Al–Mohy A H and Higham N J (2011) Improved inverse scaling and squaring algorithms for the matrix logarithm SIAM J. Sci. Comput.34(4) C152–C169
Al–Mohy A H, Higham N J and Relton S D (2012) Computing the Fréchet derivative of the matrix logarithm and estimating the condition number SIAM J. Sci. Comput.35(4) C394–C410
Higham N J (2008) Functions of Matrices: Theory and Computation SIAM, Philadelphia, PA, USA
1: – IntegerInput
On entry: , the order of the matrix .
2: – doubleInput/Output
Note: the dimension, dim, of the array a
must be at least
The th element of the matrix is stored in .
On entry: the matrix .
On exit: the principal matrix logarithm, .
3: – IntegerInput
On entry: the stride separating matrix row elements in the array a.
4: – double *Output
On exit: with NE_NOERROR or NW_SOME_PRECISION_LOSS, an estimate of the relative condition number of the matrix logarithm, . Alternatively, if NE_RCOND, contains the absolute condition number of the matrix logarithm.
5: – NagError *Input/Output
The NAG error argument (see Section 7 in the Introduction to the NAG Library CL Interface).
6Error Indicators and Warnings
Dynamic memory allocation failed.
See Section 3.1.2 in the Introduction to the NAG Library CL Interface for further information.
On entry, argument had an illegal value.
On entry, .
On entry, and .
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.
See Section 7.5 in the Introduction to the NAG Library CL Interface for further information.
has eigenvalues on the negative real line. The principal logarithm is not defined in this case; f01kjc can be used to return a complex, non-principal log.
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library CL Interface for further information.
The relative condition number is infinite. The absolute condition number was returned instead.
is singular so the logarithm cannot be computed.
has been computed using an IEEE double precision Padé approximant, although the arithmetic precision is higher than IEEE double precision.
f01jjc uses the norm estimation function f04ydc to produce an estimate of a quantity , such that . For further details on the accuracy of norm estimation, see the documentation for f04ydc.
For a normal matrix (for which ), the Schur decomposition is diagonal and the computation of the matrix logarithm reduces to evaluating the logarithm of the eigenvalues of and then constructing using the Schur vectors. This should give a very accurate result. In general, however, no error bounds are available for the algorithm. The sensitivity of the computation of is worst when has an eigenvalue of very small modulus or has a complex conjugate pair of eigenvalues lying close to the negative real axis. See Al–Mohy and Higham (2011) and Section 11.2 of Higham (2008) for details and further discussion.
8Parallelism and Performance
Background information to multithreading can be found in the Multithreading documentation.
f01jjc is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
f01jjc makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.
Please consult the X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this function. Please also consult the Users' Note for your implementation for any additional implementation-specific information.
f01jac uses a similar algorithm to f01jjc to compute an estimate of the absolute condition number (which is related to the relative condition number by a factor of ). However, the required Fréchet derivatives are computed in a more efficient and stable manner by f01jjc and so its use is recommended over f01jac.
The amount of real allocatable memory required by the algorithm is typically of the order .
If the matrix logarithm alone is required, without an estimate of the condition number, then f01ejc should be used. If the Fréchet derivative of the matrix logarithm is required then f01jkc should be used. If has negative real eigenvalues then f01kjc can be used to return a complex, non-principal matrix logarithm and its condition number.
This example estimates the relative condition number of the matrix logarithm , where