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diff --git a/src/main/java/org/apache/commons/math/optimization/fitting/HarmonicFunction.java b/src/main/java/org/apache/commons/math/optimization/fitting/HarmonicFunction.java
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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.commons.math.optimization.fitting;
+
+import org.apache.commons.math.analysis.DifferentiableUnivariateRealFunction;
+import org.apache.commons.math.util.FastMath;
+
+/** Harmonic function of the form <code>f (t) = a cos (&omega; t + &phi;)</code>.
+ * @version $Revision: 990655 $ $Date: 2010-08-29 23:49:40 +0200 (dim. 29 août 2010) $
+ * @since 2.0
+ */
+public class HarmonicFunction implements DifferentiableUnivariateRealFunction {
+
+    /** Amplitude a. */
+    private final double a;
+
+    /** Pulsation &omega;. */
+    private final double omega;
+
+    /** Phase &phi;. */
+    private final double phi;
+
+    /** Simple constructor.
+     * @param a amplitude
+     * @param omega pulsation
+     * @param phi phase
+     */
+    public HarmonicFunction(double a, double omega, double phi) {
+        this.a     = a;
+        this.omega = omega;
+        this.phi   = phi;
+    }
+
+    /** {@inheritDoc} */
+    public double value(double x) {
+        return a * FastMath.cos(omega * x + phi);
+    }
+
+    /** {@inheritDoc} */
+    public HarmonicFunction derivative() {
+        return new HarmonicFunction(a * omega, omega, phi + FastMath.PI / 2);
+    }
+
+    /** Get the amplitude a.
+     * @return amplitude a;
+     */
+    public double getAmplitude() {
+        return a;
+    }
+
+    /** Get the pulsation &omega;.
+     * @return pulsation &omega;
+     */
+    public double getPulsation() {
+        return omega;
+    }
+
+    /** Get the phase &phi;.
+     * @return phase &phi;
+     */
+    public double getPhase() {
+        return phi;
+    }
+
+}