SUBROUTINE ZLATBS( UPLO, TRANS, DIAG, NORMIN, N, KD, AB, LDAB, X, $ SCALE, CNORM, INFO ) * * -- LAPACK auxiliary routine (version 3.1) -- * Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd.. * November 2006 * * .. Scalar Arguments .. CHARACTER DIAG, NORMIN, TRANS, UPLO INTEGER INFO, KD, LDAB, N DOUBLE PRECISION SCALE * .. * .. Array Arguments .. DOUBLE PRECISION CNORM( * ) COMPLEX*16 AB( LDAB, * ), X( * ) * .. * * Purpose * ======= * * ZLATBS solves one of the triangular systems * * A * x = s*b, A**T * x = s*b, or A**H * x = s*b, * * with scaling to prevent overflow, where A is an upper or lower * triangular band matrix. Here A' denotes the transpose of A, x and b * are n-element vectors, and s is a scaling factor, usually less than * or equal to 1, chosen so that the components of x will be less than * the overflow threshold. If the unscaled problem will not cause * overflow, the Level 2 BLAS routine ZTBSV is called. If the matrix A * is singular (A(j,j) = 0 for some j), then s is set to 0 and a * non-trivial solution to A*x = 0 is returned. * * Arguments * ========= * * UPLO (input) CHARACTER*1 * Specifies whether the matrix A is upper or lower triangular. * = 'U': Upper triangular * = 'L': Lower triangular * * TRANS (input) CHARACTER*1 * Specifies the operation applied to A. * = 'N': Solve A * x = s*b (No transpose) * = 'T': Solve A**T * x = s*b (Transpose) * = 'C': Solve A**H * x = s*b (Conjugate transpose) * * DIAG (input) CHARACTER*1 * Specifies whether or not the matrix A is unit triangular. * = 'N': Non-unit triangular * = 'U': Unit triangular * * NORMIN (input) CHARACTER*1 * Specifies whether CNORM has been set or not. * = 'Y': CNORM contains the column norms on entry * = 'N': CNORM is not set on entry. On exit, the norms will * be computed and stored in CNORM. * * N (input) INTEGER * The order of the matrix A. N >= 0. * * KD (input) INTEGER * The number of subdiagonals or superdiagonals in the * triangular matrix A. KD >= 0. * * AB (input) COMPLEX*16 array, dimension (LDAB,N) * The upper or lower triangular band matrix A, stored in the * first KD+1 rows of the array. The j-th column of A is stored * in the j-th column of the array AB as follows: * if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,j-kd)<=i<=j; * if UPLO = 'L', AB(1+i-j,j) = A(i,j) for j<=i<=min(n,j+kd). * * LDAB (input) INTEGER * The leading dimension of the array AB. LDAB >= KD+1. * * X (input/output) COMPLEX*16 array, dimension (N) * On entry, the right hand side b of the triangular system. * On exit, X is overwritten by the solution vector x. * * SCALE (output) DOUBLE PRECISION * The scaling factor s for the triangular system * A * x = s*b, A**T * x = s*b, or A**H * x = s*b. * If SCALE = 0, the matrix A is singular or badly scaled, and * the vector x is an exact or approximate solution to A*x = 0. * * CNORM (input or output) DOUBLE PRECISION array, dimension (N) * * If NORMIN = 'Y', CNORM is an input argument and CNORM(j) * contains the norm of the off-diagonal part of the j-th column * of A. If TRANS = 'N', CNORM(j) must be greater than or equal * to the infinity-norm, and if TRANS = 'T' or 'C', CNORM(j) * must be greater than or equal to the 1-norm. * * If NORMIN = 'N', CNORM is an output argument and CNORM(j) * returns the 1-norm of the offdiagonal part of the j-th column * of A. * * INFO (output) INTEGER * = 0: successful exit * < 0: if INFO = -k, the k-th argument had an illegal value * * Further Details * ======= ======= * * A rough bound on x is computed; if that is less than overflow, ZTBSV * is called, otherwise, specific code is used which checks for possible * overflow or divide-by-zero at every operation. * * A columnwise scheme is used for solving A*x = b. The basic algorithm * if A is lower triangular is * * x[1:n] := b[1:n] * for j = 1, ..., n * x(j) := x(j) / A(j,j) * x[j+1:n] := x[j+1:n] - x(j) * A[j+1:n,j] * end * * Define bounds on the components of x after j iterations of the loop: * M(j) = bound on x[1:j] * G(j) = bound on x[j+1:n] * Initially, let M(0) = 0 and G(0) = max{x(i), i=1,...,n}. * * Then for iteration j+1 we have * M(j+1) <= G(j) / | A(j+1,j+1) | * G(j+1) <= G(j) + M(j+1) * | A[j+2:n,j+1] | * <= G(j) ( 1 + CNORM(j+1) / | A(j+1,j+1) | ) * * where CNORM(j+1) is greater than or equal to the infinity-norm of * column j+1 of A, not counting the diagonal. Hence * * G(j) <= G(0) product ( 1 + CNORM(i) / | A(i,i) | ) * 1<=i<=j * and * * |x(j)| <= ( G(0) / |A(j,j)| ) product ( 1 + CNORM(i) / |A(i,i)| ) * 1<=i< j * * Since |x(j)| <= M(j), we use the Level 2 BLAS routine ZTBSV if the * reciprocal of the largest M(j), j=1,..,n, is larger than * max(underflow, 1/overflow). * * The bound on x(j) is also used to determine when a step in the * columnwise method can be performed without fear of overflow. If * the computed bound is greater than a large constant, x is scaled to * prevent overflow, but if the bound overflows, x is set to 0, x(j) to * 1, and scale to 0, and a non-trivial solution to A*x = 0 is found. * * Similarly, a row-wise scheme is used to solve A**T *x = b or * A**H *x = b. The basic algorithm for A upper triangular is * * for j = 1, ..., n * x(j) := ( b(j) - A[1:j-1,j]' * x[1:j-1] ) / A(j,j) * end * * We simultaneously compute two bounds * G(j) = bound on ( b(i) - A[1:i-1,i]' * x[1:i-1] ), 1<=i<=j * M(j) = bound on x(i), 1<=i<=j * * The initial values are G(0) = 0, M(0) = max{b(i), i=1,..,n}, and we * add the constraint G(j) >= G(j-1) and M(j) >= M(j-1) for j >= 1. * Then the bound on x(j) is * * M(j) <= M(j-1) * ( 1 + CNORM(j) ) / | A(j,j) | * * <= M(0) * product ( ( 1 + CNORM(i) ) / |A(i,i)| ) * 1<=i<=j * * and we can safely call ZTBSV if 1/M(n) and 1/G(n) are both greater * than max(underflow, 1/overflow). * * ===================================================================== * * .. Parameters .. DOUBLE PRECISION ZERO, HALF, ONE, TWO PARAMETER ( ZERO = 0.0D+0, HALF = 0.5D+0, ONE = 1.0D+0, $ TWO = 2.0D+0 ) * .. * .. Local Scalars .. LOGICAL NOTRAN, NOUNIT, UPPER INTEGER I, IMAX, J, JFIRST, JINC, JLAST, JLEN, MAIND DOUBLE PRECISION BIGNUM, GROW, REC, SMLNUM, TJJ, TMAX, TSCAL, $ XBND, XJ, XMAX COMPLEX*16 CSUMJ, TJJS, USCAL, ZDUM * .. * .. External Functions .. LOGICAL LSAME INTEGER IDAMAX, IZAMAX DOUBLE PRECISION DLAMCH, DZASUM COMPLEX*16 ZDOTC, ZDOTU, ZLADIV EXTERNAL LSAME, IDAMAX, IZAMAX, DLAMCH, DZASUM, ZDOTC, $ ZDOTU, ZLADIV * .. * .. External Subroutines .. EXTERNAL DSCAL, XERBLA, ZAXPY, ZDSCAL, ZTBSV * .. * .. Intrinsic Functions .. INTRINSIC ABS, DBLE, DCMPLX, DCONJG, DIMAG, MAX, MIN * .. * .. Statement Functions .. DOUBLE PRECISION CABS1, CABS2 * .. * .. Statement Function definitions .. CABS1( ZDUM ) = ABS( DBLE( ZDUM ) ) + ABS( DIMAG( ZDUM ) ) CABS2( ZDUM ) = ABS( DBLE( ZDUM ) / 2.D0 ) + $ ABS( DIMAG( ZDUM ) / 2.D0 ) * .. * .. Executable Statements .. * INFO = 0 UPPER = LSAME( UPLO, 'U' ) NOTRAN = LSAME( TRANS, 'N' ) NOUNIT = LSAME( DIAG, 'N' ) * * Test the input parameters. * IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN INFO = -1 ELSE IF( .NOT.NOTRAN .AND. .NOT.LSAME( TRANS, 'T' ) .AND. .NOT. $ LSAME( TRANS, 'C' ) ) THEN INFO = -2 ELSE IF( .NOT.NOUNIT .AND. .NOT.LSAME( DIAG, 'U' ) ) THEN INFO = -3 ELSE IF( .NOT.LSAME( NORMIN, 'Y' ) .AND. .NOT. $ LSAME( NORMIN, 'N' ) ) THEN INFO = -4 ELSE IF( N.LT.0 ) THEN INFO = -5 ELSE IF( KD.LT.0 ) THEN INFO = -6 ELSE IF( LDAB.LT.KD+1 ) THEN INFO = -8 END IF IF( INFO.NE.0 ) THEN CALL XERBLA( 'ZLATBS', -INFO ) RETURN END IF * * Quick return if possible * IF( N.EQ.0 ) $ RETURN * * Determine machine dependent parameters to control overflow. * SMLNUM = DLAMCH( 'Safe minimum' ) BIGNUM = ONE / SMLNUM CALL DLABAD( SMLNUM, BIGNUM ) SMLNUM = SMLNUM / DLAMCH( 'Precision' ) BIGNUM = ONE / SMLNUM SCALE = ONE * IF( LSAME( NORMIN, 'N' ) ) THEN * * Compute the 1-norm of each column, not including the diagonal. * IF( UPPER ) THEN * * A is upper triangular. * DO 10 J = 1, N JLEN = MIN( KD, J-1 ) CNORM( J ) = DZASUM( JLEN, AB( KD+1-JLEN, J ), 1 ) 10 CONTINUE ELSE * * A is lower triangular. * DO 20 J = 1, N JLEN = MIN( KD, N-J ) IF( JLEN.GT.0 ) THEN CNORM( J ) = DZASUM( JLEN, AB( 2, J ), 1 ) ELSE CNORM( J ) = ZERO END IF 20 CONTINUE END IF END IF * * Scale the column norms by TSCAL if the maximum element in CNORM is * greater than BIGNUM/2. * IMAX = IDAMAX( N, CNORM, 1 ) TMAX = CNORM( IMAX ) IF( TMAX.LE.BIGNUM*HALF ) THEN TSCAL = ONE ELSE TSCAL = HALF / ( SMLNUM*TMAX ) CALL DSCAL( N, TSCAL, CNORM, 1 ) END IF * * Compute a bound on the computed solution vector to see if the * Level 2 BLAS routine ZTBSV can be used. * XMAX = ZERO DO 30 J = 1, N XMAX = MAX( XMAX, CABS2( X( J ) ) ) 30 CONTINUE XBND = XMAX IF( NOTRAN ) THEN * * Compute the growth in A * x = b. * IF( UPPER ) THEN JFIRST = N JLAST = 1 JINC = -1 MAIND = KD + 1 ELSE JFIRST = 1 JLAST = N JINC = 1 MAIND = 1 END IF * IF( TSCAL.NE.ONE ) THEN GROW = ZERO GO TO 60 END IF * IF( NOUNIT ) THEN * * A is non-unit triangular. * * Compute GROW = 1/G(j) and XBND = 1/M(j). * Initially, G(0) = max{x(i), i=1,...,n}. * GROW = HALF / MAX( XBND, SMLNUM ) XBND = GROW DO 40 J = JFIRST, JLAST, JINC * * Exit the loop if the growth factor is too small. * IF( GROW.LE.SMLNUM ) $ GO TO 60 * TJJS = AB( MAIND, J ) TJJ = CABS1( TJJS ) * IF( TJJ.GE.SMLNUM ) THEN * * M(j) = G(j-1) / abs(A(j,j)) * XBND = MIN( XBND, MIN( ONE, TJJ )*GROW ) ELSE * * M(j) could overflow, set XBND to 0. * XBND = ZERO END IF * IF( TJJ+CNORM( J ).GE.SMLNUM ) THEN * * G(j) = G(j-1)*( 1 + CNORM(j) / abs(A(j,j)) ) * GROW = GROW*( TJJ / ( TJJ+CNORM( J ) ) ) ELSE * * G(j) could overflow, set GROW to 0. * GROW = ZERO END IF 40 CONTINUE GROW = XBND ELSE * * A is unit triangular. * * Compute GROW = 1/G(j), where G(0) = max{x(i), i=1,...,n}. * GROW = MIN( ONE, HALF / MAX( XBND, SMLNUM ) ) DO 50 J = JFIRST, JLAST, JINC * * Exit the loop if the growth factor is too small. * IF( GROW.LE.SMLNUM ) $ GO TO 60 * * G(j) = G(j-1)*( 1 + CNORM(j) ) * GROW = GROW*( ONE / ( ONE+CNORM( J ) ) ) 50 CONTINUE END IF 60 CONTINUE * ELSE * * Compute the growth in A**T * x = b or A**H * x = b. * IF( UPPER ) THEN JFIRST = 1 JLAST = N JINC = 1 MAIND = KD + 1 ELSE JFIRST = N JLAST = 1 JINC = -1 MAIND = 1 END IF * IF( TSCAL.NE.ONE ) THEN GROW = ZERO GO TO 90 END IF * IF( NOUNIT ) THEN * * A is non-unit triangular. * * Compute GROW = 1/G(j) and XBND = 1/M(j). * Initially, M(0) = max{x(i), i=1,...,n}. * GROW = HALF / MAX( XBND, SMLNUM ) XBND = GROW DO 70 J = JFIRST, JLAST, JINC * * Exit the loop if the growth factor is too small. * IF( GROW.LE.SMLNUM ) $ GO TO 90 * * G(j) = max( G(j-1), M(j-1)*( 1 + CNORM(j) ) ) * XJ = ONE + CNORM( J ) GROW = MIN( GROW, XBND / XJ ) * TJJS = AB( MAIND, J ) TJJ = CABS1( TJJS ) * IF( TJJ.GE.SMLNUM ) THEN * * M(j) = M(j-1)*( 1 + CNORM(j) ) / abs(A(j,j)) * IF( XJ.GT.TJJ ) $ XBND = XBND*( TJJ / XJ ) ELSE * * M(j) could overflow, set XBND to 0. * XBND = ZERO END IF 70 CONTINUE GROW = MIN( GROW, XBND ) ELSE * * A is unit triangular. * * Compute GROW = 1/G(j), where G(0) = max{x(i), i=1,...,n}. * GROW = MIN( ONE, HALF / MAX( XBND, SMLNUM ) ) DO 80 J = JFIRST, JLAST, JINC * * Exit the loop if the growth factor is too small. * IF( GROW.LE.SMLNUM ) $ GO TO 90 * * G(j) = ( 1 + CNORM(j) )*G(j-1) * XJ = ONE + CNORM( J ) GROW = GROW / XJ 80 CONTINUE END IF 90 CONTINUE END IF * IF( ( GROW*TSCAL ).GT.SMLNUM ) THEN * * Use the Level 2 BLAS solve if the reciprocal of the bound on * elements of X is not too small. * CALL ZTBSV( UPLO, TRANS, DIAG, N, KD, AB, LDAB, X, 1 ) ELSE * * Use a Level 1 BLAS solve, scaling intermediate results. * IF( XMAX.GT.BIGNUM*HALF ) THEN * * Scale X so that its components are less than or equal to * BIGNUM in absolute value. * SCALE = ( BIGNUM*HALF ) / XMAX CALL ZDSCAL( N, SCALE, X, 1 ) XMAX = BIGNUM ELSE XMAX = XMAX*TWO END IF * IF( NOTRAN ) THEN * * Solve A * x = b * DO 120 J = JFIRST, JLAST, JINC * * Compute x(j) = b(j) / A(j,j), scaling x if necessary. * XJ = CABS1( X( J ) ) IF( NOUNIT ) THEN TJJS = AB( MAIND, J )*TSCAL ELSE TJJS = TSCAL IF( TSCAL.EQ.ONE ) $ GO TO 110 END IF TJJ = CABS1( TJJS ) IF( TJJ.GT.SMLNUM ) THEN * * abs(A(j,j)) > SMLNUM: * IF( TJJ.LT.ONE ) THEN IF( XJ.GT.TJJ*BIGNUM ) THEN * * Scale x by 1/b(j). * REC = ONE / XJ CALL ZDSCAL( N, REC, X, 1 ) SCALE = SCALE*REC XMAX = XMAX*REC END IF END IF X( J ) = ZLADIV( X( J ), TJJS ) XJ = CABS1( X( J ) ) ELSE IF( TJJ.GT.ZERO ) THEN * * 0 < abs(A(j,j)) <= SMLNUM: * IF( XJ.GT.TJJ*BIGNUM ) THEN * * Scale x by (1/abs(x(j)))*abs(A(j,j))*BIGNUM * to avoid overflow when dividing by A(j,j). * REC = ( TJJ*BIGNUM ) / XJ IF( CNORM( J ).GT.ONE ) THEN * * Scale by 1/CNORM(j) to avoid overflow when * multiplying x(j) times column j. * REC = REC / CNORM( J ) END IF CALL ZDSCAL( N, REC, X, 1 ) SCALE = SCALE*REC XMAX = XMAX*REC END IF X( J ) = ZLADIV( X( J ), TJJS ) XJ = CABS1( X( J ) ) ELSE * * A(j,j) = 0: Set x(1:n) = 0, x(j) = 1, and * scale = 0, and compute a solution to A*x = 0. * DO 100 I = 1, N X( I ) = ZERO 100 CONTINUE X( J ) = ONE XJ = ONE SCALE = ZERO XMAX = ZERO END IF 110 CONTINUE * * Scale x if necessary to avoid overflow when adding a * multiple of column j of A. * IF( XJ.GT.ONE ) THEN REC = ONE / XJ IF( CNORM( J ).GT.( BIGNUM-XMAX )*REC ) THEN * * Scale x by 1/(2*abs(x(j))). * REC = REC*HALF CALL ZDSCAL( N, REC, X, 1 ) SCALE = SCALE*REC END IF ELSE IF( XJ*CNORM( J ).GT.( BIGNUM-XMAX ) ) THEN * * Scale x by 1/2. * CALL ZDSCAL( N, HALF, X, 1 ) SCALE = SCALE*HALF END IF * IF( UPPER ) THEN IF( J.GT.1 ) THEN * * Compute the update * x(max(1,j-kd):j-1) := x(max(1,j-kd):j-1) - * x(j)* A(max(1,j-kd):j-1,j) * JLEN = MIN( KD, J-1 ) CALL ZAXPY( JLEN, -X( J )*TSCAL, $ AB( KD+1-JLEN, J ), 1, X( J-JLEN ), 1 ) I = IZAMAX( J-1, X, 1 ) XMAX = CABS1( X( I ) ) END IF ELSE IF( J.LT.N ) THEN * * Compute the update * x(j+1:min(j+kd,n)) := x(j+1:min(j+kd,n)) - * x(j) * A(j+1:min(j+kd,n),j) * JLEN = MIN( KD, N-J ) IF( JLEN.GT.0 ) $ CALL ZAXPY( JLEN, -X( J )*TSCAL, AB( 2, J ), 1, $ X( J+1 ), 1 ) I = J + IZAMAX( N-J, X( J+1 ), 1 ) XMAX = CABS1( X( I ) ) END IF 120 CONTINUE * ELSE IF( LSAME( TRANS, 'T' ) ) THEN * * Solve A**T * x = b * DO 170 J = JFIRST, JLAST, JINC * * Compute x(j) = b(j) - sum A(k,j)*x(k). * k<>j * XJ = CABS1( X( J ) ) USCAL = TSCAL REC = ONE / MAX( XMAX, ONE ) IF( CNORM( J ).GT.( BIGNUM-XJ )*REC ) THEN * * If x(j) could overflow, scale x by 1/(2*XMAX). * REC = REC*HALF IF( NOUNIT ) THEN TJJS = AB( MAIND, J )*TSCAL ELSE TJJS = TSCAL END IF TJJ = CABS1( TJJS ) IF( TJJ.GT.ONE ) THEN * * Divide by A(j,j) when scaling x if A(j,j) > 1. * REC = MIN( ONE, REC*TJJ ) USCAL = ZLADIV( USCAL, TJJS ) END IF IF( REC.LT.ONE ) THEN CALL ZDSCAL( N, REC, X, 1 ) SCALE = SCALE*REC XMAX = XMAX*REC END IF END IF * CSUMJ = ZERO IF( USCAL.EQ.DCMPLX( ONE ) ) THEN * * If the scaling needed for A in the dot product is 1, * call ZDOTU to perform the dot product. * IF( UPPER ) THEN JLEN = MIN( KD, J-1 ) CSUMJ = ZDOTU( JLEN, AB( KD+1-JLEN, J ), 1, $ X( J-JLEN ), 1 ) ELSE JLEN = MIN( KD, N-J ) IF( JLEN.GT.1 ) $ CSUMJ = ZDOTU( JLEN, AB( 2, J ), 1, X( J+1 ), $ 1 ) END IF ELSE * * Otherwise, use in-line code for the dot product. * IF( UPPER ) THEN JLEN = MIN( KD, J-1 ) DO 130 I = 1, JLEN CSUMJ = CSUMJ + ( AB( KD+I-JLEN, J )*USCAL )* $ X( J-JLEN-1+I ) 130 CONTINUE ELSE JLEN = MIN( KD, N-J ) DO 140 I = 1, JLEN CSUMJ = CSUMJ + ( AB( I+1, J )*USCAL )*X( J+I ) 140 CONTINUE END IF END IF * IF( USCAL.EQ.DCMPLX( TSCAL ) ) THEN * * Compute x(j) := ( x(j) - CSUMJ ) / A(j,j) if 1/A(j,j) * was not used to scale the dotproduct. * X( J ) = X( J ) - CSUMJ XJ = CABS1( X( J ) ) IF( NOUNIT ) THEN * * Compute x(j) = x(j) / A(j,j), scaling if necessary. * TJJS = AB( MAIND, J )*TSCAL ELSE TJJS = TSCAL IF( TSCAL.EQ.ONE ) $ GO TO 160 END IF TJJ = CABS1( TJJS ) IF( TJJ.GT.SMLNUM ) THEN * * abs(A(j,j)) > SMLNUM: * IF( TJJ.LT.ONE ) THEN IF( XJ.GT.TJJ*BIGNUM ) THEN * * Scale X by 1/abs(x(j)). * REC = ONE / XJ CALL ZDSCAL( N, REC, X, 1 ) SCALE = SCALE*REC XMAX = XMAX*REC END IF END IF X( J ) = ZLADIV( X( J ), TJJS ) ELSE IF( TJJ.GT.ZERO ) THEN * * 0 < abs(A(j,j)) <= SMLNUM: * IF( XJ.GT.TJJ*BIGNUM ) THEN * * Scale x by (1/abs(x(j)))*abs(A(j,j))*BIGNUM. * REC = ( TJJ*BIGNUM ) / XJ CALL ZDSCAL( N, REC, X, 1 ) SCALE = SCALE*REC XMAX = XMAX*REC END IF X( J ) = ZLADIV( X( J ), TJJS ) ELSE * * A(j,j) = 0: Set x(1:n) = 0, x(j) = 1, and * scale = 0 and compute a solution to A**T *x = 0. * DO 150 I = 1, N X( I ) = ZERO 150 CONTINUE X( J ) = ONE SCALE = ZERO XMAX = ZERO END IF 160 CONTINUE ELSE * * Compute x(j) := x(j) / A(j,j) - CSUMJ if the dot * product has already been divided by 1/A(j,j). * X( J ) = ZLADIV( X( J ), TJJS ) - CSUMJ END IF XMAX = MAX( XMAX, CABS1( X( J ) ) ) 170 CONTINUE * ELSE * * Solve A**H * x = b * DO 220 J = JFIRST, JLAST, JINC * * Compute x(j) = b(j) - sum A(k,j)*x(k). * k<>j * XJ = CABS1( X( J ) ) USCAL = TSCAL REC = ONE / MAX( XMAX, ONE ) IF( CNORM( J ).GT.( BIGNUM-XJ )*REC ) THEN * * If x(j) could overflow, scale x by 1/(2*XMAX). * REC = REC*HALF IF( NOUNIT ) THEN TJJS = DCONJG( AB( MAIND, J ) )*TSCAL ELSE TJJS = TSCAL END IF TJJ = CABS1( TJJS ) IF( TJJ.GT.ONE ) THEN * * Divide by A(j,j) when scaling x if A(j,j) > 1. * REC = MIN( ONE, REC*TJJ ) USCAL = ZLADIV( USCAL, TJJS ) END IF IF( REC.LT.ONE ) THEN CALL ZDSCAL( N, REC, X, 1 ) SCALE = SCALE*REC XMAX = XMAX*REC END IF END IF * CSUMJ = ZERO IF( USCAL.EQ.DCMPLX( ONE ) ) THEN * * If the scaling needed for A in the dot product is 1, * call ZDOTC to perform the dot product. * IF( UPPER ) THEN JLEN = MIN( KD, J-1 ) CSUMJ = ZDOTC( JLEN, AB( KD+1-JLEN, J ), 1, $ X( J-JLEN ), 1 ) ELSE JLEN = MIN( KD, N-J ) IF( JLEN.GT.1 ) $ CSUMJ = ZDOTC( JLEN, AB( 2, J ), 1, X( J+1 ), $ 1 ) END IF ELSE * * Otherwise, use in-line code for the dot product. * IF( UPPER ) THEN JLEN = MIN( KD, J-1 ) DO 180 I = 1, JLEN CSUMJ = CSUMJ + ( DCONJG( AB( KD+I-JLEN, J ) )* $ USCAL )*X( J-JLEN-1+I ) 180 CONTINUE ELSE JLEN = MIN( KD, N-J ) DO 190 I = 1, JLEN CSUMJ = CSUMJ + ( DCONJG( AB( I+1, J ) )*USCAL ) $ *X( J+I ) 190 CONTINUE END IF END IF * IF( USCAL.EQ.DCMPLX( TSCAL ) ) THEN * * Compute x(j) := ( x(j) - CSUMJ ) / A(j,j) if 1/A(j,j) * was not used to scale the dotproduct. * X( J ) = X( J ) - CSUMJ XJ = CABS1( X( J ) ) IF( NOUNIT ) THEN * * Compute x(j) = x(j) / A(j,j), scaling if necessary. * TJJS = DCONJG( AB( MAIND, J ) )*TSCAL ELSE TJJS = TSCAL IF( TSCAL.EQ.ONE ) $ GO TO 210 END IF TJJ = CABS1( TJJS ) IF( TJJ.GT.SMLNUM ) THEN * * abs(A(j,j)) > SMLNUM: * IF( TJJ.LT.ONE ) THEN IF( XJ.GT.TJJ*BIGNUM ) THEN * * Scale X by 1/abs(x(j)). * REC = ONE / XJ CALL ZDSCAL( N, REC, X, 1 ) SCALE = SCALE*REC XMAX = XMAX*REC END IF END IF X( J ) = ZLADIV( X( J ), TJJS ) ELSE IF( TJJ.GT.ZERO ) THEN * * 0 < abs(A(j,j)) <= SMLNUM: * IF( XJ.GT.TJJ*BIGNUM ) THEN * * Scale x by (1/abs(x(j)))*abs(A(j,j))*BIGNUM. * REC = ( TJJ*BIGNUM ) / XJ CALL ZDSCAL( N, REC, X, 1 ) SCALE = SCALE*REC XMAX = XMAX*REC END IF X( J ) = ZLADIV( X( J ), TJJS ) ELSE * * A(j,j) = 0: Set x(1:n) = 0, x(j) = 1, and * scale = 0 and compute a solution to A**H *x = 0. * DO 200 I = 1, N X( I ) = ZERO 200 CONTINUE X( J ) = ONE SCALE = ZERO XMAX = ZERO END IF 210 CONTINUE ELSE * * Compute x(j) := x(j) / A(j,j) - CSUMJ if the dot * product has already been divided by 1/A(j,j). * X( J ) = ZLADIV( X( J ), TJJS ) - CSUMJ END IF XMAX = MAX( XMAX, CABS1( X( J ) ) ) 220 CONTINUE END IF SCALE = SCALE / TSCAL END IF * * Scale the column norms by 1/TSCAL for return. * IF( TSCAL.NE.ONE ) THEN CALL DSCAL( N, ONE / TSCAL, CNORM, 1 ) END IF * RETURN * * End of ZLATBS * END