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diff --git a/src/main/java/org/apache/commons/math/analysis/DifferentiableMultivariateRealFunction.java b/src/main/java/org/apache/commons/math/analysis/DifferentiableMultivariateRealFunction.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.analysis;
+
+/**
+ * Extension of {@link MultivariateRealFunction} representing a differentiable
+ * multivariate real function.
+ * @version $Revision: 811685 $ $Date: 2009-09-05 19:36:48 +0200 (sam. 05 sept. 2009) $
+ * @since 2.0
+ */
+public interface DifferentiableMultivariateRealFunction extends MultivariateRealFunction {
+
+    /**
+     * Returns the partial derivative of the function with respect to a point coordinate.
+     * <p>
+     * The partial derivative is defined with respect to point coordinate
+     * x<sub>k</sub>. If the partial derivatives with respect to all coordinates are
+     * needed, it may be more efficient to use the {@link #gradient()} method which will
+     * compute them all at once.
+     * </p>
+     * @param k index of the coordinate with respect to which the partial
+     * derivative is computed
+     * @return the partial derivative function with respect to k<sup>th</sup> point coordinate
+     */
+    MultivariateRealFunction partialDerivative(int k);
+
+    /**
+     * Returns the gradient function.
+     * <p>If only one partial derivative with respect to a specific coordinate is
+     * needed, it may be more efficient to use the {@link #partialDerivative(int)} method
+     * which will compute only the specified component.</p>
+     * @return the gradient function
+     */
+    MultivariateVectorialFunction gradient();
+
+}