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diff --git a/spec/ruby/core/math/erf_spec.rb b/spec/ruby/core/math/erf_spec.rb
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+++ b/spec/ruby/core/math/erf_spec.rb
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+require_relative '../../spec_helper'
+require_relative 'fixtures/classes'
+
+# erf method is the "error function" encountered in integrating the normal
+# distribution (which is a normalized form of the Gaussian function).
+describe "Math.erf" do
+  it "returns a float" do
+    Math.erf(1).should.is_a?(Float)
+  end
+
+  it "returns the error function of the argument" do
+    Math.erf(0).should be_close(0.0, TOLERANCE)
+    Math.erf(1).should be_close(0.842700792949715, TOLERANCE)
+    Math.erf(-1).should be_close(-0.842700792949715, TOLERANCE)
+    Math.erf(0.5).should be_close(0.520499877813047, TOLERANCE)
+    Math.erf(-0.5).should be_close(-0.520499877813047, TOLERANCE)
+    Math.erf(10000).should be_close(1.0, TOLERANCE)
+    Math.erf(-10000).should be_close(-1.0, TOLERANCE)
+    Math.erf(0.00000000000001).should be_close(0.0, TOLERANCE)
+    Math.erf(-0.00000000000001).should be_close(0.0, TOLERANCE)
+  end
+
+  it "raises a TypeError if the argument cannot be coerced with Float()" do
+    -> { Math.erf("test") }.should.raise(TypeError)
+  end
+
+  it "returns NaN given NaN" do
+    Math.erf(nan_value).nan?.should == true
+  end
+
+  it "raises a TypeError if the argument is nil" do
+    -> { Math.erf(nil) }.should.raise(TypeError)
+  end
+
+  it "accepts any argument that can be coerced with Float()" do
+    Math.erf(MathSpecs::Float.new).should be_close(0.842700792949715, TOLERANCE)
+  end
+end
+
+describe "Math#erf" do
+  it "is accessible as a private instance method" do
+    IncludesMath.new.send(:erf, 3.1415).should be_close(0.999991118444483, TOLERANCE)
+  end
+end