Project Euler 21

gistfile1.rb

require 'prime'
arr = Array.new(10001){1}
sum = 0
2.upto(10000) do |i|
  i.prime_division.each do |j|
    a = 0
    (j[1]+1).times do |k|
      a += j[0]**k
    end
    arr[i] *= a
  end
  arr[i] -= i
end
1.upto(10000) do |i|
  (i+1).upto(10000) do |j|
    sum += i + j if arr[i] == j && arr[j] == i
  end
end
puts sum # 31626