FindRoots: explicit formula for chebychev polynomials (1st and 2nd kind)

src/Numerics/FindRoots.cs

@@ -105,5 +105,63 @@ namespace MathNet.Numerics
 
             return new Tuple<Complex, Complex>(q/a, c/q);
         }
+
+        /// <summary>
+        /// Find all roots of the Chebychev polynomial of the first kind.
+        /// </summary>
+        /// <param name="degree">The polynomial order and therefore the number of roots.</param>
+        /// <param name="intervalBegin">The real domain interval begin where to start sampling.</param>
+        /// <param name="intervalEnd">The real domain interval end where to stop sampling.</param>
+        /// <returns>Samples in [a,b] at (b+a)/2+(b-1)/2*cos(pi*(2i-1)/(2n))</returns>
+        public static double[] ChebychevPolynomialFirstKind(int degree, double intervalBegin = -1d, double intervalEnd = 1d)
+        {
+            if (degree < 1)
+            {
+                return new double[0];
+            }
+
+            // transform to map to [-1..1] interval
+            double location = 0.5*(intervalBegin + intervalEnd);
+            double scale = 0.5*(intervalEnd - intervalBegin);
+
+            // evaluate first kind chebyshev nodes
+            double angleFactor = Constants.Pi/(2*degree);
+
+            var samples = new double[degree];
+            for (int i = 0; i < samples.Length; i++)
+            {
+                samples[i] = location + scale*Math.Cos(((2*i) + 1)*angleFactor);
+            }
+            return samples;
+        }
+
+        /// <summary>
+        /// Find all roots of the Chebychev polynomial of the second kind.
+        /// </summary>
+        /// <param name="degree">The polynomial order and therefore the number of roots.</param>
+        /// <param name="intervalBegin">The real domain interval begin where to start sampling.</param>
+        /// <param name="intervalEnd">The real domain interval end where to stop sampling.</param>
+        /// <returns>Samples in [a,b] at (b+a)/2+(b-1)/2*cos(pi*i/(n-1))</returns>
+        public static double[] ChebychevPolynomialSecondKind(int degree, double intervalBegin = -1d, double intervalEnd = 1d)
+        {
+            if (degree < 1)
+            {
+                return new double[0];
+            }
+
+            // transform to map to [-1..1] interval
+            double location = 0.5*(intervalBegin + intervalEnd);
+            double scale = 0.5*(intervalEnd - intervalBegin);
+
+            // evaluate second kind chebyshev nodes
+            double angleFactor = Constants.Pi/(degree + 1);
+
+            var samples = new double[degree];
+            for (int i = 0; i < samples.Length; i++)
+            {
+                samples[i] = location + scale*Math.Cos((i + 1)*angleFactor);
+            }
+            return samples;
+        }
     }
 }

src/Numerics/Signals/SignalGenerator.Chebyshev.cs

@@ -48,36 +48,19 @@ namespace MathNet.Numerics.Signals
         /// <returns>Vector of the function sampled in [a,b] at (b+a)/2+(b-1)/2*cos(pi*(2i-1)/(2n))</returns>
         /// <exception cref="ArgumentNullException" />
         /// <exception cref="ArgumentOutOfRangeException" />
+        [Obsolete]
         public static T[] ChebyshevNodesFirstKind<T>(
             Func<double, T> function,
             double intervalBegin,
             double intervalEnd,
             int sampleCount)
         {
-            if (ReferenceEquals(function, null))
+            var roots = FindRoots.ChebychevPolynomialFirstKind(sampleCount, intervalBegin, intervalEnd);
-            {
+            var samples = new T[roots.Length];
-                throw new ArgumentNullException("function");
-            }
-
-            if (sampleCount < 1)
-            {
-                throw new ArgumentOutOfRangeException("sampleCount");
-            }
-
-            // transform to map to [-1..1] interval
-            double transformSummand = 0.5 * (intervalBegin + intervalEnd);
-            double transformFactor = 0.5 * (intervalEnd - intervalBegin);
-
-            // evaluate first kind chebyshev nodes
-            double angleFactor = Constants.Pi / (2 * sampleCount);
-
-            var samples = new T[sampleCount];
-
             for (int i = 0; i < samples.Length; i++)
             {
-                samples[i] = function(transformSummand + transformFactor * Math.Cos(((2 * i) + 1) * angleFactor));
+                samples[i] = function(roots[i]);
             }
-
             return samples;
         }
 
@@ -92,36 +75,19 @@ namespace MathNet.Numerics.Signals
         /// <returns>Vector of the function sampled in [a,b] at (b+a)/2+(b-1)/2*cos(pi*i/(n-1))</returns>
         /// <exception cref="ArgumentNullException" />
         /// <exception cref="ArgumentOutOfRangeException" />
+        [Obsolete]
         public static T[] ChebyshevNodesSecondKind<T>(
             Func<double, T> function,
             double intervalBegin,
             double intervalEnd,
             int sampleCount)
         {
-            if (ReferenceEquals(function, null))
+            var roots = FindRoots.ChebychevPolynomialSecondKind(sampleCount, intervalBegin, intervalEnd);
-            {
+            var samples = new T[roots.Length];
-                throw new ArgumentNullException("function");
-            }
-
-            if (sampleCount < 1)
-            {
-                throw new ArgumentOutOfRangeException("sampleCount");
-            }
-
-            // transform to map to [-1..1] interval
-            double transformSummand = 0.5 * (intervalBegin + intervalEnd);
-            double transformFactor = 0.5 * (intervalEnd - intervalBegin);
-
-            // evaluate second kind chebyshev nodes
-            double angleFactor = Constants.Pi / (sampleCount + 1);
-
-            var samples = new T[sampleCount];
-
             for (int i = 0; i < samples.Length; i++)
             {
-                samples[i] = function(transformSummand + transformFactor * Math.Cos((i + 1) * angleFactor));
+                samples[i] = function(roots[i]);
             }
-
             return samples;
         }
     }