LAPACK  3.6.1 LAPACK: Linear Algebra PACKage
 subroutine slagsy ( integer N, integer K, real, dimension( * ) D, real, dimension( lda, * ) A, integer LDA, integer, dimension( 4 ) ISEED, real, dimension( * ) WORK, integer INFO )

SLAGSY

Purpose:
``` SLAGSY generates a real symmetric matrix A, by pre- and post-
multiplying a real diagonal matrix D with a random orthogonal matrix:
A = U*D*U'. The semi-bandwidth may then be reduced to k by additional
orthogonal transformations.```
Parameters
 [in] N ``` N is INTEGER The order of the matrix A. N >= 0.``` [in] K ``` K is INTEGER The number of nonzero subdiagonals within the band of A. 0 <= K <= N-1.``` [in] D ``` D is REAL array, dimension (N) The diagonal elements of the diagonal matrix D.``` [out] A ``` A is REAL array, dimension (LDA,N) The generated n by n symmetric matrix A (the full matrix is stored).``` [in] LDA ``` LDA is INTEGER The leading dimension of the array A. LDA >= N.``` [in,out] ISEED ``` ISEED is INTEGER array, dimension (4) On entry, the seed of the random number generator; the array elements must be between 0 and 4095, and ISEED(4) must be odd. On exit, the seed is updated.``` [out] WORK ` WORK is REAL array, dimension (2*N)` [out] INFO ``` INFO is INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value```
Date
November 2011

Definition at line 103 of file slagsy.f.

103 *
104 * -- LAPACK auxiliary routine (version 3.4.0) --
105 * -- LAPACK is a software package provided by Univ. of Tennessee, --
106 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
107 * November 2011
108 *
109 * .. Scalar Arguments ..
110  INTEGER info, k, lda, n
111 * ..
112 * .. Array Arguments ..
113  INTEGER iseed( 4 )
114  REAL a( lda, * ), d( * ), work( * )
115 * ..
116 *
117 * =====================================================================
118 *
119 * .. Parameters ..
120  REAL zero, one, half
121  parameter ( zero = 0.0e+0, one = 1.0e+0, half = 0.5e+0 )
122 * ..
123 * .. Local Scalars ..
124  INTEGER i, j
125  REAL alpha, tau, wa, wb, wn
126 * ..
127 * .. External Subroutines ..
128  EXTERNAL saxpy, sgemv, sger, slarnv, sscal, ssymv,
129  \$ ssyr2, xerbla
130 * ..
131 * .. External Functions ..
132  REAL sdot, snrm2
133  EXTERNAL sdot, snrm2
134 * ..
135 * .. Intrinsic Functions ..
136  INTRINSIC max, sign
137 * ..
138 * .. Executable Statements ..
139 *
140 * Test the input arguments
141 *
142  info = 0
143  IF( n.LT.0 ) THEN
144  info = -1
145  ELSE IF( k.LT.0 .OR. k.GT.n-1 ) THEN
146  info = -2
147  ELSE IF( lda.LT.max( 1, n ) ) THEN
148  info = -5
149  END IF
150  IF( info.LT.0 ) THEN
151  CALL xerbla( 'SLAGSY', -info )
152  RETURN
153  END IF
154 *
155 * initialize lower triangle of A to diagonal matrix
156 *
157  DO 20 j = 1, n
158  DO 10 i = j + 1, n
159  a( i, j ) = zero
160  10 CONTINUE
161  20 CONTINUE
162  DO 30 i = 1, n
163  a( i, i ) = d( i )
164  30 CONTINUE
165 *
166 * Generate lower triangle of symmetric matrix
167 *
168  DO 40 i = n - 1, 1, -1
169 *
170 * generate random reflection
171 *
172  CALL slarnv( 3, iseed, n-i+1, work )
173  wn = snrm2( n-i+1, work, 1 )
174  wa = sign( wn, work( 1 ) )
175  IF( wn.EQ.zero ) THEN
176  tau = zero
177  ELSE
178  wb = work( 1 ) + wa
179  CALL sscal( n-i, one / wb, work( 2 ), 1 )
180  work( 1 ) = one
181  tau = wb / wa
182  END IF
183 *
184 * apply random reflection to A(i:n,i:n) from the left
185 * and the right
186 *
187 * compute y := tau * A * u
188 *
189  CALL ssymv( 'Lower', n-i+1, tau, a( i, i ), lda, work, 1, zero,
190  \$ work( n+1 ), 1 )
191 *
192 * compute v := y - 1/2 * tau * ( y, u ) * u
193 *
194  alpha = -half*tau*sdot( n-i+1, work( n+1 ), 1, work, 1 )
195  CALL saxpy( n-i+1, alpha, work, 1, work( n+1 ), 1 )
196 *
197 * apply the transformation as a rank-2 update to A(i:n,i:n)
198 *
199  CALL ssyr2( 'Lower', n-i+1, -one, work, 1, work( n+1 ), 1,
200  \$ a( i, i ), lda )
201  40 CONTINUE
202 *
203 * Reduce number of subdiagonals to K
204 *
205  DO 60 i = 1, n - 1 - k
206 *
207 * generate reflection to annihilate A(k+i+1:n,i)
208 *
209  wn = snrm2( n-k-i+1, a( k+i, i ), 1 )
210  wa = sign( wn, a( k+i, i ) )
211  IF( wn.EQ.zero ) THEN
212  tau = zero
213  ELSE
214  wb = a( k+i, i ) + wa
215  CALL sscal( n-k-i, one / wb, a( k+i+1, i ), 1 )
216  a( k+i, i ) = one
217  tau = wb / wa
218  END IF
219 *
220 * apply reflection to A(k+i:n,i+1:k+i-1) from the left
221 *
222  CALL sgemv( 'Transpose', n-k-i+1, k-1, one, a( k+i, i+1 ), lda,
223  \$ a( k+i, i ), 1, zero, work, 1 )
224  CALL sger( n-k-i+1, k-1, -tau, a( k+i, i ), 1, work, 1,
225  \$ a( k+i, i+1 ), lda )
226 *
227 * apply reflection to A(k+i:n,k+i:n) from the left and the right
228 *
229 * compute y := tau * A * u
230 *
231  CALL ssymv( 'Lower', n-k-i+1, tau, a( k+i, k+i ), lda,
232  \$ a( k+i, i ), 1, zero, work, 1 )
233 *
234 * compute v := y - 1/2 * tau * ( y, u ) * u
235 *
236  alpha = -half*tau*sdot( n-k-i+1, work, 1, a( k+i, i ), 1 )
237  CALL saxpy( n-k-i+1, alpha, a( k+i, i ), 1, work, 1 )
238 *
239 * apply symmetric rank-2 update to A(k+i:n,k+i:n)
240 *
241  CALL ssyr2( 'Lower', n-k-i+1, -one, a( k+i, i ), 1, work, 1,
242  \$ a( k+i, k+i ), lda )
243 *
244  a( k+i, i ) = -wa
245  DO 50 j = k + i + 1, n
246  a( j, i ) = zero
247  50 CONTINUE
248  60 CONTINUE
249 *
250 * Store full symmetric matrix
251 *
252  DO 80 j = 1, n
253  DO 70 i = j + 1, n
254  a( j, i ) = a( i, j )
255  70 CONTINUE
256  80 CONTINUE
257  RETURN
258 *
259 * End of SLAGSY
260 *
subroutine ssyr2(UPLO, N, ALPHA, X, INCX, Y, INCY, A, LDA)
SSYR2
Definition: ssyr2.f:149
subroutine sger(M, N, ALPHA, X, INCX, Y, INCY, A, LDA)
SGER
Definition: sger.f:132
real function sdot(N, SX, INCX, SY, INCY)
SDOT
Definition: sdot.f:53
subroutine slarnv(IDIST, ISEED, N, X)
SLARNV returns a vector of random numbers from a uniform or normal distribution.
Definition: slarnv.f:99
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:62
subroutine sgemv(TRANS, M, N, ALPHA, A, LDA, X, INCX, BETA, Y, INCY)
SGEMV
Definition: sgemv.f:158
real function snrm2(N, X, INCX)
SNRM2
Definition: snrm2.f:56
subroutine saxpy(N, SA, SX, INCX, SY, INCY)
SAXPY
Definition: saxpy.f:54
subroutine sscal(N, SA, SX, INCX)
SSCAL
Definition: sscal.f:55
subroutine ssymv(UPLO, N, ALPHA, A, LDA, X, INCX, BETA, Y, INCY)
SSYMV
Definition: ssymv.f:154

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