What's a Friendly Number?

In number theory, friendly numbers are two or more natural numbers with a common abundancy index, the ratio between the sum of divisors of a number and the number itself. Two numbers with the same "abundancy" form a friendly pair; n numbers with the same "abundancy" form a friendly n-tuple.

What is an example of a friendly number?

The smallest "friendly number" is 6, forming for example, the "friendly" pair 6 and 28 with "abundancy" σ(6) / 6 = (1+2+3+6) / 6 = 2, the same as σ(28) / 28 = (1+2+4+7+14+28) / 28 = 2. ... Numbers with "abundancy" 2 are also known as perfect numbers.

What are friendly numbers 3rd grade?

Friendly numbers are numbers that end in 0. They are called friendly because once the rule for adding 0 is understood, that understanding can be extended to larger numbers that end in 0. Landmark numbers are similar to friendly numbers. Some examples are 25, 50, 75, 100.