LAPACK  3.10.0
LAPACK: Linear Algebra PACKage

◆ ssytd2()

subroutine ssytd2 ( character  UPLO,
integer  N,
real, dimension( lda, * )  A,
integer  LDA,
real, dimension( * )  D,
real, dimension( * )  E,
real, dimension( * )  TAU,
integer  INFO 
)

SSYTD2 reduces a symmetric matrix to real symmetric tridiagonal form by an orthogonal similarity transformation (unblocked algorithm).

Download SSYTD2 + dependencies [TGZ] [ZIP] [TXT]

Purpose:
 SSYTD2 reduces a real symmetric matrix A to symmetric tridiagonal
 form T by an orthogonal similarity transformation: Q**T * A * Q = T.
Parameters
[in]UPLO
          UPLO is CHARACTER*1
          Specifies whether the upper or lower triangular part of the
          symmetric matrix A is stored:
          = 'U':  Upper triangular
          = 'L':  Lower triangular
[in]N
          N is INTEGER
          The order of the matrix A.  N >= 0.
[in,out]A
          A is REAL array, dimension (LDA,N)
          On entry, the symmetric matrix A.  If UPLO = 'U', the leading
          n-by-n upper triangular part of A contains the upper
          triangular part of the matrix A, and the strictly lower
          triangular part of A is not referenced.  If UPLO = 'L', the
          leading n-by-n lower triangular part of A contains the lower
          triangular part of the matrix A, and the strictly upper
          triangular part of A is not referenced.
          On exit, if UPLO = 'U', 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
          = 'L', 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.
[in]LDA
          LDA is INTEGER
          The leading dimension of the array A.  LDA >= max(1,N).
[out]D
          D is REAL array, dimension (N)
          The diagonal elements of the tridiagonal matrix T:
          D(i) = A(i,i).
[out]E
          E is REAL 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'.
[out]TAU
          TAU is REAL array, dimension (N-1)
          The scalar factors of the elementary reflectors (see Further
          Details).
[out]INFO
          INFO is INTEGER
          = 0:  successful exit
          < 0:  if INFO = -i, the i-th argument had an illegal value.
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Further Details:
  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**T

  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
  A(1:i-1,i+1), and tau 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**T

  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 A(i+2:n,i),
  and tau in TAU(i).

  The contents of A on exit are illustrated by the following examples
  with n = 5:

  if UPLO = 'U':                       if UPLO = 'L':

    (  d   e   v2  v3  v4 )              (  d                  )
    (      d   e   v3  v4 )              (  e   d              )
    (          d   e   v4 )              (  v1  e   d          )
    (              d   e  )              (  v1  v2  e   d      )
    (                  d  )              (  v1  v2  v3  e   d  )

  where d and e denote diagonal and off-diagonal elements of T, and vi
  denotes an element of the vector defining H(i).

Definition at line 172 of file ssytd2.f.

173 *
174 * -- LAPACK computational routine --
175 * -- LAPACK is a software package provided by Univ. of Tennessee, --
176 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
177 *
178 * .. Scalar Arguments ..
179  CHARACTER UPLO
180  INTEGER INFO, LDA, N
181 * ..
182 * .. Array Arguments ..
183  REAL A( LDA, * ), D( * ), E( * ), TAU( * )
184 * ..
185 *
186 * =====================================================================
187 *
188 * .. Parameters ..
189  REAL ONE, ZERO, HALF
190  parameter( one = 1.0, zero = 0.0, half = 1.0 / 2.0 )
191 * ..
192 * .. Local Scalars ..
193  LOGICAL UPPER
194  INTEGER I
195  REAL ALPHA, TAUI
196 * ..
197 * .. External Subroutines ..
198  EXTERNAL saxpy, slarfg, ssymv, ssyr2, xerbla
199 * ..
200 * .. External Functions ..
201  LOGICAL LSAME
202  REAL SDOT
203  EXTERNAL lsame, sdot
204 * ..
205 * .. Intrinsic Functions ..
206  INTRINSIC max, min
207 * ..
208 * .. Executable Statements ..
209 *
210 * Test the input parameters
211 *
212  info = 0
213  upper = lsame( uplo, 'U' )
214  IF( .NOT.upper .AND. .NOT.lsame( uplo, 'L' ) ) THEN
215  info = -1
216  ELSE IF( n.LT.0 ) THEN
217  info = -2
218  ELSE IF( lda.LT.max( 1, n ) ) THEN
219  info = -4
220  END IF
221  IF( info.NE.0 ) THEN
222  CALL xerbla( 'SSYTD2', -info )
223  RETURN
224  END IF
225 *
226 * Quick return if possible
227 *
228  IF( n.LE.0 )
229  $ RETURN
230 *
231  IF( upper ) THEN
232 *
233 * Reduce the upper triangle of A
234 *
235  DO 10 i = n - 1, 1, -1
236 *
237 * Generate elementary reflector H(i) = I - tau * v * v**T
238 * to annihilate A(1:i-1,i+1)
239 *
240  CALL slarfg( i, a( i, i+1 ), a( 1, i+1 ), 1, taui )
241  e( i ) = a( i, i+1 )
242 *
243  IF( taui.NE.zero ) THEN
244 *
245 * Apply H(i) from both sides to A(1:i,1:i)
246 *
247  a( i, i+1 ) = one
248 *
249 * Compute x := tau * A * v storing x in TAU(1:i)
250 *
251  CALL ssymv( uplo, i, taui, a, lda, a( 1, i+1 ), 1, zero,
252  $ tau, 1 )
253 *
254 * Compute w := x - 1/2 * tau * (x**T * v) * v
255 *
256  alpha = -half*taui*sdot( i, tau, 1, a( 1, i+1 ), 1 )
257  CALL saxpy( i, alpha, a( 1, i+1 ), 1, tau, 1 )
258 *
259 * Apply the transformation as a rank-2 update:
260 * A := A - v * w**T - w * v**T
261 *
262  CALL ssyr2( uplo, i, -one, a( 1, i+1 ), 1, tau, 1, a,
263  $ lda )
264 *
265  a( i, i+1 ) = e( i )
266  END IF
267  d( i+1 ) = a( i+1, i+1 )
268  tau( i ) = taui
269  10 CONTINUE
270  d( 1 ) = a( 1, 1 )
271  ELSE
272 *
273 * Reduce the lower triangle of A
274 *
275  DO 20 i = 1, n - 1
276 *
277 * Generate elementary reflector H(i) = I - tau * v * v**T
278 * to annihilate A(i+2:n,i)
279 *
280  CALL slarfg( n-i, a( i+1, i ), a( min( i+2, n ), i ), 1,
281  $ taui )
282  e( i ) = a( i+1, i )
283 *
284  IF( taui.NE.zero ) THEN
285 *
286 * Apply H(i) from both sides to A(i+1:n,i+1:n)
287 *
288  a( i+1, i ) = one
289 *
290 * Compute x := tau * A * v storing y in TAU(i:n-1)
291 *
292  CALL ssymv( uplo, n-i, taui, a( i+1, i+1 ), lda,
293  $ a( i+1, i ), 1, zero, tau( i ), 1 )
294 *
295 * Compute w := x - 1/2 * tau * (x**T * v) * v
296 *
297  alpha = -half*taui*sdot( n-i, tau( i ), 1, a( i+1, i ),
298  $ 1 )
299  CALL saxpy( n-i, alpha, a( i+1, i ), 1, tau( i ), 1 )
300 *
301 * Apply the transformation as a rank-2 update:
302 * A := A - v * w**T - w * v**T
303 *
304  CALL ssyr2( uplo, n-i, -one, a( i+1, i ), 1, tau( i ), 1,
305  $ a( i+1, i+1 ), lda )
306 *
307  a( i+1, i ) = e( i )
308  END IF
309  d( i ) = a( i, i )
310  tau( i ) = taui
311  20 CONTINUE
312  d( n ) = a( n, n )
313  END IF
314 *
315  RETURN
316 *
317 * End of SSYTD2
318 *
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
subroutine slarfg(N, ALPHA, X, INCX, TAU)
SLARFG generates an elementary reflector (Householder matrix).
Definition: slarfg.f:106
real function sdot(N, SX, INCX, SY, INCY)
SDOT
Definition: sdot.f:82
subroutine saxpy(N, SA, SX, INCX, SY, INCY)
SAXPY
Definition: saxpy.f:89
subroutine ssymv(UPLO, N, ALPHA, A, LDA, X, INCX, BETA, Y, INCY)
SSYMV
Definition: ssymv.f:152
subroutine ssyr2(UPLO, N, ALPHA, X, INCX, Y, INCY, A, LDA)
SSYR2
Definition: ssyr2.f:147
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