Added class Cubic to find the real roots of a cubic polynomial.

Added unit test class CubicTest.

src/Numerics/Numerics.csproj

@@ -189,6 +189,7 @@
     <Compile Include="Random\SystemRandomSource.cs" />
     <Compile Include="Random\RandomSeed.cs" />
     <Compile Include="RootFinding\Broyden.cs" />
+    <Compile Include="RootFinding\Cubic.cs" />
     <Compile Include="RootFinding\NewtonRaphson.cs" />
     <Compile Include="RootFinding\RobustNewtonRaphson.cs" />
     <Compile Include="RootFinding\ZeroCrossingBracketing.cs" />

src/Numerics/RootFinding/Cubic.cs

@@ -0,0 +1,96 @@
+using System;
+
+#if !NOSYSNUMERICS
+    using Complex = System.Numerics.Complex;
+#endif
+
+namespace MathNet.Numerics.RootFinding
+{
+    /// <summary>
+    /// Finds roots to the cubic equation x^3 + a2*x^2 + a1*x + a0 = 0
+    /// Implements the cubic formula in http://mathworld.wolfram.com/CubicFormula.html
+    /// </summary>
+    public static class Cubic
+    {
+        // D = Q^3 + R^2 is the polynomial discriminant.
+        // D > 0, 1 real root
+        // D = 0, 3 real roots, at least two are equal
+        // D < 0, 3 real and unequal roots
+
+        /// <summary>
+        /// Q and R are transformed variables.
+        /// </summary>
+        private static void QR(double a2, double a1, double a0, ref double Q, ref double R)
+		{
+			Q = (3 * a1 - a2 * a2)/9.0;
+			R = (9.0 * a2 * a1 - 27 * a0 - 2 * a2 * a2 * a2)/54.0;           
+		}
+
+        /// <summary>
+        /// n^(1/3) - work around a negative double raised to (1/3)
+        /// </summary>
+        private static double PowThird(double n)
+        {
+            return Math.Pow(Math.Abs(n), 1d / 3d) * Math.Sign(n);
+        }
+
+        public static Tuple<double, double, double> RealRoots(double a2, double a1, double a0)
+        {
+            var Q = double.NaN;
+            var R = double.NaN;
+            QR(a2, a1, a0, ref Q, ref R);
+
+            var Q3 = Q * Q * Q;
+            var D = Q3 + R * R;
+            var shift = -a2 / 3d;
+
+            double x1 = double.NaN;
+            double x2 = double.NaN;
+            double x3 = double.NaN;
+
+            // when D >= 0, use eqn (54)-(56) where S and T are real
+            if (D >= 0)
+            {
+                double sqrtD = Math.Pow(D, 0.5);
+                double S = PowThird(R + sqrtD);
+                double T = PowThird(R - sqrtD);
+                x1 = shift + (S + T);
+                if (D == 0)
+                    x2 = shift - S;
+            }
+            // 3 real roots, use eqn (70)-(73) to calculate the real roots  
+            else
+            {
+                double theta = Math.Acos(R / Math.Sqrt(-Q3));
+                x1 = 2d * Math.Sqrt(-Q) * Math.Cos(theta / 3.0) + shift;
+                x2 = 2d * Math.Sqrt(-Q) * Math.Cos((theta + 2.0 * Constants.Pi) / 3d) + shift;
+                x3 = 2d * Math.Sqrt(-Q) * Math.Cos((theta - 2.0 * Constants.Pi) / 3d) + shift;
+            }
+            return Tuple.Create(x1, x2, x3);
+        }
+
+        public static Tuple<Complex, Complex, Complex> Roots(double a2, double a1, double a0)
+        {
+            // use eqn (54)-(56)
+            var Q = double.NaN;
+            var R = double.NaN;
+            QR(a2, a1, a0, ref Q, ref R);
+
+            var D = Q * Q * Q + R * R;
+
+            var rootD = Complex.Sqrt(D);
+            var S = Complex.Pow(R + rootD, 1d / 3d);
+            var T = Complex.Pow(R - rootD, 1d / 3d);
+            var shift = -a2 / 3d;
+            var sharedI = 0.5 * Complex.ImaginaryOne * Math.Sqrt(3) * (S - T);
+
+            var x1 = shift + (S + T);
+            var x2 = shift - 0.5 * (S + T);
+            var x3 = x2;
+            x2 += sharedI;
+            x3 -= sharedI;
+
+            return Tuple.Create(x1, x2, x3);
+        }
+    }
+}

src/UnitTests/RootFindingTests/CubicTest.cs

@@ -0,0 +1,61 @@
+using System;
+using MathNet.Numerics.RootFinding;
+using NUnit.Framework;
+
+namespace MathNet.Numerics.UnitTests.RootFindingTests
+{
+    [TestFixture, Category("RootFinding")]
+    internal class CubicTest
+    {
+        [Test]
+        public void RealRoots_SingleReal()
+        {
+            var a0 = 1d;
+            var a1 = 5d;
+            var a2 = 2d;
+
+            var roots = Cubic.RealRoots(a2, a1, a0);
+            var root = roots.Item1;
+            var funcValue = root * (root * (root + a2) + a1) + a0;
+            Assert.That(funcValue, Is.EqualTo(0).Within(1e-14));
+            Assert.AreEqual(double.NaN, roots.Item2);
+            Assert.AreEqual(double.NaN, roots.Item3);
+        }
+
+        [Test]
+        public void RealRoots_DoubleReal1()
+        {
+            var a0 = -2d;
+            var a1 = 5d;
+            var a2 = -4d;
+            var roots = Cubic.RealRoots(a2, a1, a0);
+            Assert.That(roots.Item1, Is.EqualTo(2).Within(1e-14));
+            Assert.That(roots.Item2, Is.EqualTo(1).Within(1e-14));
+            Assert.AreEqual(double.NaN, roots.Item3);
+        }
+        
+        [Test]
+        public void RealRoots_DoubleReal2()
+        {
+            var a0 = -4d;
+            var a1 = 8d;
+            var a2 = -5d;
+            var roots = Cubic.RealRoots(a2, a1, a0);
+            Assert.That(roots.Item1, Is.EqualTo(1).Within(1e-14));
+            Assert.That(roots.Item2, Is.EqualTo(2).Within(1e-14));
+            Assert.AreEqual(double.NaN, roots.Item3);
+        }
+
+        [Test]
+        public void RealRoots_TripleReal()
+        {
+            var a0 = 6d;
+            var a1 = -5d;
+            var a2 = -2d;
+            var roots = Cubic.RealRoots(a2, a1, a0);
+            Assert.That(roots.Item1, Is.EqualTo(3).Within(1e-14));
+            Assert.That(roots.Item2, Is.EqualTo(-2).Within(1e-14));
+            Assert.That(roots.Item3, Is.EqualTo(1).Within(1e-14));
+        }
+    }
+}

src/UnitTests/UnitTests.csproj

@@ -376,6 +376,7 @@
     <Compile Include="Random\XorshiftTests.cs" />
     <Compile Include="RootFindingTests\BrentTest.cs" />
     <Compile Include="RootFindingTests\BroydenTest.cs" />
+    <Compile Include="RootFindingTests\CubicTest.cs" />
     <Compile Include="RootFindingTests\FindRootsTest.cs" />
     <Compile Include="RootFindingTests\RobustNewtonRaphsonTest.cs" />
     <Compile Include="RootFindingTests\NewtonRaphsonTest.cs" />