Define the abundancy of a positive integer as
(1)

where is the divisor function. Then a pair of distinct numbers is a friendly pair (and is said to be a friend of ) if their abundancies are equal:
(2)

For example, (4320, 4680) is a friendly pair since , , and
(3)
 
(4)

Another example is , which has index 5/2. The first few friendly pairs, ordered by smallest maximum element are (6, 28), (30, 140), (80, 200), (40, 224), (12, 234), (84, 270), (66, 308), ... (OEIS A050972 and A050973).
Friendly triples and higherorder tuples are also possible. Friendly triples include (2160, 5400, 13104), (9360, 21600, 23400), and (4320, 4680, 26208), friendly quadruples include (6, 28, 496, 8128), (3612, 11610, 63984, 70434), (3948, 12690, 69936, 76986), and friendly quintuples include (84, 270, 1488, 1638, 24384), (30, 140, 2480, 6200, 40640), (420, 7440, 8190, 18600, 121920).
Numbers that have friends are called friendly numbers, and numbers that do not have friends are called solitary numbers. A sufficient (but not necessary) condition for to be a solitary number is that , where is the greatest common divisor of and . There are some numbers that can easily be proved to be solitary, but the status of numbers 10, 14, 15, 20, and many others remains unknown (Hickerson 2002).
Hoffman (1998, p. 45) uses the term "friendly numbers" to describe amicable pairs.