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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.nonstiff;
+
+import org.apache.commons.math3.util.FastMath;
+
+
+/**
+ * This class implements the Gill fourth order Runge-Kutta
+ * integrator for Ordinary Differential Equations .
+
+ * <p>This method is an explicit Runge-Kutta method, its Butcher-array
+ * is the following one :
+ * <pre>
+ *    0  |    0        0       0      0
+ *   1/2 |   1/2       0       0      0
+ *   1/2 | (q-1)/2  (2-q)/2    0      0
+ *    1  |    0       -q/2  (2+q)/2   0
+ *       |-------------------------------
+ *       |   1/6    (2-q)/6 (2+q)/6  1/6
+ * </pre>
+ * where q = sqrt(2)</p>
+ *
+ * @see EulerIntegrator
+ * @see ClassicalRungeKuttaIntegrator
+ * @see MidpointIntegrator
+ * @see ThreeEighthesIntegrator
+ * @see LutherIntegrator
+ * @since 1.2
+ */
+
+public class GillIntegrator extends RungeKuttaIntegrator {
+
+  /** Time steps Butcher array. */
+  private static final double[] STATIC_C = {
+    1.0 / 2.0, 1.0 / 2.0, 1.0
+  };
+
+  /** Internal weights Butcher array. */
+  private static final double[][] STATIC_A = {
+    { 1.0 / 2.0 },
+    { (FastMath.sqrt(2.0) - 1.0) / 2.0, (2.0 - FastMath.sqrt(2.0)) / 2.0 },
+    { 0.0, -FastMath.sqrt(2.0) / 2.0, (2.0 + FastMath.sqrt(2.0)) / 2.0 }
+  };
+
+  /** Propagation weights Butcher array. */
+  private static final double[] STATIC_B = {
+    1.0 / 6.0, (2.0 - FastMath.sqrt(2.0)) / 6.0, (2.0 + FastMath.sqrt(2.0)) / 6.0, 1.0 / 6.0
+  };
+
+  /** Simple constructor.
+   * Build a fourth-order Gill integrator with the given step.
+   * @param step integration step
+   */
+  public GillIntegrator(final double step) {
+    super("Gill", STATIC_C, STATIC_A, STATIC_B, new GillStepInterpolator(), step);
+  }
+
+}