*> \brief \b ZGBTF2 computes the LU factorization of a general band matrix using the unblocked version of the algorithm. * * =========== DOCUMENTATION =========== * * Online html documentation available at * http://www.netlib.org/lapack/explore-html/ * *> \htmlonly *> Download ZGBTF2 + dependencies *> *> [TGZ] *> *> [ZIP] *> *> [TXT] *> \endhtmlonly * * Definition: * =========== * * SUBROUTINE ZGBTF2( M, N, KL, KU, AB, LDAB, IPIV, INFO ) * * .. Scalar Arguments .. * INTEGER INFO, KL, KU, LDAB, M, N * .. * .. Array Arguments .. * INTEGER IPIV( * ) * COMPLEX*16 AB( LDAB, * ) * .. * * *> \par Purpose: * ============= *> *> \verbatim *> *> ZGBTF2 computes an LU factorization of a complex m-by-n band matrix *> A using partial pivoting with row interchanges. *> *> This is the unblocked version of the algorithm, calling Level 2 BLAS. *> \endverbatim * * Arguments: * ========== * *> \param[in] M *> \verbatim *> M is INTEGER *> The number of rows of the matrix A. M >= 0. *> \endverbatim *> *> \param[in] N *> \verbatim *> N is INTEGER *> The number of columns of the matrix A. N >= 0. *> \endverbatim *> *> \param[in] KL *> \verbatim *> KL is INTEGER *> The number of subdiagonals within the band of A. KL >= 0. *> \endverbatim *> *> \param[in] KU *> \verbatim *> KU is INTEGER *> The number of superdiagonals within the band of A. KU >= 0. *> \endverbatim *> *> \param[in,out] AB *> \verbatim *> AB is COMPLEX*16 array, dimension (LDAB,N) *> On entry, the matrix A in band storage, in rows KL+1 to *> 2*KL+KU+1; rows 1 to KL of the array need not be set. *> The j-th column of A is stored in the j-th column of the *> array AB as follows: *> AB(kl+ku+1+i-j,j) = A(i,j) for max(1,j-ku)<=i<=min(m,j+kl) *> *> On exit, details of the factorization: U is stored as an *> upper triangular band matrix with KL+KU superdiagonals in *> rows 1 to KL+KU+1, and the multipliers used during the *> factorization are stored in rows KL+KU+2 to 2*KL+KU+1. *> See below for further details. *> \endverbatim *> *> \param[in] LDAB *> \verbatim *> LDAB is INTEGER *> The leading dimension of the array AB. LDAB >= 2*KL+KU+1. *> \endverbatim *> *> \param[out] IPIV *> \verbatim *> IPIV is INTEGER array, dimension (min(M,N)) *> The pivot indices; for 1 <= i <= min(M,N), row i of the *> matrix was interchanged with row IPIV(i). *> \endverbatim *> *> \param[out] INFO *> \verbatim *> INFO is INTEGER *> = 0: successful exit *> < 0: if INFO = -i, the i-th argument had an illegal value *> > 0: if INFO = +i, U(i,i) is exactly zero. The factorization *> has been completed, but the factor U is exactly *> singular, and division by zero will occur if it is used *> to solve a system of equations. *> \endverbatim * * Authors: * ======== * *> \author Univ. of Tennessee *> \author Univ. of California Berkeley *> \author Univ. of Colorado Denver *> \author NAG Ltd. * *> \ingroup complex16GBcomputational * *> \par Further Details: * ===================== *> *> \verbatim *> *> The band storage scheme is illustrated by the following example, when *> M = N = 6, KL = 2, KU = 1: *> *> On entry: On exit: *> *> * * * + + + * * * u14 u25 u36 *> * * + + + + * * u13 u24 u35 u46 *> * a12 a23 a34 a45 a56 * u12 u23 u34 u45 u56 *> a11 a22 a33 a44 a55 a66 u11 u22 u33 u44 u55 u66 *> a21 a32 a43 a54 a65 * m21 m32 m43 m54 m65 * *> a31 a42 a53 a64 * * m31 m42 m53 m64 * * *> *> Array elements marked * are not used by the routine; elements marked *> + need not be set on entry, but are required by the routine to store *> elements of U, because of fill-in resulting from the row *> interchanges. *> \endverbatim *> * ===================================================================== SUBROUTINE ZGBTF2( M, N, KL, KU, AB, LDAB, IPIV, INFO ) * * -- LAPACK computational routine -- * -- LAPACK is a software package provided by Univ. of Tennessee, -- * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..-- * * .. Scalar Arguments .. INTEGER INFO, KL, KU, LDAB, M, N * .. * .. Array Arguments .. INTEGER IPIV( * ) COMPLEX*16 AB( LDAB, * ) * .. * * ===================================================================== * * .. Parameters .. COMPLEX*16 ONE, ZERO PARAMETER ( ONE = ( 1.0D+0, 0.0D+0 ), \$ ZERO = ( 0.0D+0, 0.0D+0 ) ) * .. * .. Local Scalars .. INTEGER I, J, JP, JU, KM, KV * .. * .. External Functions .. INTEGER IZAMAX EXTERNAL IZAMAX * .. * .. External Subroutines .. EXTERNAL XERBLA, ZGERU, ZSCAL, ZSWAP * .. * .. Intrinsic Functions .. INTRINSIC MAX, MIN * .. * .. Executable Statements .. * * KV is the number of superdiagonals in the factor U, allowing for * fill-in. * KV = KU + KL * * Test the input parameters. * INFO = 0 IF( M.LT.0 ) THEN INFO = -1 ELSE IF( N.LT.0 ) THEN INFO = -2 ELSE IF( KL.LT.0 ) THEN INFO = -3 ELSE IF( KU.LT.0 ) THEN INFO = -4 ELSE IF( LDAB.LT.KL+KV+1 ) THEN INFO = -6 END IF IF( INFO.NE.0 ) THEN CALL XERBLA( 'ZGBTF2', -INFO ) RETURN END IF * * Quick return if possible * IF( M.EQ.0 .OR. N.EQ.0 ) \$ RETURN * * Gaussian elimination with partial pivoting * * Set fill-in elements in columns KU+2 to KV to zero. * DO 20 J = KU + 2, MIN( KV, N ) DO 10 I = KV - J + 2, KL AB( I, J ) = ZERO 10 CONTINUE 20 CONTINUE * * JU is the index of the last column affected by the current stage * of the factorization. * JU = 1 * DO 40 J = 1, MIN( M, N ) * * Set fill-in elements in column J+KV to zero. * IF( J+KV.LE.N ) THEN DO 30 I = 1, KL AB( I, J+KV ) = ZERO 30 CONTINUE END IF * * Find pivot and test for singularity. KM is the number of * subdiagonal elements in the current column. * KM = MIN( KL, M-J ) JP = IZAMAX( KM+1, AB( KV+1, J ), 1 ) IPIV( J ) = JP + J - 1 IF( AB( KV+JP, J ).NE.ZERO ) THEN JU = MAX( JU, MIN( J+KU+JP-1, N ) ) * * Apply interchange to columns J to JU. * IF( JP.NE.1 ) \$ CALL ZSWAP( JU-J+1, AB( KV+JP, J ), LDAB-1, \$ AB( KV+1, J ), LDAB-1 ) IF( KM.GT.0 ) THEN * * Compute multipliers. * CALL ZSCAL( KM, ONE / AB( KV+1, J ), AB( KV+2, J ), 1 ) * * Update trailing submatrix within the band. * IF( JU.GT.J ) \$ CALL ZGERU( KM, JU-J, -ONE, AB( KV+2, J ), 1, \$ AB( KV, J+1 ), LDAB-1, AB( KV+1, J+1 ), \$ LDAB-1 ) END IF ELSE * * If pivot is zero, set INFO to the index of the pivot * unless a zero pivot has already been found. * IF( INFO.EQ.0 ) \$ INFO = J END IF 40 CONTINUE RETURN * * End of ZGBTF2 * END