SUBROUTINE SSYTRF( UPLO, N, A, LDA, IPIV, WORK, LWORK, INFO )
*
* -- LAPACK routine (version 3.1) --
* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
* November 2006
*
* .. Scalar Arguments ..
CHARACTER UPLO
INTEGER INFO, LDA, LWORK, N
* ..
* .. Array Arguments ..
INTEGER IPIV( * )
REAL A( LDA, * ), WORK( * )
* ..
*
* Purpose
* =======
*
* SSYTRF computes the factorization of a real symmetric matrix A using
* the Bunch-Kaufman diagonal pivoting method. The form of the
* factorization is
*
* A = U*D*U**T or A = L*D*L**T
*
* where U (or L) is a product of permutation and unit upper (lower)
* triangular matrices, and D is symmetric and block diagonal with
* 1-by-1 and 2-by-2 diagonal blocks.
*
* This is the blocked version of the algorithm, calling Level 3 BLAS.
*
* 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.
*
* A (input/output) REAL array, dimension (LDA,N)
* On entry, the symmetric matrix A. If UPLO = 'U', the leading
* N-by-N upper triangular part of A contains the upper
* triangular part of the matrix A, and the strictly lower
* triangular part of A is not referenced. If UPLO = 'L', the
* leading N-by-N lower triangular part of A contains the lower
* triangular part of the matrix A, and the strictly upper
* triangular part of A is not referenced.
*
* On exit, the block diagonal matrix D and the multipliers used
* to obtain the factor U or L (see below for further details).
*
* LDA (input) INTEGER
* The leading dimension of the array A. LDA >= max(1,N).
*
* IPIV (output) INTEGER array, dimension (N)
* Details of the interchanges and the block structure of D.
* If IPIV(k) > 0, then rows and columns k and IPIV(k) were
* interchanged and D(k,k) is a 1-by-1 diagonal block.
* If UPLO = 'U' and IPIV(k) = IPIV(k-1) < 0, then rows and
* columns k-1 and -IPIV(k) were interchanged and D(k-1:k,k-1:k)
* is a 2-by-2 diagonal block. If UPLO = 'L' and IPIV(k) =
* IPIV(k+1) < 0, then rows and columns k+1 and -IPIV(k) were
* interchanged and D(k:k+1,k:k+1) is a 2-by-2 diagonal block.
*
* WORK (workspace/output) REAL array, dimension (MAX(1,LWORK))
* On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
*
* LWORK (input) INTEGER
* The length of WORK. LWORK >=1. For best performance
* LWORK >= N*NB, where NB is the block size returned by ILAENV.
*
* If LWORK = -1, then a workspace query is assumed; the routine
* only calculates the optimal size of the WORK array, returns
* this value as the first entry of the WORK array, and no error
* message related to LWORK is issued by XERBLA.
*
* INFO (output) INTEGER
* = 0: successful exit
* < 0: if INFO = -i, the i-th argument had an illegal value
* > 0: if INFO = i, D(i,i) is exactly zero. The factorization
* has been completed, but the block diagonal matrix D is
* exactly singular, and division by zero will occur if it
* is used to solve a system of equations.
*
* Further Details
* ===============
*
* If UPLO = 'U', then A = U*D*U', where
* U = P(n)*U(n)* ... *P(k)U(k)* ...,
* i.e., U is a product of terms P(k)*U(k), where k decreases from n to
* 1 in steps of 1 or 2, and D is a block diagonal matrix with 1-by-1
* and 2-by-2 diagonal blocks D(k). P(k) is a permutation matrix as
* defined by IPIV(k), and U(k) is a unit upper triangular matrix, such
* that if the diagonal block D(k) is of order s (s = 1 or 2), then
*
* ( I v 0 ) k-s
* U(k) = ( 0 I 0 ) s
* ( 0 0 I ) n-k
* k-s s n-k
*
* If s = 1, D(k) overwrites A(k,k), and v overwrites A(1:k-1,k).
* If s = 2, the upper triangle of D(k) overwrites A(k-1,k-1), A(k-1,k),
* and A(k,k), and v overwrites A(1:k-2,k-1:k).
*
* If UPLO = 'L', then A = L*D*L', where
* L = P(1)*L(1)* ... *P(k)*L(k)* ...,
* i.e., L is a product of terms P(k)*L(k), where k increases from 1 to
* n in steps of 1 or 2, and D is a block diagonal matrix with 1-by-1
* and 2-by-2 diagonal blocks D(k). P(k) is a permutation matrix as
* defined by IPIV(k), and L(k) is a unit lower triangular matrix, such
* that if the diagonal block D(k) is of order s (s = 1 or 2), then
*
* ( I 0 0 ) k-1
* L(k) = ( 0 I 0 ) s
* ( 0 v I ) n-k-s+1
* k-1 s n-k-s+1
*
* If s = 1, D(k) overwrites A(k,k), and v overwrites A(k+1:n,k).
* If s = 2, the lower triangle of D(k) overwrites A(k,k), A(k+1,k),
* and A(k+1,k+1), and v overwrites A(k+2:n,k:k+1).
*
* =====================================================================
*
* .. Local Scalars ..
LOGICAL LQUERY, UPPER
INTEGER IINFO, IWS, J, K, KB, LDWORK, LWKOPT, NB, NBMIN
* ..
* .. External Functions ..
LOGICAL LSAME
INTEGER ILAENV
EXTERNAL LSAME, ILAENV
* ..
* .. External Subroutines ..
EXTERNAL SLASYF, SSYTF2, XERBLA
* ..
* .. Intrinsic Functions ..
INTRINSIC MAX
* ..
* .. Executable Statements ..
*
* Test the input parameters.
*
INFO = 0
UPPER = LSAME( UPLO, 'U' )
LQUERY = ( LWORK.EQ.-1 )
IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
INFO = -1
ELSE IF( N.LT.0 ) THEN
INFO = -2
ELSE IF( LDA.LT.MAX( 1, N ) ) THEN
INFO = -4
ELSE IF( LWORK.LT.1 .AND. .NOT.LQUERY ) THEN
INFO = -7
END IF
*
IF( INFO.EQ.0 ) THEN
*
* Determine the block size
*
NB = ILAENV( 1, 'SSYTRF', UPLO, N, -1, -1, -1 )
LWKOPT = N*NB
WORK( 1 ) = LWKOPT
END IF
*
IF( INFO.NE.0 ) THEN
CALL XERBLA( 'SSYTRF', -INFO )
RETURN
ELSE IF( LQUERY ) THEN
RETURN
END IF
*
NBMIN = 2
LDWORK = N
IF( NB.GT.1 .AND. NB.LT.N ) THEN
IWS = LDWORK*NB
IF( LWORK.LT.IWS ) THEN
NB = MAX( LWORK / LDWORK, 1 )
NBMIN = MAX( 2, ILAENV( 2, 'SSYTRF', UPLO, N, -1, -1, -1 ) )
END IF
ELSE
IWS = 1
END IF
IF( NB.LT.NBMIN )
$ NB = N
*
IF( UPPER ) THEN
*
* Factorize A as U*D*U' using the upper triangle of A
*
* K is the main loop index, decreasing from N to 1 in steps of
* KB, where KB is the number of columns factorized by SLASYF;
* KB is either NB or NB-1, or K for the last block
*
K = N
10 CONTINUE
*
* If K < 1, exit from loop
*
IF( K.LT.1 )
$ GO TO 40
*
IF( K.GT.NB ) THEN
*
* Factorize columns k-kb+1:k of A and use blocked code to
* update columns 1:k-kb
*
CALL SLASYF( UPLO, K, NB, KB, A, LDA, IPIV, WORK, LDWORK,
$ IINFO )
ELSE
*
* Use unblocked code to factorize columns 1:k of A
*
CALL SSYTF2( UPLO, K, A, LDA, IPIV, IINFO )
KB = K
END IF
*
* Set INFO on the first occurrence of a zero pivot
*
IF( INFO.EQ.0 .AND. IINFO.GT.0 )
$ INFO = IINFO
*
* Decrease K and return to the start of the main loop
*
K = K - KB
GO TO 10
*
ELSE
*
* Factorize A as L*D*L' using the lower triangle of A
*
* K is the main loop index, increasing from 1 to N in steps of
* KB, where KB is the number of columns factorized by SLASYF;
* KB is either NB or NB-1, or N-K+1 for the last block
*
K = 1
20 CONTINUE
*
* If K > N, exit from loop
*
IF( K.GT.N )
$ GO TO 40
*
IF( K.LE.N-NB ) THEN
*
* Factorize columns k:k+kb-1 of A and use blocked code to
* update columns k+kb:n
*
CALL SLASYF( UPLO, N-K+1, NB, KB, A( K, K ), LDA, IPIV( K ),
$ WORK, LDWORK, IINFO )
ELSE
*
* Use unblocked code to factorize columns k:n of A
*
CALL SSYTF2( UPLO, N-K+1, A( K, K ), LDA, IPIV( K ), IINFO )
KB = N - K + 1
END IF
*
* Set INFO on the first occurrence of a zero pivot
*
IF( INFO.EQ.0 .AND. IINFO.GT.0 )
$ INFO = IINFO + K - 1
*
* Adjust IPIV
*
DO 30 J = K, K + KB - 1
IF( IPIV( J ).GT.0 ) THEN
IPIV( J ) = IPIV( J ) + K - 1
ELSE
IPIV( J ) = IPIV( J ) - K + 1
END IF
30 CONTINUE
*
* Increase K and return to the start of the main loop
*
K = K + KB
GO TO 20
*
END IF
*
40 CONTINUE
WORK( 1 ) = LWKOPT
RETURN
*
* End of SSYTRF
*
END