problem21-solved.rb

#Let d(n) be defined as the sum of proper divisors of n (numbers less than n which divide evenly into n).
#If d(a) = b and d(b) = a, where a  b, then a and b are an amicable pair and each of a and b are called amicable numbers.
#For example, the proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55 and 110; therefore d(220) = 284. The proper divisors of 284 are 1, 2, 4, 71 and 142; so d(284) = 220.
#Evaluate the sum of all the amicable numbers under 10000.

require "mathn"

class Integer
    def fsum
        (1..self-1).select{|n| (self % n).zero?}.reduce(:+)
    end
    def amicable?
        if !self.fsum.nil? && !self.fsum.fsum.nil? && self == self.fsum.fsum
            puts "#{self} == #{self.fsum}"
            return true
        end
    end
end

puts (1..1E4).select{|n| n.amicable?}.reduce(:+)