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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.ode;
+
+import org.apache.commons.math.ode.DerivativeException;
+
+/** This interface represents a second order differential equations set.
+
+ * <p>This interface should be implemented by all real second order
+ * differential equation problems before they can be handled by the
+ * integrators {@link SecondOrderIntegrator#integrate} method.</p>
+ *
+ * <p>A second order differential equations problem, as seen by an
+ * integrator is the second time derivative <code>d2Y/dt^2</code> of a
+ * state vector <code>Y</code>, both being one dimensional
+ * arrays. From the integrator point of view, this derivative depends
+ * only on the current time <code>t</code>, on the state vector
+ * <code>Y</code> and on the first time derivative of the state
+ * vector.</p>
+ *
+ * <p>For real problems, the derivative depends also on parameters
+ * that do not belong to the state vector (dynamical model constants
+ * for example). These constants are completely outside of the scope
+ * of this interface, the classes that implement it are allowed to
+ * handle them as they want.</p>
+ *
+ * @see SecondOrderIntegrator
+ * @see FirstOrderConverter
+ * @see FirstOrderDifferentialEquations
+ * @version $Revision: 1073158 $ $Date: 2011-02-21 22:46:52 +0100 (lun. 21 févr. 2011) $
+ * @since 1.2
+ */
+
+public interface SecondOrderDifferentialEquations {
+
+    /** Get the dimension of the problem.
+     * @return dimension of the problem
+     */
+    int getDimension();
+
+    /** Get the current time derivative of the state vector.
+     * @param t current value of the independent <I>time</I> variable
+     * @param y array containing the current value of the state vector
+     * @param yDot array containing the current value of the first derivative
+     * of the state vector
+     * @param yDDot placeholder array where to put the second time derivative
+     * of the state vector
+     * @throws DerivativeException this user-defined exception should be used if an error is
+     * is triggered by user code
+     */
+    void computeSecondDerivatives(double t, double[] y, double[] yDot, double[] yDDot)
+        throws DerivativeException;
+
+}