 LAPACK 3.11.0 LAPACK: Linear Algebra PACKage
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## ◆ strtrs()

 subroutine strtrs ( character UPLO, character TRANS, character DIAG, integer N, integer NRHS, real, dimension( lda, * ) A, integer LDA, real, dimension( ldb, * ) B, integer LDB, integer INFO )

STRTRS

Purpose:
``` STRTRS solves a triangular system of the form

A * X = B  or  A**T * X = B,

where A is a triangular matrix of order N, and B is an N-by-NRHS
matrix.  A check is made to verify that A is nonsingular.```
Parameters
 [in] UPLO ``` UPLO is CHARACTER*1 = 'U': A is upper triangular; = 'L': A is lower triangular.``` [in] TRANS ``` TRANS is CHARACTER*1 Specifies the form of the system of equations: = 'N': A * X = B (No transpose) = 'T': A**T * X = B (Transpose) = 'C': A**H * X = B (Conjugate transpose = Transpose)``` [in] DIAG ``` DIAG is CHARACTER*1 = 'N': A is non-unit triangular; = 'U': A is unit triangular.``` [in] N ``` N is INTEGER The order of the matrix A. N >= 0.``` [in] NRHS ``` NRHS is INTEGER The number of right hand sides, i.e., the number of columns of the matrix B. NRHS >= 0.``` [in] A ``` A is REAL array, dimension (LDA,N) The triangular matrix A. If UPLO = 'U', the leading N-by-N upper triangular part of the array A contains the upper triangular matrix, and the strictly lower triangular part of A is not referenced. If UPLO = 'L', the leading N-by-N lower triangular part of the array A contains the lower triangular matrix, and the strictly upper triangular part of A is not referenced. If DIAG = 'U', the diagonal elements of A are also not referenced and are assumed to be 1.``` [in] LDA ``` LDA is INTEGER The leading dimension of the array A. LDA >= max(1,N).``` [in,out] B ``` B is REAL array, dimension (LDB,NRHS) On entry, the right hand side matrix B. On exit, if INFO = 0, the solution matrix X.``` [in] LDB ``` LDB is INTEGER The leading dimension of the array B. LDB >= max(1,N).``` [out] INFO ``` INFO is INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value > 0: if INFO = i, the i-th diagonal element of A is zero, indicating that the matrix is singular and the solutions X have not been computed.```

Definition at line 138 of file strtrs.f.

140*
141* -- LAPACK computational routine --
142* -- LAPACK is a software package provided by Univ. of Tennessee, --
143* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
144*
145* .. Scalar Arguments ..
146 CHARACTER DIAG, TRANS, UPLO
147 INTEGER INFO, LDA, LDB, N, NRHS
148* ..
149* .. Array Arguments ..
150 REAL A( LDA, * ), B( LDB, * )
151* ..
152*
153* =====================================================================
154*
155* .. Parameters ..
156 REAL ZERO, ONE
157 parameter( zero = 0.0e+0, one = 1.0e+0 )
158* ..
159* .. Local Scalars ..
160 LOGICAL NOUNIT
161* ..
162* .. External Functions ..
163 LOGICAL LSAME
164 EXTERNAL lsame
165* ..
166* .. External Subroutines ..
167 EXTERNAL strsm, xerbla
168* ..
169* .. Intrinsic Functions ..
170 INTRINSIC max
171* ..
172* .. Executable Statements ..
173*
174* Test the input parameters.
175*
176 info = 0
177 nounit = lsame( diag, 'N' )
178 IF( .NOT.lsame( uplo, 'U' ) .AND. .NOT.lsame( uplo, 'L' ) ) THEN
179 info = -1
180 ELSE IF( .NOT.lsame( trans, 'N' ) .AND. .NOT.
181 \$ lsame( trans, 'T' ) .AND. .NOT.lsame( trans, 'C' ) ) THEN
182 info = -2
183 ELSE IF( .NOT.nounit .AND. .NOT.lsame( diag, 'U' ) ) THEN
184 info = -3
185 ELSE IF( n.LT.0 ) THEN
186 info = -4
187 ELSE IF( nrhs.LT.0 ) THEN
188 info = -5
189 ELSE IF( lda.LT.max( 1, n ) ) THEN
190 info = -7
191 ELSE IF( ldb.LT.max( 1, n ) ) THEN
192 info = -9
193 END IF
194 IF( info.NE.0 ) THEN
195 CALL xerbla( 'STRTRS', -info )
196 RETURN
197 END IF
198*
199* Quick return if possible
200*
201 IF( n.EQ.0 )
202 \$ RETURN
203*
204* Check for singularity.
205*
206 IF( nounit ) THEN
207 DO 10 info = 1, n
208 IF( a( info, info ).EQ.zero )
209 \$ RETURN
210 10 CONTINUE
211 END IF
212 info = 0
213*
214* Solve A * x = b or A**T * x = b.
215*
216 CALL strsm( 'Left', uplo, trans, diag, n, nrhs, one, a, lda, b,
217 \$ ldb )
218*
219 RETURN
220*
221* End of STRTRS
222*
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
subroutine strsm(SIDE, UPLO, TRANSA, DIAG, M, N, ALPHA, A, LDA, B, LDB)
STRSM
Definition: strsm.f:181
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