Contents
dsptrd - reduce a real symmetric matrix A stored in packed
form to symmetric tridiagonal form T by an orthogonal simi-
larity transformation
SUBROUTINE DSPTRD(UPLO, N, AP, D, E, TAU, INFO)
CHARACTER * 1 UPLO
INTEGER N, INFO
DOUBLE PRECISION AP(*), D(*), E(*), TAU(*)
SUBROUTINE DSPTRD_64(UPLO, N, AP, D, E, TAU, INFO)
CHARACTER * 1 UPLO
INTEGER*8 N, INFO
DOUBLE PRECISION AP(*), D(*), E(*), TAU(*)
F95 INTERFACE
SUBROUTINE SPTRD(UPLO, [N], AP, D, E, TAU, [INFO])
CHARACTER(LEN=1) :: UPLO
INTEGER :: N, INFO
REAL(8), DIMENSION(:) :: AP, D, E, TAU
SUBROUTINE SPTRD_64(UPLO, [N], AP, D, E, TAU, [INFO])
CHARACTER(LEN=1) :: UPLO
INTEGER(8) :: N, INFO
REAL(8), DIMENSION(:) :: AP, D, E, TAU
C INTERFACE
#include <sunperf.h>
void dsptrd(char uplo, int n, double *ap, double *d, double
*e, double *tau, int *info);
void dsptrd_64(char uplo, long n, double *ap, double *d,
double *e, double *tau, long *info);
dsptrd reduces a real symmetric matrix A stored in packed
form to symmetric tridiagonal form T by an orthogonal simi-
larity transformation: Q**T * A * Q = T.
UPLO (input)
= 'U': Upper triangle of A is stored;
= 'L': Lower triangle of A is stored.
N (input) The order of the matrix A. N >= 0.
AP (input)
Double precision array, dimension (N*(N+1)/2) On
entry, the upper or lower triangle of the sym-
metric matrix A, packed columnwise in a linear
array. The j-th column of A is stored in the
array AP as follows: if UPLO = 'U', AP(i + (j-
1)*j/2) = A(i,j) for 1<=i<=j; if UPLO = 'L', AP(i
+ (j-1)*(2*n-j)/2) = A(i,j) for j<=i<=n. On exit,
if UPLO = 'U', the diagonal and first superdiago-
nal of A are overwritten by the corresponding ele-
ments of the tridiagonal matrix T, and the ele-
ments above the first superdiagonal, with the
array TAU, represent the orthogonal matrix Q as a
product of elementary reflectors; if UPLO = 'L',
the diagonal and first subdiagonal of A are over-
written by the corresponding elements of the tri-
diagonal matrix T, and the elements below the
first subdiagonal, with the array TAU, represent
the orthogonal matrix Q as a product of elementary
reflectors. See Further Details.
D (output)
Double precision array, dimension (N) The diagonal
elements of the tridiagonal matrix T: D(i) =
A(i,i).
E (output)
Double precision array, dimension (N-1) The off-
diagonal elements of the tridiagonal matrix T:
E(i) = A(i,i+1) if UPLO = 'U', E(i) = A(i+1,i) if
UPLO = 'L'.
TAU (output)
Double precision array, dimension (N-1) The scalar
factors of the elementary reflectors (see Further
Details).
INFO (output)
= 0: successful exit
< 0: if INFO = -i, the i-th argument had an ille-
gal value
If UPLO = 'U', the matrix Q is represented as a product of
elementary reflectors
Q = H(n-1) . . . H(2) H(1).
Each H(i) has the form
H(i) = I - tau * v * v'
where tau is a real scalar, and v is a real vector with
v(i+1:n) = 0 and v(i) = 1; v(1:i-1) is stored on exit in AP,
overwriting A(1:i-1,i+1), and tau is stored in TAU(i).
If UPLO = 'L', the matrix Q is represented as a product of
elementary reflectors
Q = H(1) H(2) . . . H(n-1).
Each H(i) has the form
H(i) = I - tau * v * v'
where tau is a real scalar, and v is a real vector with
v(1:i) = 0 and v(i+1) = 1; v(i+2:n) is stored on exit in AP,
overwriting A(i+2:n,i), and tau is stored in TAU(i).