public void Ln_TestInvalidArg()
 {
     Assert.Throws <ArgumentException>(() => PolyApprox.Ln(0.0));
 }
 public void Ln_Test(double x)
 {
     Assert.That(PolyApprox.Ln(x) / Math.Log(x), Is.EqualTo(1.0).Within(_tolerance));
 }