Exceptions for infinte bounds.

Author: jkalias

src/Numerics.Tests/RootFindingTests/RobustNewtonRaphsonTest.cs

@@ -136,7 +136,6 @@ public void InfinitelyManyRootsWithGivenAccuracy()
         [Test]
         public void BoundsNearMaxValueNoOverflow()
         {
-            // degenerate case with infinitely many roots
             Func<double, double> f1 = x => 0.0;
             Func<double, double> df1 = x => 0.0;
             Assert.AreEqual(double.MaxValue, RobustNewtonRaphson.FindRoot(f1, df1, double.MaxValue - 2, double.MaxValue - 1), 1e-6);
@@ -145,12 +144,29 @@ public void BoundsNearMaxValueNoOverflow()
         [Test]
         public void BoundsNearMinValueNoUnderflow()
         {
-            // degenerate case with infinitely many roots
             Func<double, double> f1 = x => 0.0;
             Func<double, double> df1 = x => 0.0;
             Assert.AreEqual(double.MinValue, RobustNewtonRaphson.FindRoot(f1, df1, double.MinValue + 1, double.MinValue + 2), 1e-6);
         }
 
+        [Test]
+        public void InfiniteLowerBound()
+        {
+            Func<double, double> f1 = x => 0.0;
+            Func<double, double> df1 = x => 0.0;
+            Assert.That(() => RobustNewtonRaphson.FindRoot(f1, df1, double.NegativeInfinity, 5), Throws.TypeOf<ArgumentOutOfRangeException>());
+            Assert.That(() => RobustNewtonRaphson.FindRoot(f1, df1, double.PositiveInfinity, 5), Throws.TypeOf<ArgumentOutOfRangeException>());
+        }
+
+        [Test]
+        public void InfiniteUpperBound()
+        {
+            Func<double, double> f1 = x => 0.0;
+            Func<double, double> df1 = x => 0.0;
+            Assert.That(() => RobustNewtonRaphson.FindRoot(f1, df1, -5, double.NegativeInfinity), Throws.TypeOf<ArgumentOutOfRangeException>());
+            Assert.That(() => RobustNewtonRaphson.FindRoot(f1, df1, -5, double.PositiveInfinity), Throws.TypeOf<ArgumentOutOfRangeException>());
+        }
+
         [Test]
         public void NoRoot()
         {

src/Numerics/RootFinding/RobustNewtonRaphson.cs

@@ -60,8 +60,8 @@ public static double FindRoot(Func<double, double> f, Func<double, double> df, d
         /// <summary>Find a solution of the equation f(x)=0.</summary>
         /// <param name="f">The function to find roots from.</param>
         /// <param name="df">The first derivative of the function to find roots from.</param>
-        /// <param name="lowerBound">The low value of the range where the root is supposed to be.</param>
-        /// <param name="upperBound">The high value of the range where the root is supposed to be.</param>
+        /// <param name="lowerBound">The low value of the range where the root is supposed to be (finite number).</param>
+        /// <param name="upperBound">The high value of the range where the root is supposed to be (finite number).</param>
         /// <param name="accuracy">Desired accuracy. The root will be refined until the accuracy or the maximum number of iterations is reached. Example: 1e-14. Must be greater than 0.</param>
         /// <param name="maxIterations">Maximum number of iterations. Example: 100.</param>
         /// <param name="subdivision">How many parts an interval should be split into for zero crossing scanning in case of lacking bracketing. Example: 20.</param>
@@ -74,6 +74,16 @@ public static bool TryFindRoot(Func<double, double> f, Func<double, double> df,
                 throw new ArgumentOutOfRangeException(nameof(accuracy), "Must be greater than zero.");
             }
 
+            if (double.IsInfinity(lowerBound))
+            {
+                throw new ArgumentOutOfRangeException(nameof(lowerBound), "Must be a finite number.");
+            }
+
+            if (double.IsInfinity(upperBound))
+            {
+                throw new ArgumentOutOfRangeException(nameof(upperBound), "Must be a finite number.");
+            }
+
             root = lowerBound + 0.5 * (upperBound - lowerBound);
             double fx = f(root);
             if (Math.Abs(fx) < accuracy)