// Take a matrix and make all the eigenvalues non-negative
private static Matrix projectToPositiveSemidefiniteMatrix(Matrix M)
{
int size = M.rows();
Utils.QL_REQUIRE(size == M.columns(), () => "matrix not square");
Matrix diagonal = new Matrix(size, size);
SymmetricSchurDecomposition jd = new SymmetricSchurDecomposition(M);
for (int i = 0; i < size; ++i)
{
diagonal[i, i] = Math.Max(jd.eigenvalues()[i], 0.0);
}
Matrix result = jd.eigenvectors() * diagonal * Matrix.transpose(jd.eigenvectors());
return(result);
}
public static Matrix rankReducedSqrt(Matrix matrix,
int maxRank,
double componentRetainedPercentage,
SalvagingAlgorithm sa)
{
int size = matrix.rows();
#if QL_EXTRA_SAFETY_CHECKS
checkSymmetry(matrix);
#else
Utils.QL_REQUIRE(size == matrix.columns(), () =>
"non square matrix: " + size + " rows, " + matrix.columns() + " columns");
#endif
Utils.QL_REQUIRE(componentRetainedPercentage > 0.0, () => "no eigenvalues retained");
Utils.QL_REQUIRE(componentRetainedPercentage <= 1.0, () => "percentage to be retained > 100%");
Utils.QL_REQUIRE(maxRank >= 1, () => "max rank required < 1");
// spectral (a.k.a Principal Component) analysis
SymmetricSchurDecomposition jd = new SymmetricSchurDecomposition(matrix);
Vector eigenValues = jd.eigenvalues();
// salvaging algorithm
switch (sa)
{
case SalvagingAlgorithm.None:
// eigenvalues are sorted in decreasing order
Utils.QL_REQUIRE(eigenValues[size - 1] >= -1e-16, () =>
"negative eigenvalue(s) (" + eigenValues[size - 1] + ")");
break;
case SalvagingAlgorithm.Spectral:
// negative eigenvalues set to zero
for (int i = 0; i < size; ++i)
{
eigenValues[i] = Math.Max(eigenValues[i], 0.0);
}
break;
case SalvagingAlgorithm.Higham:
{
int maxIterations = 40;
double tolerance = 1e-6;
Matrix adjustedMatrix = highamImplementation(matrix, maxIterations, tolerance);
jd = new SymmetricSchurDecomposition(adjustedMatrix);
eigenValues = jd.eigenvalues();
}
break;
default:
Utils.QL_FAIL("unknown or invalid salvaging algorithm");
break;
}
// factor reduction
double accumulate = 0;
eigenValues.ForEach((ii, vv) => accumulate += eigenValues[ii]);
double enough = componentRetainedPercentage * accumulate;
if (componentRetainedPercentage.IsEqual(1.0))
{
// numerical glitches might cause some factors to be discarded
enough *= 1.1;
}
// retain at least one factor
double components = eigenValues[0];
int retainedFactors = 1;
for (int i = 1; components < enough && i < size; ++i)
{
components += eigenValues[i];
retainedFactors++;
}
// output is granted to have a rank<=maxRank
retainedFactors = Math.Min(retainedFactors, maxRank);
Matrix diagonal = new Matrix(size, retainedFactors, 0.0);
for (int i = 0; i < retainedFactors; ++i)
{
diagonal[i, i] = Math.Sqrt(eigenValues[i]);
}
Matrix result = jd.eigenvectors() * diagonal;
normalizePseudoRoot(matrix, result);
return(result);
}
//! Returns the pseudo square root of a real symmetric matrix
/*! Given a matrix \f$ M \f$, the result \f$ S \f$ is defined
* as the matrix such that \f$ S S^T = M. \f$
* If the matrix is not positive semi definite, it can
* return an approximation of the pseudo square root
* using a (user selected) salvaging algorithm.
*
* For more information see: "The most general methodology to create
* a valid correlation matrix for risk management and option pricing
* purposes", by R. Rebonato and P. Jдckel.
* The Journal of Risk, 2(2), Winter 1999/2000
* http://www.rebonato.com/correlationmatrix.pdf
*
* Revised and extended in "Monte Carlo Methods in Finance",
* by Peter Jдckel, Chapter 6.
*
* \pre the given matrix must be symmetric.
*
* \relates Matrix
*
* \warning Higham algorithm only works for correlation matrices.
*
* \test
* - the correctness of the results is tested by reproducing
* known good data.
* - the correctness of the results is tested by checking
* returned values against numerical calculations.
*/
public static Matrix pseudoSqrt(Matrix matrix, SalvagingAlgorithm sa)
{
int size = matrix.rows();
#if QL_EXTRA_SAFETY_CHECKS
checkSymmetry(matrix);
#else
Utils.QL_REQUIRE(size == matrix.columns(), () =>
"non square matrix: " + size + " rows, " + matrix.columns() + " columns");
#endif
// spectral (a.k.a Principal Component) analysis
SymmetricSchurDecomposition jd = new SymmetricSchurDecomposition(matrix);
Matrix diagonal = new Matrix(size, size, 0.0);
// salvaging algorithm
Matrix result = new Matrix(size, size);
bool negative;
switch (sa)
{
case SalvagingAlgorithm.None:
// eigenvalues are sorted in decreasing order
Utils.QL_REQUIRE(jd.eigenvalues()[size - 1] >= -1e-16, () =>
"negative eigenvalue(s) (" + jd.eigenvalues()[size - 1] + ")");
result = MatrixUtilities.CholeskyDecomposition(matrix, true);
break;
case SalvagingAlgorithm.Spectral:
// negative eigenvalues set to zero
for (int i = 0; i < size; i++)
{
diagonal[i, i] = Math.Sqrt(Math.Max(jd.eigenvalues()[i], 0.0));
}
result = jd.eigenvectors() * diagonal;
normalizePseudoRoot(matrix, result);
break;
case SalvagingAlgorithm.Hypersphere:
// negative eigenvalues set to zero
negative = false;
for (int i = 0; i < size; ++i)
{
diagonal[i, i] = Math.Sqrt(Math.Max(jd.eigenvalues()[i], 0.0));
if (jd.eigenvalues()[i] < 0.0)
{
negative = true;
}
}
result = jd.eigenvectors() * diagonal;
normalizePseudoRoot(matrix, result);
if (negative)
{
result = hypersphereOptimize(matrix, result, false);
}
break;
case SalvagingAlgorithm.LowerDiagonal:
// negative eigenvalues set to zero
negative = false;
for (int i = 0; i < size; ++i)
{
diagonal[i, i] = Math.Sqrt(Math.Max(jd.eigenvalues()[i], 0.0));
if (jd.eigenvalues()[i] < 0.0)
{
negative = true;
}
}
result = jd.eigenvectors() * diagonal;
normalizePseudoRoot(matrix, result);
if (negative)
{
result = hypersphereOptimize(matrix, result, true);
}
break;
case SalvagingAlgorithm.Higham:
int maxIterations = 40;
double tol = 1e-6;
result = highamImplementation(matrix, maxIterations, tol);
result = MatrixUtilities.CholeskyDecomposition(result, true);
break;
default:
Utils.QL_FAIL("unknown salvaging algorithm");
break;
}
return(result);
}