1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * https://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17
18 /*
19 * This is not the original file distributed by the Apache Software Foundation
20 * It has been modified by the Hipparchus project
21 */
22
23 package org.hipparchus.ode;
24
25 import org.hipparchus.util.FastMath;
26
27 /**
28 * This class is used in the junit tests for the ODE integrators.
29
30 * <p>This specific problem is the following differential equation :
31 * <pre>
32 * y' = -y
33 * </pre>
34 * the solution of this equation is a simple exponential function :
35 * <pre>
36 * y (t) = y (t0) exp (t0-t)
37 * </pre>
38 * </p>
39
40 */
41 public class TestProblem1 extends TestProblemAbstract {
42
43 /**
44 * Simple constructor.
45 */
46 public TestProblem1() {
47 super(0.0, new double[] { 1.0, 0.1 }, 4.0, new double[] { 1.0, 1.0 });
48 }
49
50 @Override
51 public double[] doComputeDerivatives(double t, double[] y) {
52
53 // compute the derivatives
54 final double[] yDot = new double[getDimension()];
55 for (int i = 0; i < getDimension(); ++i) {
56 yDot[i] = -y[i];
57 }
58 return yDot;
59
60 }
61
62 @Override
63 public double[] computeTheoreticalState(double t) {
64 final double c = FastMath.exp (getInitialTime() - t);
65 final double[] y = getInitialState().getPrimaryState();
66 for (int i = 0; i < getDimension(); ++i) {
67 y[i] *= c;
68 }
69 return y;
70 }
71
72 }