LAPACK 3.12.0 LAPACK: Linear Algebra PACKage
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## ◆ sgttrs()

 subroutine sgttrs ( character trans, integer n, integer nrhs, real, dimension( * ) dl, real, dimension( * ) d, real, dimension( * ) du, real, dimension( * ) du2, integer, dimension( * ) ipiv, real, dimension( ldb, * ) b, integer ldb, integer info )

SGTTRS

Purpose:
``` SGTTRS solves one of the systems of equations
A*X = B  or  A**T*X = B,
with a tridiagonal matrix A using the LU factorization computed
by SGTTRF.```
Parameters
 [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**T* X = B (Conjugate transpose = Transpose)``` [in] N ``` N is INTEGER The order of the matrix A.``` [in] NRHS ``` NRHS is INTEGER The number of right hand sides, i.e., the number of columns of the matrix B. NRHS >= 0.``` [in] DL ``` DL is REAL array, dimension (N-1) The (n-1) multipliers that define the matrix L from the LU factorization of A.``` [in] D ``` D is REAL array, dimension (N) The n diagonal elements of the upper triangular matrix U from the LU factorization of A.``` [in] DU ``` DU is REAL array, dimension (N-1) The (n-1) elements of the first super-diagonal of U.``` [in] DU2 ``` DU2 is REAL array, dimension (N-2) The (n-2) elements of the second super-diagonal of U.``` [in] IPIV ``` IPIV is INTEGER array, dimension (N) The pivot indices; for 1 <= i <= n, row i of the matrix was interchanged with row IPIV(i). IPIV(i) will always be either i or i+1; IPIV(i) = i indicates a row interchange was not required.``` [in,out] B ``` B is REAL array, dimension (LDB,NRHS) On entry, the matrix of right hand side vectors B. On exit, B is overwritten by the solution vectors 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```

Definition at line 136 of file sgttrs.f.

138*
139* -- LAPACK computational routine --
140* -- LAPACK is a software package provided by Univ. of Tennessee, --
141* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
142*
143* .. Scalar Arguments ..
144 CHARACTER TRANS
145 INTEGER INFO, LDB, N, NRHS
146* ..
147* .. Array Arguments ..
148 INTEGER IPIV( * )
149 REAL B( LDB, * ), D( * ), DL( * ), DU( * ), DU2( * )
150* ..
151*
152* =====================================================================
153*
154* .. Local Scalars ..
155 LOGICAL NOTRAN
156 INTEGER ITRANS, J, JB, NB
157* ..
158* .. External Functions ..
159 INTEGER ILAENV
160 EXTERNAL ilaenv
161* ..
162* .. External Subroutines ..
163 EXTERNAL sgtts2, xerbla
164* ..
165* .. Intrinsic Functions ..
166 INTRINSIC max, min
167* ..
168* .. Executable Statements ..
169*
170 info = 0
171 notran = ( trans.EQ.'N' .OR. trans.EQ.'n' )
172 IF( .NOT.notran .AND. .NOT.( trans.EQ.'T' .OR. trans.EQ.
173 \$ 't' ) .AND. .NOT.( trans.EQ.'C' .OR. trans.EQ.'c' ) ) THEN
174 info = -1
175 ELSE IF( n.LT.0 ) THEN
176 info = -2
177 ELSE IF( nrhs.LT.0 ) THEN
178 info = -3
179 ELSE IF( ldb.LT.max( n, 1 ) ) THEN
180 info = -10
181 END IF
182 IF( info.NE.0 ) THEN
183 CALL xerbla( 'SGTTRS', -info )
184 RETURN
185 END IF
186*
187* Quick return if possible
188*
189 IF( n.EQ.0 .OR. nrhs.EQ.0 )
190 \$ RETURN
191*
192* Decode TRANS
193*
194 IF( notran ) THEN
195 itrans = 0
196 ELSE
197 itrans = 1
198 END IF
199*
200* Determine the number of right-hand sides to solve at a time.
201*
202 IF( nrhs.EQ.1 ) THEN
203 nb = 1
204 ELSE
205 nb = max( 1, ilaenv( 1, 'SGTTRS', trans, n, nrhs, -1, -1 ) )
206 END IF
207*
208 IF( nb.GE.nrhs ) THEN
209 CALL sgtts2( itrans, n, nrhs, dl, d, du, du2, ipiv, b, ldb )
210 ELSE
211 DO 10 j = 1, nrhs, nb
212 jb = min( nrhs-j+1, nb )
213 CALL sgtts2( itrans, n, jb, dl, d, du, du2, ipiv, b( 1, j ),
214 \$ ldb )
215 10 CONTINUE
216 END IF
217*
218* End of SGTTRS
219*
subroutine xerbla(srname, info)
Definition cblat2.f:3285
subroutine sgtts2(itrans, n, nrhs, dl, d, du, du2, ipiv, b, ldb)
SGTTS2 solves a system of linear equations with a tridiagonal matrix using the LU factorization compu...
Definition sgtts2.f:128
integer function ilaenv(ispec, name, opts, n1, n2, n3, n4)
ILAENV
Definition ilaenv.f:162
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