amicable numbers
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Article
Demonstration with Cuisenaire rods of the amicability of the pair of numbers (220,284), the first of the series.
In mathematics, the amicable numbers are two different natural numbers related in such a way that the sum of the proper divisors of each is equal to the other number. That is, s(a)=b and s(b)=a, where s(n)=σ(n) − n is equal to the sum of positive divisors of n except n itself (see also divisor function).