LAPACK 3.12.0 LAPACK: Linear Algebra PACKage
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## ◆ 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```

Definition at line 108 of file spttrs.f.

109*
110* -- LAPACK computational routine --
111* -- LAPACK is a software package provided by Univ. of Tennessee, --
112* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
113*
114* .. Scalar Arguments ..
115 INTEGER INFO, LDB, N, NRHS
116* ..
117* .. Array Arguments ..
118 REAL B( LDB, * ), D( * ), E( * )
119* ..
120*
121* =====================================================================
122*
123* .. Local Scalars ..
124 INTEGER J, JB, NB
125* ..
126* .. External Functions ..
127 INTEGER ILAENV
128 EXTERNAL ilaenv
129* ..
130* .. External Subroutines ..
131 EXTERNAL sptts2, xerbla
132* ..
133* .. Intrinsic Functions ..
134 INTRINSIC max, min
135* ..
136* .. Executable Statements ..
137*
138* Test the input arguments.
139*
140 info = 0
141 IF( n.LT.0 ) THEN
142 info = -1
143 ELSE IF( nrhs.LT.0 ) THEN
144 info = -2
145 ELSE IF( ldb.LT.max( 1, n ) ) THEN
146 info = -6
147 END IF
148 IF( info.NE.0 ) THEN
149 CALL xerbla( 'SPTTRS', -info )
150 RETURN
151 END IF
152*
153* Quick return if possible
154*
155 IF( n.EQ.0 .OR. nrhs.EQ.0 )
156 \$ RETURN
157*
158* Determine the number of right-hand sides to solve at a time.
159*
160 IF( nrhs.EQ.1 ) THEN
161 nb = 1
162 ELSE
163 nb = max( 1, ilaenv( 1, 'SPTTRS', ' ', n, nrhs, -1, -1 ) )
164 END IF
165*
166 IF( nb.GE.nrhs ) THEN
167 CALL sptts2( n, nrhs, d, e, b, ldb )
168 ELSE
169 DO 10 j = 1, nrhs, nb
170 jb = min( nrhs-j+1, nb )
171 CALL sptts2( n, jb, d, e, b( 1, j ), ldb )
172 10 CONTINUE
173 END IF
174*
175 RETURN
176*
177* End of SPTTRS
178*
subroutine xerbla(srname, info)
Definition cblat2.f:3285
integer function ilaenv(ispec, name, opts, n1, n2, n3, n4)
ILAENV
Definition ilaenv.f:162
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:102
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