      SUBROUTINE SPTRFS( N, NRHS, D, E, DF, EF, B, LDB, X, LDX, FERR,
     $                   BERR, WORK, INFO )
*
*  -- LAPACK routine (version 3.1) --
*     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
*     November 2006
*
*     .. Scalar Arguments ..
      INTEGER            INFO, LDB, LDX, N, NRHS
*     ..
*     .. Array Arguments ..
      REAL               B( LDB, * ), BERR( * ), D( * ), DF( * ),
     $                   E( * ), EF( * ), FERR( * ), WORK( * ),
     $                   X( LDX, * )
*     ..
*
*  Purpose
*  =======
*
*  SPTRFS improves the computed solution to a system of linear
*  equations when the coefficient matrix is symmetric positive definite
*  and tridiagonal, and provides error bounds and backward error
*  estimates for the solution.
*
*  Arguments
*  =========
*
*  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.
*
*  D       (input) REAL array, dimension (N)
*          The n diagonal elements of the tridiagonal matrix A.
*
*  E       (input) REAL array, dimension (N-1)
*          The (n-1) subdiagonal elements of the tridiagonal matrix A.
*
*  DF      (input) REAL array, dimension (N)
*          The n diagonal elements of the diagonal matrix D from the
*          factorization computed by SPTTRF.
*
*  EF      (input) REAL array, dimension (N-1)
*          The (n-1) subdiagonal elements of the unit bidiagonal factor
*          L from the factorization computed by SPTTRF.
*
*  B       (input) REAL 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) REAL array, dimension (LDX,NRHS)
*          On entry, the solution matrix X, as computed by SPTTRS.
*          On exit, the improved solution matrix X.
*
*  LDX     (input) INTEGER
*          The leading dimension of the array X.  LDX >= max(1,N).
*
*  FERR    (output) REAL array, dimension (NRHS)
*          The 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).
*
*  BERR    (output) REAL 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) REAL array, dimension (2*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 )
      REAL               ZERO
      PARAMETER          ( ZERO = 0.0E+0 )
      REAL               ONE
      PARAMETER          ( ONE = 1.0E+0 )
      REAL               TWO
      PARAMETER          ( TWO = 2.0E+0 )
      REAL               THREE
      PARAMETER          ( THREE = 3.0E+0 )
*     ..
*     .. Local Scalars ..
      INTEGER            COUNT, I, IX, J, NZ
      REAL               BI, CX, DX, EPS, EX, LSTRES, S, SAFE1, SAFE2,
     $                   SAFMIN
*     ..
*     .. External Subroutines ..
      EXTERNAL           SAXPY, SPTTRS, XERBLA
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          ABS, MAX
*     ..
*     .. External Functions ..
      INTEGER            ISAMAX
      REAL               SLAMCH
      EXTERNAL           ISAMAX, SLAMCH
*     ..
*     .. Executable Statements ..
*
*     Test the input parameters.
*
      INFO = 0
      IF( N.LT.0 ) THEN
         INFO = -1
      ELSE IF( NRHS.LT.0 ) THEN
         INFO = -2
      ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
         INFO = -8
      ELSE IF( LDX.LT.MAX( 1, N ) ) THEN
         INFO = -10
      END IF
      IF( INFO.NE.0 ) THEN
         CALL XERBLA( 'SPTRFS', -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 = 4
      EPS = SLAMCH( 'Epsilon' )
      SAFMIN = SLAMCH( 'Safe minimum' )
      SAFE1 = NZ*SAFMIN
      SAFE2 = SAFE1 / EPS
*
*     Do for each right hand side
*
      DO 90 J = 1, NRHS
*
         COUNT = 1
         LSTRES = THREE
   20    CONTINUE
*
*        Loop until stopping criterion is satisfied.
*
*        Compute residual R = B - A * X.  Also compute
*        abs(A)*abs(x) + abs(b) for use in the backward error bound.
*
         IF( N.EQ.1 ) THEN
            BI = B( 1, J )
            DX = D( 1 )*X( 1, J )
            WORK( N+1 ) = BI - DX
            WORK( 1 ) = ABS( BI ) + ABS( DX )
         ELSE
            BI = B( 1, J )
            DX = D( 1 )*X( 1, J )
            EX = E( 1 )*X( 2, J )
            WORK( N+1 ) = BI - DX - EX
            WORK( 1 ) = ABS( BI ) + ABS( DX ) + ABS( EX )
            DO 30 I = 2, N - 1
               BI = B( I, J )
               CX = E( I-1 )*X( I-1, J )
               DX = D( I )*X( I, J )
               EX = E( I )*X( I+1, J )
               WORK( N+I ) = BI - CX - DX - EX
               WORK( I ) = ABS( BI ) + ABS( CX ) + ABS( DX ) + ABS( EX )
   30       CONTINUE
            BI = B( N, J )
            CX = E( N-1 )*X( N-1, J )
            DX = D( N )*X( N, J )
            WORK( N+N ) = BI - CX - DX
            WORK( N ) = ABS( BI ) + ABS( CX ) + ABS( DX )
         END IF
*
*        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.
*
         S = ZERO
         DO 40 I = 1, N
            IF( WORK( I ).GT.SAFE2 ) THEN
               S = MAX( S, ABS( WORK( N+I ) ) / WORK( I ) )
            ELSE
               S = MAX( S, ( ABS( WORK( N+I ) )+SAFE1 ) /
     $             ( WORK( I )+SAFE1 ) )
            END IF
   40    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 SPTTRS( N, 1, DF, EF, WORK( N+1 ), N, INFO )
            CALL SAXPY( N, ONE, WORK( N+1 ), 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.
*
         DO 50 I = 1, N
            IF( WORK( I ).GT.SAFE2 ) THEN
               WORK( I ) = ABS( WORK( N+I ) ) + NZ*EPS*WORK( I )
            ELSE
               WORK( I ) = ABS( WORK( N+I ) ) + NZ*EPS*WORK( I ) + SAFE1
            END IF
   50    CONTINUE
         IX = ISAMAX( N, WORK, 1 )
         FERR( J ) = WORK( IX )
*
*        Estimate the norm of inv(A).
*
*        Solve M(A) * x = e, where M(A) = (m(i,j)) is given by
*
*           m(i,j) =  abs(A(i,j)), i = j,
*           m(i,j) = -abs(A(i,j)), i .ne. j,
*
*        and e = [ 1, 1, ..., 1 ]'.  Note M(A) = M(L)*D*M(L)'.
*
*        Solve M(L) * x = e.
*
         WORK( 1 ) = ONE
         DO 60 I = 2, N
            WORK( I ) = ONE + WORK( I-1 )*ABS( EF( I-1 ) )
   60    CONTINUE
*
*        Solve D * M(L)' * x = b.
*
         WORK( N ) = WORK( N ) / DF( N )
         DO 70 I = N - 1, 1, -1
            WORK( I ) = WORK( I ) / DF( I ) + WORK( I+1 )*ABS( EF( I ) )
   70    CONTINUE
*
*        Compute norm(inv(A)) = max(x(i)), 1<=i<=n.
*
         IX = ISAMAX( N, WORK, 1 )
         FERR( J ) = FERR( J )*ABS( WORK( IX ) )
*
*        Normalize error.
*
         LSTRES = ZERO
         DO 80 I = 1, N
            LSTRES = MAX( LSTRES, ABS( X( I, J ) ) )
   80    CONTINUE
         IF( LSTRES.NE.ZERO )
     $      FERR( J ) = FERR( J ) / LSTRES
*
   90 CONTINUE
*
      RETURN
*
*     End of SPTRFS
*
      END
