The function may be called by the names: f01jec or nag_matop_real_gen_matrix_cond_pow.
For a matrix with no eigenvalues on the closed negative real line, () can be defined as
where is the principal logarithm of (the unique logarithm whose spectrum lies in the strip ).
The Fréchet derivative of the matrix th power of is the unique linear mapping such that for any matrix
The derivative describes the first-order effect of perturbations in on the matrix power .
The relative condition number of the matrix th power can be defined by
where is the norm of the Fréchet derivative of the matrix power at .
f01jec uses the algorithms of Higham and Lin (2011) and Higham and Lin (2013) to compute and . The real number is expressed as where and . Then . The integer power is found using a combination of binary powering and, if necessary, matrix inversion. The fractional power is computed using a Schur decomposition, a Padé approximant and the scaling and squaring method.
To obtain an estimate of , f01jec first estimates by computing an estimate of a quantity , such that . This requires multiple Fréchet derivatives to be computed. Fréchet derivatives of are obtained by differentiating the Padé approximant. Fréchet derivatives of are then computed using a combination of the chain rule and the product rule for Fréchet derivatives.
Higham N J (2008) Functions of Matrices: Theory and Computation SIAM, Philadelphia, PA, USA
Higham N J and Lin L (2011) A Schur–Padé algorithm for fractional powers of a matrix SIAM J. Matrix Anal. Appl.32(3) 1056–1078
Higham N J and Lin L (2013) An improved Schur–Padé algorithm for fractional powers of a matrix and their Fréchet derivatives SIAM J. Matrix Anal. Appl.34(3) 1341–1360
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 th power, .
3: – IntegerInput
On entry: the stride separating matrix row elements in the array a.
4: – doubleInput
On entry: the required power of .
5: – double *Output
On exit: if NE_NOERROR or NW_SOME_PRECISION_LOSS, an estimate of the relative condition number of the matrix th power, . Alternatively, if NE_RCOND, the absolute condition number of the matrix th power.
6: – 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, . Constraint: .
On entry, and . Constraint: .
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 th power is not defined in this case; f01kec can be used to find a complex, non-principal th power.
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 th power cannot be computed.
has been computed using an IEEE double precision Padé approximant, although the arithmetic precision is higher than IEEE double precision.
f01jec 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 fractional part of the matrix power reduces to evaluating powers 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. See Higham and Lin (2011) and Higham and Lin (2013) for details and further discussion.
8Parallelism and Performance
f01jec is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
f01jec 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.
The amount of real allocatable memory required by the algorithm is typically of the order .
If the matrix th power alone is required, without an estimate of the condition number, then f01eqc should be used. If the Fréchet derivative of the matrix power is required then f01jfc should be used. If has negative real eigenvalues then f01kec can be used to return a complex, non-principal th power and its condition number.
This example estimates the relative condition number of the matrix power , where and