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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;
+
+
+/** This interface represents a first order differential equations set
+ * with a main set of equations and an extension set.
+ *
+ * <p>
+ * This interface is a simple extension on the {@link
+ * FirstOrderDifferentialEquations} that allows to identify which part
+ * of a complete set of differential equations correspond to the main
+ * set and which part correspond to the extension set.
+ * </p>
+ * <p>
+ * One typical use case is the computation of Jacobians. The main
+ * set of equations correspond to the raw ode, and we add to this set
+ * another bunch of equations which represent the jacobians of the
+ * main set. In that case, we want the integrator to use <em>only</em>
+ * the main set to estimate the errors and hence the step sizes. It should
+ * <em>not</em> use the additional equations in this computation. If the
+ * complete ode implements this interface, the {@link FirstOrderIntegrator
+ * integrator} will be able to know where the main set ends and where the
+ * extended set begins.
+ * </p>
+ * <p>
+ * We consider that the main set always corresponds to the first equations
+ * and the extended set to the last equations.
+ * </p>
+ *
+ * @see FirstOrderDifferentialEquations
+ *
+ * @version $Revision: 980981 $ $Date: 2010-07-31 00:03:04 +0200 (sam. 31 juil. 2010) $
+ * @since 2.2
+ */
+
+public interface ExtendedFirstOrderDifferentialEquations extends FirstOrderDifferentialEquations {
+
+    /** Return the dimension of the main set of equations.
+     * <p>
+     * The main set of equations represent the first part of an ODE state.
+     * The error estimations and adaptive step size computation should be
+     * done on this first part only, not on the final part of the state
+     * which represent an extension set of equations which are considered
+     * secondary.
+     * </p>
+     * @return dimension of the main set of equations, must be lesser than or
+     * equal to the {@link #getDimension() total dimension}
+     */
+    int getMainSetDimension();
+
+}