You can use the fibonacci method on positive integers an it will return the correlating fibonacci number.

fibonacci_sequence_recursion.rb

class Integer
  def fibonacci
    if self < 0
      "You can only use the fibonacci method with positive integers"
    else
      return self if (0..1).include? self
      (self -1).fibonacci + (self - 2).fibonacci 
    end
  end
end

puts "'#{8.fibonacci}' should be '21'"
puts "'#{0.fibonacci}' should be '0'"
puts "'#{1.fibonacci}' should be '1'"
puts "'#{2.fibonacci}' should be '1'"
puts "'#{3.fibonacci}' should be '2'"
puts "'#{4.fibonacci}' should be '3'"
puts "'#{5.fibonacci}' should be '5'"
puts "'#{-1.fibonacci}' should be 'You can only use the fibonacci method with positive integers'"
puts "'#{10.fibonacci}' should be '55'"