Integer#σ∞: Add

lib/numb/sum_of_infinitary_divisors.rb

@@ -0,0 +1,9 @@
+# coding: utf-8
+class Integer
+  def σ∞
+    infinitary_divisors.reduce :+
+  end
+
+  alias :sum_of_infinitary_divisors :σ∞
+  alias :isigma :σ∞
+end

spec/sum_of_infinitary_divisors_spec.rb

@@ -0,0 +1,15 @@
+# coding: utf-8
+describe Integer, "#σ∞" do
+  # A049417 
+  @seq = [1,3,4,5,6,12,8,15,10,18,12,20,14,24,24,17,18,30,
+          20,30,32,36,24,60,26,42,40,40,30,72,32,51,48,54,
+          48,50,38,60,56,90,42,96,44,60,60,72,48,68,50,78,
+          72,70,54,120,72,120,80,90,60,120,62,96,80,85,84,
+          144,68,90]
+
+  @seq.to_enum.with_index(1).each do |sum, n|
+    it "returns #{sum} for #{n}" do
+      n.σ∞.should == sum
+    end
+  end
+end