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* Licensed to the Apache Software Foundation (ASF) under one or more
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* contributor license agreements. See the NOTICE file distributed with
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* this work for additional information regarding copyright ownership.
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* The ASF licenses this file to You under the Apache License, Version 2.0
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* (the "License"); you may not use this file except in compliance with
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* the License. You may obtain a copy of the License at
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* http://www.apache.org/licenses/LICENSE-2.0
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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package org.apache.commons.math.random;
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import org.apache.commons.math.DimensionMismatchException;
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import org.apache.commons.math.linear.RealMatrix;
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import org.apache.commons.math.linear.RealMatrixImpl;
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* A {@link RandomVectorGenerator} that generates vectors with with
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* correlated components.
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* <p>Random vectors with correlated components are built by combining
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* the uncorrelated components of another random vector in such a way that
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* the resulting correlations are the ones specified by a positive
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* definite covariance matrix.</p>
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* <p>The main use for correlated random vector generation is for Monte-Carlo
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* simulation of physical problems with several variables, for example to
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* generate error vectors to be added to a nominal vector. A particularly
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* interesting case is when the generated vector should be drawn from a <a
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* href="http://en.wikipedia.org/wiki/Multivariate_normal_distribution">
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* Multivariate Normal Distribution</a>. The approach using a Cholesky
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* decomposition is quite usual in this case. However, it cas be extended
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* to other cases as long as the underlying random generator provides
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* {@link NormalizedRandomGenerator normalized values} like {@link
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* GaussianRandomGenerator} or {@link UniformRandomGenerator}.</p>
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* <p>Sometimes, the covariance matrix for a given simulation is not
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* strictly positive definite. This means that the correlations are
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* not all independent from each other. In this case, however, the non
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* strictly positive elements found during the Cholesky decomposition
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* of the covariance matrix should not be negative either, they
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* should be null. Another non-conventional extension handling this case
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* is used here. Rather than computing <code>C = U<sup>T</sup>.U</code>
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* where <code>C</code> is the covariance matrix and <code>U</code>
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* is an uppertriangular matrix, we compute <code>C = B.B<sup>T</sup></code>
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* where <code>B</code> is a rectangular matrix having
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* more rows than columns. The number of columns of <code>B</code> is
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* the rank of the covariance matrix, and it is the dimension of the
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* uncorrelated random vector that is needed to compute the component
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* of the correlated vector. This class handles this situation
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* @version $Revision: 620312 $ $Date: 2008-02-10 12:28:59 -0700 (Sun, 10 Feb 2008) $
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public class CorrelatedRandomVectorGenerator
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implements RandomVectorGenerator {
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/** Simple constructor.
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* <p>Build a correlated random vector generator from its mean
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* vector and covariance matrix.</p>
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* @param mean expected mean values for all components
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* @param covariance covariance matrix
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* @param small diagonal elements threshold under which column are
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* considered to be dependent on previous ones and are discarded
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* @param generator underlying generator for uncorrelated normalized
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* @exception IllegalArgumentException if there is a dimension
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* mismatch between the mean vector and the covariance matrix
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* @exception NotPositiveDefiniteMatrixException if the
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* covariance matrix is not strictly positive definite
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* @exception DimensionMismatchException if the mean and covariance
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* arrays dimensions don't match
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public CorrelatedRandomVectorGenerator(double[] mean,
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RealMatrix covariance, double small,
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NormalizedRandomGenerator generator)
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throws NotPositiveDefiniteMatrixException, DimensionMismatchException {
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int order = covariance.getRowDimension();
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if (mean.length != order) {
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throw new DimensionMismatchException(mean.length, order);
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this.mean = (double[]) mean.clone();
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decompose(covariance, small);
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this.generator = generator;
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normalized = new double[rank];
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/** Simple constructor.
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* <p>Build a null mean random correlated vector generator from its
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* covariance matrix.</p>
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* @param covariance covariance matrix
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* @param small diagonal elements threshold under which column are
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* considered to be dependent on previous ones and are discarded
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* @param generator underlying generator for uncorrelated normalized
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* @exception NotPositiveDefiniteMatrixException if the
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* covariance matrix is not strictly positive definite
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public CorrelatedRandomVectorGenerator(RealMatrix covariance, double small,
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NormalizedRandomGenerator generator)
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throws NotPositiveDefiniteMatrixException {
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int order = covariance.getRowDimension();
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mean = new double[order];
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for (int i = 0; i < order; ++i) {
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decompose(covariance, small);
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this.generator = generator;
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normalized = new double[rank];
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/** Get the underlying normalized components generator.
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* @return underlying uncorrelated components generator
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public NormalizedRandomGenerator getGenerator() {
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/** Get the root of the covariance matrix.
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* The root is the rectangular matrix <code>B</code> such that
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* the covariance matrix is equal to <code>B.B<sup>T</sup></code>
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* @return root of the square matrix
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public RealMatrix getRootMatrix() {
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/** Get the rank of the covariance matrix.
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* The rank is the number of independent rows in the covariance
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* matrix, it is also the number of columns of the rectangular
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* matrix of the decomposition.
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* @return rank of the square matrix.
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* @see #getRootMatrix()
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public int getRank() {
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/** Decompose the original square matrix.
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* <p>The decomposition is based on a Choleski decomposition
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* where additional transforms are performed:
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* <li>the rows of the decomposed matrix are permuted</li>
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* <li>columns with the too small diagonal element are discarded</li>
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* <li>the matrix is permuted</li>
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* This means that rather than computing M = U<sup>T</sup>.U where U
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* is an upper triangular matrix, this method computed M=B.B<sup>T</sup>
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* where B is a rectangular matrix.
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* @param covariance covariance matrix
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* @param small diagonal elements threshold under which column are
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* considered to be dependent on previous ones and are discarded
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* @exception NotPositiveDefiniteMatrixException if the
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* covariance matrix is not strictly positive definite
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private void decompose(RealMatrix covariance, double small)
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throws NotPositiveDefiniteMatrixException {
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int order = covariance.getRowDimension();
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double[][] c = covariance.getData();
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double[][] b = new double[order][order];
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int[] swap = new int[order];
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int[] index = new int[order];
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for (int i = 0; i < order; ++i) {
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for (boolean loop = true; loop;) {
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// find maximal diagonal element
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for (int i = rank + 1; i < order; ++i) {
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int isi = index[swap[i]];
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if (c[ii][ii] > c[isi][isi]) {
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if (swap[rank] != rank) {
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int tmp = index[rank];
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index[rank] = index[swap[rank]];
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index[swap[rank]] = tmp;
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// check diagonal element
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int ir = index[rank];
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if (c[ir][ir] < small) {
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throw new NotPositiveDefiniteMatrixException();
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// check remaining diagonal elements
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for (int i = rank; i < order; ++i) {
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if (c[index[i]][index[i]] < -small) {
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// there is at least one sufficiently negative diagonal element,
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// the covariance matrix is wrong
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throw new NotPositiveDefiniteMatrixException();
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// all remaining diagonal elements are close to zero,
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// we consider we have found the rank of the covariance matrix
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// transform the matrix
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double sqrt = Math.sqrt(c[ir][ir]);
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b[rank][rank] = sqrt;
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double inverse = 1 / sqrt;
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for (int i = rank + 1; i < order; ++i) {
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double e = inverse * c[ii][ir];
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for (int j = rank + 1; j < i; ++j) {
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double f = c[ii][ij] - e * b[j][rank];
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// prepare next iteration
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loop = ++rank < order;
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// build the root matrix
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root = new RealMatrixImpl(order, rank);
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for (int i = 0; i < order; ++i) {
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System.arraycopy(b[i], 0, root.getDataRef()[swap[i]], 0, rank);
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/** Generate a correlated random vector.
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* @return a random vector as an array of double. The returned array
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* is created at each call, the caller can do what it wants with it.
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public double[] nextVector() {
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// generate uncorrelated vector
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for (int i = 0; i < rank; ++i) {
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normalized[i] = generator.nextNormalizedDouble();
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// compute correlated vector
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double[] correlated = new double[mean.length];
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for (int i = 0; i < correlated.length; ++i) {
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correlated[i] = mean[i];
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for (int j = 0; j < rank; ++j) {
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correlated[i] += root.getEntry(i, j) * normalized[j];
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private double[] mean;
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/** Permutated Cholesky root of the covariance matrix. */
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private RealMatrixImpl root;
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/** Rank of the covariance matrix. */
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/** Underlying generator. */
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private NormalizedRandomGenerator generator;
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/** Storage for the normalized vector. */
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private double[] normalized;