ssptrd (l)  Linux Manuals
ssptrd: reduces a real symmetric matrix A stored in packed form to symmetric tridiagonal form T by an orthogonal similarity transformation
NAME
SSPTRD  reduces a real symmetric matrix A stored in packed form to symmetric tridiagonal form T by an orthogonal similarity transformationSYNOPSIS
 SUBROUTINE SSPTRD(
 UPLO, N, AP, D, E, TAU, INFO )
 CHARACTER UPLO
 INTEGER INFO, N
 REAL AP( * ), D( * ), E( * ), TAU( * )
PURPOSE
SSPTRD reduces a real symmetric matrix A stored in packed form to symmetric tridiagonal form T by an orthogonal similarity transformation: Q**T * A * Q = T.ARGUMENTS
 UPLO (input) CHARACTER*1

= aqUaq: Upper triangle of A is stored;
= aqLaq: Lower triangle of A is stored.  N (input) INTEGER
 The order of the matrix A. N >= 0.
 AP (input/output) REAL array, dimension (N*(N+1)/2)
 On entry, the upper or lower triangle of the symmetric matrix A, packed columnwise in a linear array. The jth column of A is stored in the array AP as follows: if UPLO = aqUaq, AP(i + (j1)*j/2) = A(i,j) for 1<=i<=j; if UPLO = aqLaq, AP(i + (j1)*(2*nj)/2) = A(i,j) for j<=i<=n. On exit, if UPLO = aqUaq, the diagonal and first superdiagonal of A are overwritten by the corresponding elements of the tridiagonal matrix T, and the elements above the first superdiagonal, with the array TAU, represent the orthogonal matrix Q as a product of elementary reflectors; if UPLO = aqLaq, the diagonal and first subdiagonal of A are over written by the corresponding elements of the tridiagonal 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) REAL array, dimension (N) The diagonal elements of the tridiagonal matrix T: D(i) = A(i,i).
 E (output) REAL array, dimension (N1)
 The offdiagonal elements of the tridiagonal matrix T: E(i) = A(i,i+1) if UPLO = aqUaq, E(i) = A(i+1,i) if UPLO = aqLaq.
 TAU (output) REAL array, dimension (N1)
 The scalar factors of the elementary reflectors (see Further Details).
 INFO (output) INTEGER

= 0: successful exit
< 0: if INFO = i, the ith argument had an illegal value
FURTHER DETAILS
If UPLO = aqUaq, the matrix Q is represented as a product of elementary reflectorsQ
Each H(i) has the form
H(i)
where tau is a real scalar, and v is a real vector with
v(i+1:n) = 0 and v(i) = 1; v(1:i1) is stored on exit in AP, overwriting A(1:i1,i+1), and tau is stored in TAU(i).
If UPLO = aqLaq, the matrix Q is represented as a product of elementary reflectors
Q
Each H(i) has the form
H(i)
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).