When you want to step through all numbers that are divisible by either of two digits there is a palindrome pattern in that sequence (much like the patterns in frequencies that overlap). This gem discovers that palindrome and creates a custom enumerator like object that lets you step through each of those numbers. danielpclark/multiples

This may help save you some time by not looping over non-multiples when iterating through any range of numbers.

require "multiples"
x = Multiples.new(3,5)

x.lazy.take(4).to_a
# => [3, 5, 6, 9] 
x.next
# => 10 
x.prev
# => 9 

x.lazy.select {|v| v % 7 == 0 }.take(14).to_a
# => [21, 35, 42, 63, 70, 84, 105, 126, 140, 147, 168, 175, 189, 210]