nag_zgttrf (f07crc) uses Gaussian elimination with partial pivoting and row interchanges to factorize the matrix
as
where
is a permutation matrix,
is unit lower triangular with at most one nonzero subdiagonal element in each column, and
is an upper triangular band matrix, with two superdiagonals.
Anderson E, Bai Z, Bischof C, Blackford S, Demmel J, Dongarra J J, Du Croz J J, Greenbaum A, Hammarling S, McKenney A and Sorensen D (1999)
LAPACK Users' Guide (3rd Edition) SIAM, Philadelphia
http://www.netlib.org/lapack/lug
- 1:
– IntegerInput
-
On entry: , the order of the matrix .
Constraint:
.
- 2:
– ComplexInput/Output
-
Note: the dimension,
dim, of the array
dl
must be at least
.
On entry: must contain the subdiagonal elements of the matrix .
On exit: is overwritten by the multipliers that define the matrix of the factorization of .
- 3:
– ComplexInput/Output
-
Note: the dimension,
dim, of the array
d
must be at least
.
On entry: must contain the diagonal elements of the matrix .
On exit: is overwritten by the diagonal elements of the upper triangular matrix from the factorization of .
- 4:
– ComplexInput/Output
-
Note: the dimension,
dim, of the array
du
must be at least
.
On entry: must contain the superdiagonal elements of the matrix .
On exit: is overwritten by the elements of the first superdiagonal of .
- 5:
– ComplexOutput
-
On exit: contains the elements of the second superdiagonal of .
- 6:
– IntegerOutput
-
On exit: contains the pivot indices that define the permutation matrix . At the th step, row of the matrix was interchanged with row . will always be either or , indicating that a row interchange was not performed.
- 7:
– NagError *Input/Output
-
The NAG error argument (see
Section 3.7 in How to Use the NAG Library and its Documentation).
The computed factorization satisfies an equation of the form
where
and
is the
machine precision.
Following the use of this function,
nag_zgttrs (f07csc) can be used to solve systems of equations
or
or
, and
nag_zgtcon (f07cuc) can be used to estimate the condition number of
.
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 real analogue of this function is
nag_dgttrf (f07cdc).
This example factorizes the tridiagonal matrix
given by