text: /*
   * 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 $Revision$ $Date$
   * @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();
  
  }
  