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diff --git a/src/main/java/org/apache/commons/math3/ode/FieldSecondaryEquations.java b/src/main/java/org/apache/commons/math3/ode/FieldSecondaryEquations.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.math3.ode;
+
+import org.apache.commons.math3.RealFieldElement;
+import org.apache.commons.math3.exception.DimensionMismatchException;
+import org.apache.commons.math3.exception.MaxCountExceededException;
+
+/**
+ * This interface allows users to add secondary differential equations to a primary set of
+ * differential equations.
+ *
+ * <p>In some cases users may need to integrate some problem-specific equations along with a primary
+ * set of differential equations. One example is optimal control where adjoined parameters linked to
+ * the minimized Hamiltonian must be integrated.
+ *
+ * <p>This interface allows users to add such equations to a primary set of {@link
+ * FirstOrderFieldDifferentialEquations first order differential equations} thanks to the {@link
+ * FieldExpandableODE#addSecondaryEquations(FieldSecondaryEquations)} method.
+ *
+ * @see FirstOrderFieldDifferentialEquations
+ * @see FieldExpandableODE
+ * @param <T> the type of the field elements
+ * @since 3.6
+ */
+public interface FieldSecondaryEquations<T extends RealFieldElement<T>> {
+
+    /**
+     * Get the dimension of the secondary state parameters.
+     *
+     * @return dimension of the secondary state parameters
+     */
+    int getDimension();
+
+    /**
+     * Initialize equations at the start of an ODE integration.
+     *
+     * <p>This method is called once at the start of the integration. It may be used by the
+     * equations to initialize some internal data if needed.
+     *
+     * @param t0 value of the independent <I>time</I> variable at integration start
+     * @param primary0 array containing the value of the primary state vector at integration start
+     * @param secondary0 array containing the value of the secondary state vector at integration
+     *     start
+     * @param finalTime target time for the integration
+     */
+    void init(T t0, T[] primary0, T[] secondary0, T finalTime);
+
+    /**
+     * Compute the derivatives related to the secondary state parameters.
+     *
+     * @param t current value of the independent <I>time</I> variable
+     * @param primary array containing the current value of the primary state vector
+     * @param primaryDot array containing the derivative of the primary state vector
+     * @param secondary array containing the current value of the secondary state vector
+     * @return derivative of the secondary state vector
+     * @exception MaxCountExceededException if the number of functions evaluations is exceeded
+     * @exception DimensionMismatchException if arrays dimensions do not match equations settings
+     */
+    T[] computeDerivatives(T t, T[] primary, T[] primaryDot, T[] secondary)
+            throws MaxCountExceededException, DimensionMismatchException;
+}