diff --git a/src/Numerics/RootFinding/Brent.cs b/src/Numerics/RootFinding/Brent.cs
index 291c33e2..87ece682 100644
--- a/src/Numerics/RootFinding/Brent.cs
+++ b/src/Numerics/RootFinding/Brent.cs
@@ -72,6 +72,7 @@ namespace MathNet.Numerics.RootFinding
             double d = 0.0, e = 0.0;
 
             root = upperBound;
+            double xMid = double.NaN;
 
             for (int i = 0; i <= maxIterations; i++)
             {
@@ -95,12 +96,20 @@ namespace MathNet.Numerics.RootFinding
 
                 // convergence check
                 double xAcc1 = 2.0*Precision.DoubleMachinePrecision*Math.Abs(root) + 0.5*accuracy;
-                double xMid = (upperBound - root)/2.0;
+                double xMidOld = xMid;
+                xMid = (upperBound - root)/2.0;
+
                 if (Math.Abs(xMid) <= xAcc1 && froot.AlmostEqualWithAbsoluteError(0, froot, accuracy))
                 {
                     return true;
                 }
 
+                if (xMid == xMidOld)
+                {
+                    // accuracy not sufficient, but cannot be improved further
+                    return false;
+                }
+
                 if (Math.Abs(e) >= xAcc1 && Math.Abs(fmin) > Math.Abs(froot))
                 {
                     // Attempt inverse quadratic interpolation
diff --git a/src/UnitTests/RootFindingTests/BisectionTest.cs b/src/UnitTests/RootFindingTests/BisectionTest.cs
index 67d93d68..9919a275 100644
--- a/src/UnitTests/RootFindingTests/BisectionTest.cs
+++ b/src/UnitTests/RootFindingTests/BisectionTest.cs
@@ -497,9 +497,15 @@ namespace MathNet.Numerics.UnitTests.RootFindingTests
                 return 1.296 * (T - Tp) + 10369 * k * Pp;
             };
 
-            double x = Bisection.FindRoot(f1, 500, 1000);
+            double x = Bisection.FindRoot(f1, 500, 725);
             Assert.AreEqual(690.4486645013260, x, 1e-5);
             Assert.AreEqual(0, f1(x), 1e-12);
+            x = Bisection.FindRoot(f1, 725, 850, 1e-12);
+            Assert.AreEqual(758.3286948959860, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-12);
+            x = Bisection.FindRoot(f1, 850, 1000, 1e-12);
+            Assert.AreEqual(912.7964911381540, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-12);
         }
 
         [Test]
diff --git a/src/UnitTests/RootFindingTests/BrentTest.cs b/src/UnitTests/RootFindingTests/BrentTest.cs
index 9c06c2f4..451840a6 100644
--- a/src/UnitTests/RootFindingTests/BrentTest.cs
+++ b/src/UnitTests/RootFindingTests/BrentTest.cs
@@ -120,5 +120,636 @@ namespace MathNet.Numerics.UnitTests.RootFindingTests
             Assert.AreEqual(58.7000023925600, x, 1e-5);
             Assert.AreEqual(0, f1(x), 1e-14);
         }
+
+        [Test]
+        public void Oneeq2b()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq2b.htm
+            Func<double, double> f1 = T =>
+            {
+                const double x1 = 0.6763397;
+                const double x2 = 1 - x1;
+                const double A = 0.75807689;
+                const double B = 1.1249035;
+                double P2 = Math.Pow(10, 6.9024 - 1268.115 / (216.9 + T));
+                double P1 = Math.Pow(10, 8.04494 - 1554.3 / (222.65 + T));
+                double gamma2 = Math.Pow(10, B * Math.Pow(x1, 2) / Math.Pow(x1 + B * x2 / A, 2));
+                double gamma1 = Math.Pow(10, A * Math.Pow(x2, 2) / Math.Pow(A * x1 / B + x2, 2));
+                double k1 = gamma1 * P1 / 760;
+                double k2 = gamma2 * P2 / 760;
+                return 1 - k1 * x1 - k2 * x2;
+            };
+
+            double x = Brent.FindRoot(f1, 0, 100, 1e-14);
+            Assert.AreEqual(70.9000000710300, x, 1e-9);
+            Assert.AreEqual(0, f1(x), 1e-14);
+        }
+
+        [Test]
+        public void Oneeq3()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq3.htm
+            // not solvable with this method
+            Func<double, double> f1 = T => Math.Exp(21000 / T) / (T * T) - 1.11e11;
+            Assert.Throws<NonConvergenceException>(() => Brent.FindRoot(f1, 550, 560, 1e-2));
+        }
+
+        [Test]
+        public void Oneeq4()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq4.htm
+            Func<double, double> f1 = x =>
+            {
+                const double A = 0.38969;
+                const double B = 0.55954;
+                return A * B * (B * Math.Pow(1 - x, 2) - A * x * x) / Math.Pow(x * (A - B) + B, 2) + 0.14845;
+            };
+
+            double r = Brent.FindRoot(f1, 0, 1, 1.0e-14);
+            Assert.AreEqual(0.691473747542323, r, 1e-5);
+            Assert.AreEqual(0, f1(r), 1e-14);
+        }
+
+        [Test]
+        public void Oneeq5()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq5.htm
+            // only solvable with reduced accuracy (1e-3 instead of 1e-10 with Bisection)
+            Func<double, double> f1 = TR =>
+            {
+                const double ALPHA = 7.256;
+                const double BETA = 2.298E-3;
+                const double GAMMA = 0.283E-6;
+                const double DH = -57798.0;
+                return DH + TR * (ALPHA + TR * (BETA / 2 + TR * GAMMA / 3)) - 298.0 * (ALPHA + 298.0 * (BETA / 2 + 298.0 * GAMMA / 3));
+            };
+
+            double x = Brent.FindRoot(f1, 3000, 5000, 1.0e-3);
+            Assert.AreEqual(4305.30991366774, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-3);
+        }
+
+        [Test]
+        public void Oneeq6a()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq6a.htm
+            // not solvable with this method
+            Func<double, double> f1 = V =>
+            {
+                const double R = 0.08205;
+                const double T = 273.0;
+                const double B0 = 0.05587;
+                const double A0 = 2.2769;
+                const double C = 128300.0;
+                const double A = 0.01855;
+                const double B = -0.01587;
+                const double P = 100.0;
+                const double Beta = R * T * B0 - A0 - R * C / (T * T);
+                const double Gama = -R * T * B0 * B + A0 * A - R * C * B0 / (T * T);
+                const double Delta = R * B0 * B * C / (T * T);
+
+                return R * T / V + Beta / (V * V) + Gama / (V * V * V) + Delta / (V * V * V * V) - P;
+            };
+
+            Assert.Throws<NonConvergenceException>(() => Brent.FindRoot(f1, 0.1, 1));
+        }
+
+        [Test]
+        public void Oneeq6b()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq6b.htm
+            Func<double, double> f1 = V =>
+            {
+                const double R = 0.08205;
+                const double T = 273.0;
+                const double B0 = 0.05587;
+                const double A0 = 2.2769;
+                const double C = 128300.0;
+                const double A = 0.01855;
+                const double B = -0.01587;
+                const double P = 100.0;
+                const double Beta = R * T * B0 - A0 - R * C / (T * T);
+                const double Gama = -R * T * B0 * B + A0 * A - R * C * B0 / (T * T);
+                const double Delta = R * B0 * B * C / (T * T);
+
+                return R * T * V * V * V + Beta * V * V + Gama * V + Delta - P * V * V * V * V;
+            };
+
+            double x = Brent.FindRoot(f1, 0.1, 1, 1e-14);
+            Assert.AreEqual(0.174749600398905, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-13);
+        }
+
+        [Test]
+        public void Oneeq7()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq7.htm
+            // not solvable with this method
+            Func<double, double> f1 = x => x / (1 - x) - 5 * Math.Log(0.4 * (1 - x) / (0.4 - 0.5 * x)) + 4.45977;
+            Assert.Throws<NonConvergenceException>(() => Brent.FindRoot(f1, 0, 0.79, 1e-2));
+        }
+
+        [Test]
+        public void Oneeq8()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq8.htm
+            // not solvable with this method
+            Func<double, double> f1 = v =>
+            {
+                const double a = 240;
+                const double b = 40;
+                const double c = 200;
+
+                return a * v * v + b * Math.Pow(v, 7 / 4) - c;
+            };
+
+            Assert.Throws<NonConvergenceException>(() => Brent.FindRoot(f1, 0.01, 1, 1e-2));
+        }
+
+        [Test]
+        public void Oneeq9()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq9.htm
+            Func<double, double> f1 = P =>
+            {
+                const double At = 0.1;
+                const double A = 0.12;
+                const double gama = 1.41;
+                const double Pd = 100;
+
+                return Math.Pow((gama + 1) / 2, (gama + 1) / (gama - 1)) * (2 / (gama - 1)) * (Math.Pow(P / Pd, 2 / gama) - Math.Pow(P / Pd, (gama + 1) / gama)) - Math.Pow(At / A, 2);
+            };
+
+            double x = Brent.FindRoot(f1, 1, 50, 1e-14);
+            Assert.AreEqual(25.743977294088, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-14);
+            x = Brent.FindRoot(f1, 50, 100, 1e-14);
+            Assert.AreEqual(78.905412035983, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-14);
+        }
+
+        [Test]
+        public void Oneeq10()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq10.htm
+            Func<double, double> f1 = x => (1.0 / 63.0) * Math.Log(x) + (64.0 / 63.0) * Math.Log(1 / (1 - x)) + Math.Log(0.95 - x) - Math.Log(0.9);
+
+            double r = Brent.FindRoot(f1, .01, 0.35, 1e-14);
+            Assert.AreEqual(0.036210083704, r, 1e-5);
+            Assert.AreEqual(0, f1(r), 1e-14);
+            r = Brent.FindRoot(f1, 0.35, 0.7);
+            Assert.AreEqual(0.5, r, 1e-5);
+            Assert.AreEqual(0, f1(r), 1e-14);
+        }
+
+        [Test]
+        public void Oneeq11a()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq11a.htm
+            Func<double, double> f1 = x =>
+            {
+                const double a = 0.5;
+                const double b = 0.8;
+                const double c = 0.3;
+                const double kp = 604500;
+                const double P = 0.00243;
+
+                return a - Math.Pow((c + 2 * x) * (a + b + c - 2 * x), 2) / (kp * P * P * Math.Pow(b - 3 * x, 3)) - x;
+            };
+
+            double r = Brent.FindRoot(f1, 0, 0.25, 1e-14);
+            Assert.AreEqual(0.058654581076661, r, 1e-5);
+            Assert.AreEqual(0, f1(r), 1e-14);
+            r = Brent.FindRoot(f1, 0.3, 1, 1e-14);
+            Assert.AreEqual(0.600323116977323, r, 1e-5);
+            Assert.AreEqual(0, f1(r), 1e-14);
+        }
+
+        [Test]
+        public void Oneeq11b()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq11b.htm
+            Func<double, double> f1 = x =>
+            {
+                const double a = 0.5;
+                const double b = 0.8;
+                const double c = 0.3;
+                const double kp = 604500;
+                const double P = 0.00243;
+
+                return (b - Math.Pow(Math.Pow((c + 2 * x) * (a + b + c - 2 * x), 2) / (kp * P * P * (a - x)), 1.0 / 3.0)) / 3.0 - x;
+            };
+
+            double r = Brent.FindRoot(f1, 0.01, 0.45, 1e-14);
+            Assert.AreEqual(0.058654571042804, r, 1e-5);
+            Assert.AreEqual(0, f1(r), 1e-14);
+            // Could not test the following root, since Math.Pow(y,1/3) does not work for y<0  
+            // r = Brent.FindRoot(f1, 0.55, 0.7);
+            // Assert.AreEqual(0.600323117488527, r, 1e-5);
+            // Assert.AreEqual(0, f1(r), 1e-14);
+        }
+
+        [Test]
+        public void Oneeq12()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq12.htm
+            Func<double, double> f1 = v =>
+            {
+                const double b = -159;
+                const double c = 9000;
+                const double P = 75;
+                const double T = 430.85;
+                const double R = 82.05;
+
+                return (R * T / P) * (1 + b / v + c / (v * v)) - v;
+            };
+
+            double x = Brent.FindRoot(f1, 100, 300, 1e-14);
+            Assert.AreEqual(213.001818272273, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-14);
+        }
+
+        [Test]
+        public void Oneeq13()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq13.htm
+            Func<double, double> f1 = V =>
+            {
+                const double Pc = 45.8;
+                const double Tc = 191.0;
+                const double P = 100;
+                const double T = 210.0;
+                const double R = 0.08205;
+                double A = 0.42748 * (R * R * Math.Pow(Tc, 2.5)) / Pc;
+                const double B = 0.08664 * R * Tc / Pc;
+
+                return R * Math.Pow(T, 1.5) * V * (V + B) / (P * Math.Sqrt(T) * V * (V + B) + A) + B - V;
+            };
+
+            double x = Brent.FindRoot(f1, .01, .3, 1e-14);
+            Assert.AreEqual(0.075699587993613, x, 1e-9);
+            Assert.AreEqual(0, f1(x), 1e-14);
+        }
+
+        [Test]
+        public void Oneeq14()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq14.htm
+            Func<double, double> f1 = Q =>
+            {
+                const double M = 43800.0;
+                const double m = 149000.0;
+                const double Cp = 0.605;
+                const double U = 55.0;
+                const double A = 662.0;
+                const double T1 = 390.0;
+                const double t1 = 100.0;
+                const double cp = 0.49;
+                double DT1 = T1 - t1 - Q / (M * Cp);
+                double DT2 = T1 - t1 - Q / (m * cp);
+
+                return U * A * (DT2 - DT1) / Math.Log(DT2 / DT1) - Q;
+            };
+
+            double x = Brent.FindRoot(f1, 1000000, 7000000, 1e-9);
+            Assert.AreEqual(5281024.23857737, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-9);
+        }
+
+        [Test]
+        public void Oneeq15a()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq15a.htm
+            Func<double, double> f1 = h => h * (1 + h * (3 - h)) - 2.4 - h;
+
+            double x = Brent.FindRoot(f1, -2, 0, 1e-14);
+            Assert.AreEqual(-0.795219754733581, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-14);
+            x = Brent.FindRoot(f1, 0, 2, 1e-14);
+            Assert.AreEqual(1.13413784566692, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-14);
+            x = Brent.FindRoot(f1, 2, 5, 1e-14);
+            Assert.AreEqual(2.66108192383197, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-14);
+        }
+
+        [Test]
+        public void Oneeq15b()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq15b.htm
+            Func<double, double> f1 = h => 2.4 / (h * (3 - h)) - h;
+
+            double x = Brent.FindRoot(f1, -2, -0.5, 1e-14);
+            Assert.AreEqual(-0.795219745444464, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-14);
+            x = Brent.FindRoot(f1, 0.5, 2, 1e-14);
+            Assert.AreEqual(1.134137843811, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-14);
+            x = Brent.FindRoot(f1, 2, 2.9, 1e-14);
+            Assert.AreEqual(2.66108190354552, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-14);
+        }
+
+        [Test]
+        public void Oneeq16()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq16.htm
+            Func<double, double> f1 = T =>
+            {
+                const double y = 0.7;
+                const double x = 1.7;
+                const double z = 1 - y - 0.02;
+                const double CH4 = 0;
+                const double C2H6 = 0;
+                const double COtwo = y + 2 * z;
+                const double H2O = 2 * y + 3 * z;
+                const double N2 = 0.02 + 3.76 * (2 * y + 7 * z / 2) * x;
+                const double O2 = (2 * y + 7 * z / 2) * (x - 1);
+                const double alp = 3.381 * CH4 + 2.247 * C2H6 + 6.214 * COtwo + 7.256 * H2O + 6.524 * N2 + 6.148 * O2;
+                const double bet = 18.044 * CH4 + 38.201 * C2H6 + 10.396 * COtwo + 2.298 * H2O + 1.25 * N2 + 3.102 * O2;
+                const double gam = -4.3 * CH4 - 11.049 * C2H6 - 3.545 * COtwo + 0.283 * H2O - 0.001 * N2 - 0.923 * O2;
+                const double H0 = alp * 298 + bet * 0.001 * 298 * 298 / 2 + gam * 1e-6 * 298 * 298 * 298 / 3;
+                double Hf = alp * T + bet * 0.001 * T * T / 2 + gam * 1e-6 * T * T * T / 3;
+                double xx = 1;
+
+                return 212798 * y * xx + 372820 * z * xx + H0 - Hf;
+            };
+
+            double r = Brent.FindRoot(f1, 1000, 3000);
+            Assert.AreEqual(1779.48406483697, r, 1e-5);
+            Assert.AreEqual(0, f1(r), 1e-9);
+        }
+
+        [Test]
+        public void Oneeq17()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq17.htm
+            Func<double, double> f1 = rp => rp - 0.327 * Math.Pow(0.06 - 161 * rp, 0.804) * Math.Exp(-5230 / (1.987 * (373 + 1.84e6 * rp)));
+
+            double x = Brent.FindRoot(f1, 0, 0.00035, 1e-13);
+            Assert.AreEqual(0.000340568862275, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 2e-14);
+        }
+
+        [Test]
+        public void Oneeq18a()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq18a.htm
+            Func<double, double> f1 = T =>
+            {
+                double Tp = (269.267 * T - 1752) / 266.667;
+                double k = 0.0006 * Math.Exp(20.7 - 15000 / Tp);
+                double Pp = -(0.1 / (321 * (320.0 / 321.0 - (1 + k))));
+                double P = (320 * Pp + 0.1) / 321;
+
+                return 1.296 * (T - Tp) + 10369 * k * Pp;
+            };
+
+            double x = Brent.FindRoot(f1, 500, 725);
+            Assert.AreEqual(690.4486645013260, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-12);
+            x = Brent.FindRoot(f1, 725, 850, 1e-12);
+            Assert.AreEqual(758.3286948959860, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-12);
+            x = Brent.FindRoot(f1, 850, 1000, 1e-12);
+            Assert.AreEqual(912.7964911381540, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-12);
+        }
+
+        [Test]
+        public void Oneeq18b()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq18b.htm
+            Func<double, double> f1 = T =>
+            {
+                double Tp = (269 * T - 1752) / 267;
+                double k = 0.0006 * Math.Exp(20.7 - 15000 / Tp);
+                double Pp = -(0.1 / (321 * (320.0 / 321.0 - (1 + k))));
+                double P = (320 * Pp + 0.1) / 321;
+
+                return 1.3 * (T - Tp) + 1.04e4 * k * Pp;
+            };
+
+            double x = Brent.FindRoot(f1, 500, 1500);
+            Assert.AreEqual(1208.2863599396200, x, 1e-4);
+            Assert.AreEqual(0, f1(x), 1e-12);
+        }
+
+        [Test]
+        public void Oneeq19a()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq19a.htm
+            Func<double, double> f1 = z =>
+            {
+                const double P = 200;
+                const double Pc = 33.5;
+                const double T = 631 * 2;
+                const double Tc = 126.2;
+                const double Pr = P / Pc;
+                const double Tr = T / Tc;
+                double Asqr = 0.4278 * Pr / Math.Pow(Tr, 2.5);
+                const double B = 0.0867 * Pr / Tr;
+                double Q = B * B + B - Asqr;
+                double r = Asqr * B;
+                double p = (-3 * Q - 1) / 3;
+                double q = (-27 * r - 9 * Q - 2) / 27;
+                double R = Math.Pow(p / 3, 3) + Math.Pow(q / 2, 2);
+                double V = (R > 0) ? Math.Pow(-q / 2 + Math.Sqrt(R), 1 / 3) : 0;
+                double WW = (R > 0) ? (-q / 2 - Math.Sqrt(R)) : (0);
+                double psi1 = (R < 0) ? (Math.Acos(Math.Sqrt((q * q / 4) / (-p * p * p / 27)))) : (0);
+                double W = (R > 0) ? (Math.Sign(WW) * Math.Pow(Math.Abs(WW), 1 / 3)) : (0);
+                double z1 = (R < 0) ? (2 * Math.Sqrt(-p / 3) * Math.Cos((psi1 / 3) + 2 * 3.1416 * 0 / 3) + 1 / 3) : (0);
+                double z2 = (R < 0) ? (2 * Math.Sqrt(-p / 3) * Math.Cos((psi1 / 3) + 2 * 3.1416 * 1 / 3) + 1 / 3) : (0);
+                double z3 = (R < 0) ? (2 * Math.Sqrt(-p / 3) * Math.Cos((psi1 / 3) + 2 * 3.1416 * 2 / 3) + 1 / 3) : (0);
+                double z0 = (R > 0) ? (V + W + 1 / 3) : (0);
+
+                return z * z * z - z * z - Q * z - r;
+            };
+
+            double x = Brent.FindRoot(f1, -0.5, -0.02, 1e-14);
+            Assert.AreEqual(-0.03241692071380, x, 1e-7);
+            Assert.AreEqual(0, f1(x), 1e-14);
+            x = Brent.FindRoot(f1, -0.02, 0.5, 1e-14);
+            Assert.AreEqual(-0.01234360708030, x, 1e-7);
+            Assert.AreEqual(0, f1(x), 1e-14);
+            x = Brent.FindRoot(f1, 0.5, 1.2, 1e-14);
+            Assert.AreEqual(1.04476050281300, x, 1e-7);
+            Assert.AreEqual(0, f1(x), 1e-14);
+        }
+
+        [Test]
+        public void Oneeq19b()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq19b.htm
+            Func<double, double> f1 = z =>
+            {
+                const double P = 200;
+                const double Pc = 33.5;
+                const double T = 631;
+                const double Tc = 126.2;
+                const double Pr = P / Pc;
+                const double Tr = T / Tc;
+                double Asqr = 0.4278 * Pr / Math.Pow(Tr, 2.5);
+                const double B = 0.0867 * Pr / Tr;
+                double Q = B * B + B - Asqr;
+                double r = Asqr * B;
+                double p = (-3 * Q - 1) / 3;
+                double q = (-27 * r - 9 * Q - 2) / 27;
+                double R = Math.Pow(p / 3, 3) + Math.Pow(q / 2, 2);
+                double V = (R > 0) ? Math.Pow(-q / 2 + Math.Sqrt(R), 1 / 3) : 0;
+                double WW = (R > 0) ? (-q / 2 - Math.Sqrt(R)) : (0);
+                double psi1 = (R < 0) ? (Math.Acos(Math.Sqrt((q * q / 4) / (-p * p * p / 27)))) : (0);
+                double W = (R > 0) ? (Math.Sign(WW) * Math.Pow(Math.Abs(WW), 1 / 3)) : (0);
+                double z1 = (R < 0) ? (2 * Math.Sqrt(-p / 3) * Math.Cos((psi1 / 3) + 2 * 3.1416 * 0 / 3) + 1 / 3) : (0);
+                double z2 = (R < 0) ? (2 * Math.Sqrt(-p / 3) * Math.Cos((psi1 / 3) + 2 * 3.1416 * 1 / 3) + 1 / 3) : (0);
+                double z3 = (R < 0) ? (2 * Math.Sqrt(-p / 3) * Math.Cos((psi1 / 3) + 2 * 3.1416 * 2 / 3) + 1 / 3) : (0);
+                double z0 = (R > 0) ? (V + W + 1 / 3) : (0);
+
+                return z * z * z - z * z - Q * z - r;
+            };
+
+            double x = Brent.FindRoot(f1, -0.5, 1.2, 1e-14);
+            Assert.AreEqual(1.06831213384200, x, 1e-7);
+            Assert.AreEqual(0, f1(x), 1e-14);
+        }
+
+        [Test]
+        public void Oneeq20a()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq20a.htm
+            Func<double, double> f1 = xa =>
+            {
+                const double T = 313;
+                const double P0 = 10;
+                const double FA0 = 20 * P0 / (0.082 * 450);
+                double k1 = 1.277 * 1.0e9 * Math.Exp(-90000 / (8.31 * T));
+                double k2 = 1.29 * 1.0e11 * Math.Exp(-135000 / (8.31 * T));
+                double den = 1 + 7 * P0 * (1 - xa) / (1 + xa);
+                double xa1 = 1 + xa;
+                double ra = (-k1 * P0 * (1 - xa) / xa1 + k2 * P0 * P0 * xa * xa / (xa1 * xa1)) / den;
+
+                return -ra / FA0;
+            };
+
+            double x = Brent.FindRoot(f1, 0.75, 1.02);
+            Assert.AreEqual(0.999251497006000, x, 1e-3);
+            Assert.AreEqual(0, f1(x), 1e-14);
+        }
+
+        [Test]
+        public void Oneeq20b()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq20b.htm
+            Func<double, double> f1 = xa =>
+            {
+                const double T = 313;
+                const double P0 = 10;
+                const double FA0 = 20 * P0 / (0.082 * 450);
+                double k1 = 1.277 * 1.0e9 * Math.Exp(-90000 / (8.31 * T));
+                double k2 = 1.29 * 1.0e11 * Math.Exp(-135000 / (8.31 * T));
+                double xa1 = 1 + xa;
+                double ra = (-k1 * P0 * (1 - xa) / xa1 + k2 * P0 * P0 * xa * xa / (xa1 * xa1));
+
+                return -ra / FA0;
+            };
+
+            double x = Brent.FindRoot(f1, 0.75, 1.02);
+            Assert.AreEqual(0.999984253901100, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-14);
+        }
+
+        [Test]
+        public void Oneeq21a()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq21a.htm
+            Func<double, double> f1 = h =>
+            {
+                double sg = Math.Acos(2 * h - 1);
+                double si = Math.Pow(1 - Math.Pow(2 * h - 1, 2), 0.5);
+                double ag = (sg - (2 * h - 1) * si) / 4;
+                double al = 3.1415927 / 4 - ag;
+                const double d = 1.44;
+                double dg = 4 * ag / (sg + si) * d;
+                const double ugs = -0.0295;
+                double ug = ugs * 3.1415927 / 4 / ag;
+                const double uls = -0.00001;
+                double ul = uls * 3.1415927 / 4 / al;
+                double sl = 3.1415927 - sg;
+                double dl = 4 * al / (sl) * d;
+                const double rol = 1;
+                const double rog = 0.835;
+                const double bt = .6;
+                double g = 980 * Math.Sin(bt * 3.1415927 / 180);
+                double dpg = (rol * al + rog * ag) * g / 3.1415927 * 4;
+                const double vig = 1.0;
+                double reg = Math.Abs(ug) * rog * dg / vig;
+                double tg = -16 / reg * (.5 * rog * ug * Math.Abs(ug));
+                const double vil = .01;
+                double rel = Math.Abs(ul) * rol * dl / vil;
+                double tl = -16 / rel * (.5 * rol * ul * Math.Abs(ul));
+                const double it = 1;
+                double ti = -16 / reg * rog * (.5 * (ug - ul) * Math.Abs(ug - ul)) * it;
+                double dpf = (tl * sl + tg * sg) * d / (al + ag) / (d * d) * 4;
+                double dp = dpg + dpf;
+                double tic = (Math.Abs(ul) < Math.Abs(ug)) ? (rog / reg) : (rol / rel);
+                double y = (rol - rog) * g * Math.Pow(d / 2, 2) / (8 * vig * ugs);
+
+                return -tg * sg * al + tl * sl * ag - ti * si * (al + ag) + d * (rol - rog) * al * ag * g;
+            };
+
+            double x = Brent.FindRoot(f1, 0, 0.3, 1e-14);
+            Assert.AreEqual(0.00596133169486, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-14);
+        }
+
+        [Test]
+        public void Oneeq21b()
+        {
+            // Test case from http://www.polymath-software.com/library/nle/Oneeq21b.htm
+            Func<double, double> f1 = h =>
+            {
+                double sg = Math.Acos(2 * h - 1);
+                double si = Math.Pow(1 - Math.Pow(2 * h - 1, 2), 0.5);
+                double ag = (sg - (2 * h - 1) * si) / 4;
+                double al = 3.1415927 / 4 - ag;
+                const double d = 1.44;
+                double dg = 4 * ag / (sg + si) * d;
+                const double ugs = -0.029;
+                double ug = ugs * 3.1415927 / 4 / ag;
+                const double uls = -0.00001;
+                double ul = uls * 3.1415927 / 4 / al;
+                double sl = 3.1415927 - sg;
+                double dl = 4 * al / (sl) * d;
+                const double rol = 1;
+                const double rog = 0.835;
+                const double bt = .6;
+                double g = 980 * Math.Sin(bt * 3.1415927 / 180);
+                double dpg = (rol * al + rog * ag) * g / 3.1415927 * 4;
+                const double vig = 1.0;
+                double reg = Math.Abs(ug) * rog * dg / vig;
+                double tg = -16 / reg * (.5 * rog * ug * Math.Abs(ug));
+                const double vil = .01;
+                double rel = Math.Abs(ul) * rol * dl / vil;
+                double tl = -16 / rel * (.5 * rol * ul * Math.Abs(ul));
+                const double it = 1;
+                double ti = -16 / reg * rog * (.5 * (ug - ul) * Math.Abs(ug - ul)) * it;
+                double dpf = (tl * sl + tg * sg) * d / (al + ag) / (d * d) * 4;
+                double dp = dpg + dpf;
+                double tic = (Math.Abs(ul) < Math.Abs(ug)) ? (rog / reg) : (rol / rel);
+                double y = (rol - rog) * g * Math.Pow(d / 2, 2) / (8 * vig * ugs);
+
+                return -tg * sg * al + tl * sl * ag - ti * si * (al + ag) + d * (rol - rog) * al * ag * g;
+            };
+
+            double x = Brent.FindRoot(f1, 0, 0.1, 1e-14);
+            Assert.AreEqual(0.00602268958797, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-14);
+            x = Brent.FindRoot(f1, 0.1, 0.24, 1e-14);
+            Assert.AreEqual(0.19925189173123, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-14);
+            x = Brent.FindRoot(f1, 0.24, 0.3, 1e-14);
+            Assert.AreEqual(0.26405262123160, x, 1e-5);
+            Assert.AreEqual(0, f1(x), 1e-14);
+        }
     }
 }