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SUBROUTINE SPPTRS( UPLO, N, NRHS, AP, B, LDB, INFO )
*
* -- LAPACK routine (version 3.0) --
* Univ. of Tennessee, Univ. of California Berkeley, NAG Ltd.,
* Courant Institute, Argonne National Lab, and Rice University
* March 31, 1993
*
* .. Scalar Arguments ..
CHARACTER UPLO
INTEGER INFO, LDB, N, NRHS
* ..
* .. Array Arguments ..
REAL AP( * ), B( LDB, * )
* ..
*
* Purpose
* =======
*
* SPPTRS solves a system of linear equations A*X = B with a symmetric
* positive definite matrix A in packed storage using the Cholesky
* factorization A = U**T*U or A = L*L**T computed by SPPTRF.
*
* Arguments
* =========
*
* UPLO (input) CHARACTER*1
* = 'U': Upper triangle of A is stored;
* = 'L': Lower triangle of A is stored.
*
* N (input) INTEGER
* The order of the matrix A. N >= 0.
*
* NRHS (input) INTEGER
* The number of right hand sides, i.e., the number of columns
* of the matrix B. NRHS >= 0.
*
* AP (input) REAL array, dimension (N*(N+1)/2)
* The triangular factor U or L from the Cholesky factorization
* A = U**T*U or A = L*L**T, packed columnwise in a linear
* array. The j-th column of U or L is stored in the array AP
* as follows:
* if UPLO = 'U', AP(i + (j-1)*j/2) = U(i,j) for 1<=i<=j;
* if UPLO = 'L', AP(i + (j-1)*(2n-j)/2) = L(i,j) for j<=i<=n.
*
* B (input/output) REAL array, dimension (LDB,NRHS)
* On entry, the right hand side matrix B.
* On exit, the solution matrix X.
*
* LDB (input) INTEGER
* The leading dimension of the array B. LDB >= max(1,N).
*
* INFO (output) INTEGER
* = 0: successful exit
* < 0: if INFO = -i, the i-th argument had an illegal value
*
* =====================================================================
*
* .. Local Scalars ..
LOGICAL UPPER
INTEGER I
* ..
* .. External Functions ..
LOGICAL LSAME
EXTERNAL LSAME
* ..
* .. External Subroutines ..
EXTERNAL STPSV, XERBLA
* ..
* .. Intrinsic Functions ..
INTRINSIC MAX
* ..
* .. Executable Statements ..
*
* Test the input parameters.
*
INFO = 0
UPPER = LSAME( UPLO, 'U' )
IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
INFO = -1
ELSE IF( N.LT.0 ) THEN
INFO = -2
ELSE IF( NRHS.LT.0 ) THEN
INFO = -3
ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
INFO = -6
END IF
IF( INFO.NE.0 ) THEN
CALL XERBLA( 'SPPTRS', -INFO )
RETURN
END IF
*
* Quick return if possible
*
IF( N.EQ.0 .OR. NRHS.EQ.0 )
$ RETURN
*
IF( UPPER ) THEN
*
* Solve A*X = B where A = U'*U.
*
DO 10 I = 1, NRHS
*
* Solve U'*X = B, overwriting B with X.
*
CALL STPSV( 'Upper', 'Transpose', 'Non-unit', N, AP,
$ B( 1, I ), 1 )
*
* Solve U*X = B, overwriting B with X.
*
CALL STPSV( 'Upper', 'No transpose', 'Non-unit', N, AP,
$ B( 1, I ), 1 )
10 CONTINUE
ELSE
*
* Solve A*X = B where A = L*L'.
*
DO 20 I = 1, NRHS
*
* Solve L*Y = B, overwriting B with X.
*
CALL STPSV( 'Lower', 'No transpose', 'Non-unit', N, AP,
$ B( 1, I ), 1 )
*
* Solve L'*X = Y, overwriting B with X.
*
CALL STPSV( 'Lower', 'Transpose', 'Non-unit', N, AP,
$ B( 1, I ), 1 )
20 CONTINUE
END IF
*
RETURN
*
* End of SPPTRS
*
END
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