LAPACK  3.8.0 LAPACK: Linear Algebra PACKage

## ◆ spttrs()

 subroutine spttrs ( integer N, integer NRHS, real, dimension( * ) D, real, dimension( * ) E, real, dimension( ldb, * ) B, integer LDB, integer INFO )

SPTTRS

Purpose:
``` SPTTRS solves a tridiagonal system of the form
A * X = B
using the L*D*L**T factorization of A computed by SPTTRF.  D is a
diagonal matrix specified in the vector D, L is a unit bidiagonal
matrix whose subdiagonal is specified in the vector E, and X and B
are N by NRHS matrices.```
Parameters
 [in] N ``` N is INTEGER The order of the tridiagonal 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] D ``` D is REAL array, dimension (N) The n diagonal elements of the diagonal matrix D from the L*D*L**T factorization of A.``` [in] E ``` E is REAL array, dimension (N-1) The (n-1) subdiagonal elements of the unit bidiagonal factor L from the L*D*L**T factorization of A. E can also be regarded as the superdiagonal of the unit bidiagonal factor U from the factorization A = U**T*D*U.``` [in,out] B ``` B is REAL array, dimension (LDB,NRHS) On entry, the right hand side vectors B for the system of linear equations. On exit, 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 = -k, the k-th argument had an illegal value```
Date
December 2016

Definition at line 111 of file spttrs.f.

111 *
112 * -- LAPACK computational routine (version 3.7.0) --
113 * -- LAPACK is a software package provided by Univ. of Tennessee, --
114 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
115 * December 2016
116 *
117 * .. Scalar Arguments ..
118  INTEGER info, ldb, n, nrhs
119 * ..
120 * .. Array Arguments ..
121  REAL b( ldb, * ), d( * ), e( * )
122 * ..
123 *
124 * =====================================================================
125 *
126 * .. Local Scalars ..
127  INTEGER j, jb, nb
128 * ..
129 * .. External Functions ..
130  INTEGER ilaenv
131  EXTERNAL ilaenv
132 * ..
133 * .. External Subroutines ..
134  EXTERNAL sptts2, xerbla
135 * ..
136 * .. Intrinsic Functions ..
137  INTRINSIC max, min
138 * ..
139 * .. Executable Statements ..
140 *
141 * Test the input arguments.
142 *
143  info = 0
144  IF( n.LT.0 ) THEN
145  info = -1
146  ELSE IF( nrhs.LT.0 ) THEN
147  info = -2
148  ELSE IF( ldb.LT.max( 1, n ) ) THEN
149  info = -6
150  END IF
151  IF( info.NE.0 ) THEN
152  CALL xerbla( 'SPTTRS', -info )
153  RETURN
154  END IF
155 *
156 * Quick return if possible
157 *
158  IF( n.EQ.0 .OR. nrhs.EQ.0 )
159  \$ RETURN
160 *
161 * Determine the number of right-hand sides to solve at a time.
162 *
163  IF( nrhs.EQ.1 ) THEN
164  nb = 1
165  ELSE
166  nb = max( 1, ilaenv( 1, 'SPTTRS', ' ', n, nrhs, -1, -1 ) )
167  END IF
168 *
169  IF( nb.GE.nrhs ) THEN
170  CALL sptts2( n, nrhs, d, e, b, ldb )
171  ELSE
172  DO 10 j = 1, nrhs, nb
173  jb = min( nrhs-j+1, nb )
174  CALL sptts2( n, jb, d, e, b( 1, j ), ldb )
175  10 CONTINUE
176  END IF
177 *
178  RETURN
179 *
180 * End of SPTTRS
181 *
subroutine sptts2(N, NRHS, D, E, B, LDB)
SPTTS2 solves a tridiagonal system of the form AX=B using the L D LH factorization computed by spttrf...
Definition: sptts2.f:104
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:62
integer function ilaenv(ISPEC, NAME, OPTS, N1, N2, N3, N4)
ILAENV
Definition: tstiee.f:83
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