the other's aliquot parts; and the members of aliquot cycles of length greater than two are often called sociable numbers. This page, housed at (but not officially affiliated with) the Institute for Materials Science at the University of Connecticut, defines and describes perfect, amicable, and sociable numbers and introduces aliquot sequences. The text has links to a bibliography and to numeric tables. This site might be interesting to college-level mathematics students or anyone into mathematical puzzles.