```      SUBROUTINE ZPPRFS( UPLO, N, NRHS, AP, AFP, B, LDB, X, LDX, FERR,
\$                   BERR, WORK, RWORK, INFO )
*
*  -- LAPACK routine (version 3.1) --
*     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
*     November 2006
*
*     Modified to call ZLACN2 in place of ZLACON, 10 Feb 03, SJH.
*
*     .. Scalar Arguments ..
CHARACTER          UPLO
INTEGER            INFO, LDB, LDX, N, NRHS
*     ..
*     .. Array Arguments ..
DOUBLE PRECISION   BERR( * ), FERR( * ), RWORK( * )
COMPLEX*16         AFP( * ), AP( * ), B( LDB, * ), WORK( * ),
\$                   X( LDX, * )
*     ..
*
*  Purpose
*  =======
*
*  ZPPRFS improves the computed solution to a system of linear
*  equations when the coefficient matrix is Hermitian positive definite
*  and packed, and provides error bounds and backward error estimates
*  for the solution.
*
*  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 matrices B and X.  NRHS >= 0.
*
*  AP      (input) COMPLEX*16 array, dimension (N*(N+1)/2)
*          The upper or lower triangle of the Hermitian matrix A, packed
*          columnwise in a linear array.  The j-th column of A is stored
*          in the array AP as follows:
*          if UPLO = 'U', AP(i + (j-1)*j/2) = A(i,j) for 1<=i<=j;
*          if UPLO = 'L', AP(i + (j-1)*(2n-j)/2) = A(i,j) for j<=i<=n.
*
*  AFP     (input) COMPLEX*16 array, dimension (N*(N+1)/2)
*          The triangular factor U or L from the Cholesky factorization
*          A = U**H*U or A = L*L**H, as computed by DPPTRF/ZPPTRF,
*          packed columnwise in a linear array in the same format as A
*          (see AP).
*
*  B       (input) COMPLEX*16 array, dimension (LDB,NRHS)
*          The right hand side matrix B.
*
*  LDB     (input) INTEGER
*          The leading dimension of the array B.  LDB >= max(1,N).
*
*  X       (input/output) COMPLEX*16 array, dimension (LDX,NRHS)
*          On entry, the solution matrix X, as computed by ZPPTRS.
*          On exit, the improved solution matrix X.
*
*  LDX     (input) INTEGER
*          The leading dimension of the array X.  LDX >= max(1,N).
*
*  FERR    (output) DOUBLE PRECISION array, dimension (NRHS)
*          The estimated forward error bound for each solution vector
*          X(j) (the j-th column of the solution matrix X).
*          If XTRUE is the true solution corresponding to X(j), FERR(j)
*          is an estimated upper bound for the magnitude of the largest
*          element in (X(j) - XTRUE) divided by the magnitude of the
*          largest element in X(j).  The estimate is as reliable as
*          the estimate for RCOND, and is almost always a slight
*          overestimate of the true error.
*
*  BERR    (output) DOUBLE PRECISION array, dimension (NRHS)
*          The componentwise relative backward error of each solution
*          vector X(j) (i.e., the smallest relative change in
*          any element of A or B that makes X(j) an exact solution).
*
*  WORK    (workspace) COMPLEX*16 array, dimension (2*N)
*
*  RWORK   (workspace) DOUBLE PRECISION array, dimension (N)
*
*  INFO    (output) INTEGER
*          = 0:  successful exit
*          < 0:  if INFO = -i, the i-th argument had an illegal value
*
*  Internal Parameters
*  ===================
*
*  ITMAX is the maximum number of steps of iterative refinement.
*
*  ====================================================================
*
*     .. Parameters ..
INTEGER            ITMAX
PARAMETER          ( ITMAX = 5 )
DOUBLE PRECISION   ZERO
PARAMETER          ( ZERO = 0.0D+0 )
COMPLEX*16         CONE
PARAMETER          ( CONE = ( 1.0D+0, 0.0D+0 ) )
DOUBLE PRECISION   TWO
PARAMETER          ( TWO = 2.0D+0 )
DOUBLE PRECISION   THREE
PARAMETER          ( THREE = 3.0D+0 )
*     ..
*     .. Local Scalars ..
LOGICAL            UPPER
INTEGER            COUNT, I, IK, J, K, KASE, KK, NZ
DOUBLE PRECISION   EPS, LSTRES, S, SAFE1, SAFE2, SAFMIN, XK
COMPLEX*16         ZDUM
*     ..
*     .. Local Arrays ..
INTEGER            ISAVE( 3 )
*     ..
*     .. External Subroutines ..
EXTERNAL           XERBLA, ZAXPY, ZCOPY, ZHPMV, ZLACN2, ZPPTRS
*     ..
*     .. Intrinsic Functions ..
INTRINSIC          ABS, DBLE, DIMAG, MAX
*     ..
*     .. External Functions ..
LOGICAL            LSAME
DOUBLE PRECISION   DLAMCH
EXTERNAL           LSAME, DLAMCH
*     ..
*     .. Statement Functions ..
DOUBLE PRECISION   CABS1
*     ..
*     .. Statement Function definitions ..
CABS1( ZDUM ) = ABS( DBLE( ZDUM ) ) + ABS( DIMAG( ZDUM ) )
*     ..
*     .. 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 = -7
ELSE IF( LDX.LT.MAX( 1, N ) ) THEN
INFO = -9
END IF
IF( INFO.NE.0 ) THEN
CALL XERBLA( 'ZPPRFS', -INFO )
RETURN
END IF
*
*     Quick return if possible
*
IF( N.EQ.0 .OR. NRHS.EQ.0 ) THEN
DO 10 J = 1, NRHS
FERR( J ) = ZERO
BERR( J ) = ZERO
10    CONTINUE
RETURN
END IF
*
*     NZ = maximum number of nonzero elements in each row of A, plus 1
*
NZ = N + 1
EPS = DLAMCH( 'Epsilon' )
SAFMIN = DLAMCH( 'Safe minimum' )
SAFE1 = NZ*SAFMIN
SAFE2 = SAFE1 / EPS
*
*     Do for each right hand side
*
DO 140 J = 1, NRHS
*
COUNT = 1
LSTRES = THREE
20    CONTINUE
*
*        Loop until stopping criterion is satisfied.
*
*        Compute residual R = B - A * X
*
CALL ZCOPY( N, B( 1, J ), 1, WORK, 1 )
CALL ZHPMV( UPLO, N, -CONE, AP, X( 1, J ), 1, CONE, WORK, 1 )
*
*        Compute componentwise relative backward error from formula
*
*        max(i) ( abs(R(i)) / ( abs(A)*abs(X) + abs(B) )(i) )
*
*        where abs(Z) is the componentwise absolute value of the matrix
*        or vector Z.  If the i-th component of the denominator is less
*        than SAFE2, then SAFE1 is added to the i-th components of the
*        numerator and denominator before dividing.
*
DO 30 I = 1, N
RWORK( I ) = CABS1( B( I, J ) )
30    CONTINUE
*
*        Compute abs(A)*abs(X) + abs(B).
*
KK = 1
IF( UPPER ) THEN
DO 50 K = 1, N
S = ZERO
XK = CABS1( X( K, J ) )
IK = KK
DO 40 I = 1, K - 1
RWORK( I ) = RWORK( I ) + CABS1( AP( IK ) )*XK
S = S + CABS1( AP( IK ) )*CABS1( X( I, J ) )
IK = IK + 1
40          CONTINUE
RWORK( K ) = RWORK( K ) + ABS( DBLE( AP( KK+K-1 ) ) )*
\$                      XK + S
KK = KK + K
50       CONTINUE
ELSE
DO 70 K = 1, N
S = ZERO
XK = CABS1( X( K, J ) )
RWORK( K ) = RWORK( K ) + ABS( DBLE( AP( KK ) ) )*XK
IK = KK + 1
DO 60 I = K + 1, N
RWORK( I ) = RWORK( I ) + CABS1( AP( IK ) )*XK
S = S + CABS1( AP( IK ) )*CABS1( X( I, J ) )
IK = IK + 1
60          CONTINUE
RWORK( K ) = RWORK( K ) + S
KK = KK + ( N-K+1 )
70       CONTINUE
END IF
S = ZERO
DO 80 I = 1, N
IF( RWORK( I ).GT.SAFE2 ) THEN
S = MAX( S, CABS1( WORK( I ) ) / RWORK( I ) )
ELSE
S = MAX( S, ( CABS1( WORK( I ) )+SAFE1 ) /
\$             ( RWORK( I )+SAFE1 ) )
END IF
80    CONTINUE
BERR( J ) = S
*
*        Test stopping criterion. Continue iterating if
*           1) The residual BERR(J) is larger than machine epsilon, and
*           2) BERR(J) decreased by at least a factor of 2 during the
*              last iteration, and
*           3) At most ITMAX iterations tried.
*
IF( BERR( J ).GT.EPS .AND. TWO*BERR( J ).LE.LSTRES .AND.
\$       COUNT.LE.ITMAX ) THEN
*
*           Update solution and try again.
*
CALL ZPPTRS( UPLO, N, 1, AFP, WORK, N, INFO )
CALL ZAXPY( N, CONE, WORK, 1, X( 1, J ), 1 )
LSTRES = BERR( J )
COUNT = COUNT + 1
GO TO 20
END IF
*
*        Bound error from formula
*
*        norm(X - XTRUE) / norm(X) .le. FERR =
*        norm( abs(inv(A))*
*           ( abs(R) + NZ*EPS*( abs(A)*abs(X)+abs(B) ))) / norm(X)
*
*        where
*          norm(Z) is the magnitude of the largest component of Z
*          inv(A) is the inverse of A
*          abs(Z) is the componentwise absolute value of the matrix or
*             vector Z
*          NZ is the maximum number of nonzeros in any row of A, plus 1
*          EPS is machine epsilon
*
*        The i-th component of abs(R)+NZ*EPS*(abs(A)*abs(X)+abs(B))
*        is incremented by SAFE1 if the i-th component of
*        abs(A)*abs(X) + abs(B) is less than SAFE2.
*
*        Use ZLACN2 to estimate the infinity-norm of the matrix
*           inv(A) * diag(W),
*        where W = abs(R) + NZ*EPS*( abs(A)*abs(X)+abs(B) )))
*
DO 90 I = 1, N
IF( RWORK( I ).GT.SAFE2 ) THEN
RWORK( I ) = CABS1( WORK( I ) ) + NZ*EPS*RWORK( I )
ELSE
RWORK( I ) = CABS1( WORK( I ) ) + NZ*EPS*RWORK( I ) +
\$                      SAFE1
END IF
90    CONTINUE
*
KASE = 0
100    CONTINUE
CALL ZLACN2( N, WORK( N+1 ), WORK, FERR( J ), KASE, ISAVE )
IF( KASE.NE.0 ) THEN
IF( KASE.EQ.1 ) THEN
*
*              Multiply by diag(W)*inv(A').
*
CALL ZPPTRS( UPLO, N, 1, AFP, WORK, N, INFO )
DO 110 I = 1, N
WORK( I ) = RWORK( I )*WORK( I )
110          CONTINUE
ELSE IF( KASE.EQ.2 ) THEN
*
*              Multiply by inv(A)*diag(W).
*
DO 120 I = 1, N
WORK( I ) = RWORK( I )*WORK( I )
120          CONTINUE
CALL ZPPTRS( UPLO, N, 1, AFP, WORK, N, INFO )
END IF
GO TO 100
END IF
*
*        Normalize error.
*
LSTRES = ZERO
DO 130 I = 1, N
LSTRES = MAX( LSTRES, CABS1( X( I, J ) ) )
130    CONTINUE
IF( LSTRES.NE.ZERO )
\$      FERR( J ) = FERR( J ) / LSTRES
*
140 CONTINUE
*
RETURN
*
*     End of ZPPRFS
*
END

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