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.linear;
24
25 import org.hipparchus.exception.MathIllegalArgumentException;
26
27 /**
28 * This class defines a linear operator operating on real ({@code double})
29 * vector spaces. No direct access to the coefficients of the underlying matrix
30 * is provided.
31 * <p>
32 * The motivation for such an interface is well stated by
33 * <a href="#BARR1994">Barrett et al. (1994)</a>:
34 * </p>
35 * <blockquote>
36 * We restrict ourselves to iterative methods, which work by repeatedly
37 * improving an approximate solution until it is accurate enough. These
38 * methods access the coefficient matrix A of the linear system only via the
39 * matrix-vector product y = A · x
40 * (and perhaps z = A<sup>T</sup> · x). Thus the user need only
41 * supply a subroutine for computing y (and perhaps z) given x, which permits
42 * full exploitation of the sparsity or other special structure of A.
43 * </blockquote>
44 * <dl>
45 * <dt>Barret et al. (1994)</dt>
46 * <dd>
47 * R. Barrett, M. Berry, T. F. Chan, J. Demmel, J. M. Donato, J. Dongarra,
48 * V. Eijkhout, R. Pozo, C. Romine and H. Van der Vorst,
49 * <em>Templates for the Solution of Linear Systems: Building Blocks for
50 * Iterative Methods</em>, SIAM
51 * </dd>
52 * </dl>
53 */
54 public interface RealLinearOperator {
55 /**
56 * Returns the dimension of the codomain of this operator.
57 *
58 * @return the number of rows of the underlying matrix
59 */
60 int getRowDimension();
61
62 /**
63 * Returns the dimension of the domain of this operator.
64 *
65 * @return the number of columns of the underlying matrix
66 */
67 int getColumnDimension();
68
69 /**
70 * Returns the result of multiplying {@code this} by the vector {@code x}.
71 *
72 * @param x the vector to operate on
73 * @return the product of {@code this} instance with {@code x}
74 * @throws MathIllegalArgumentException if the column dimension does not match
75 * the size of {@code x}
76 */
77 RealVector operate(RealVector x)
78 throws MathIllegalArgumentException;
79
80 /**
81 * Returns the result of multiplying the transpose of {@code this} operator
82 * by the vector {@code x} (optional operation).
83 * <p>
84 * The default implementation throws an {@link UnsupportedOperationException}.
85 * Users overriding this method must also override {@link #isTransposable()}.
86 *
87 * @param x the vector to operate on
88 * @return the product of the transpose of {@code this} instance with {@code x}
89 * @throws MathIllegalArgumentException if the row dimension does not match the
90 * size of {@code x}
91 * @throws UnsupportedOperationException if this operation is not supported
92 * by {@code this} operator
93 */
94 default RealVector operateTranspose(final RealVector x)
95 throws MathIllegalArgumentException, UnsupportedOperationException {
96 throw new UnsupportedOperationException();
97 }
98
99 /**
100 * Returns {@code true} if this operator supports {@link #operateTranspose(RealVector)}.
101 * <p>
102 * If {@code true} is returned, {@link #operateTranspose(RealVector)}
103 * should not throw {@code UnsupportedOperationException}.
104 * <p>
105 * The default implementation returns {@code false}.
106 *
107 * @return {@code false}
108 */
109 default boolean isTransposable() {
110 return false;
111 }
112 }