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NAME
     dtrrfs - provide error bounds and backward  error  estimates
     for the solution to a system of linear equations with a tri-
     angular coefficient matrix

SYNOPSIS
     SUBROUTINE DTRRFS( UPLO, TRANS, DIAG, N, NRHS,  A,  LDA,  B,
               LDB, X, LDX, FERR, BERR, WORK, IWORK, INFO )

     CHARACTER DIAG, TRANS, UPLO

     INTEGER INFO, LDA, LDB, LDX, N, NRHS

     INTEGER IWORK( * )

     DOUBLE PRECISION A( LDA, * ), B( LDB, * ), BERR( * ),  FERR(
               * ), WORK( * ), X( LDX, * )



     #include <sunperf.h>

     void dtrrfs(char uplo, char trans, char  diag,  int  n,  int
               nrhs,  double  *da,  int lda, double *db, int ldb,
               double *dx, int ldx, double *ferr,  double  *berr,
               int *info) ;

PURPOSE
     DTRRFS provides error bounds and  backward  error  estimates
     for the solution to a system of linear equations with a tri-
     angular coefficient matrix.

     The solution matrix X must be computed  by  DTRTRS  or  some
     other  means  before entering this routine.  DTRRFS does not
     do iterative refinement because doing so cannot improve  the
     backward error.


ARGUMENTS
     UPLO      (input) CHARACTER*1
               = 'U':  A is upper triangular;
               = 'L':  A is lower triangular.

     TRANS     (input) CHARACTER*1
               Specifies the form of the system of equations:
               = 'N':  A * X = B  (No transpose)
               = 'T':  A**T * X = B  (Transpose)
               = 'C':  A**H * X = B  (Conjugate transpose = Tran-
               spose)

     DIAG      (input) CHARACTER*1
               = 'N':  A is non-unit triangular;
               = 'U':  A is unit triangular.

     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.

     A         (input) DOUBLE PRECISION array, dimension (LDA,N)
               The triangular matrix A.  If UPLO = 'U', the lead-
               ing  N-by-N  upper  triangular part of the array A
               contains the  upper  triangular  matrix,  and  the
               strictly  lower triangular part of A is not refer-
               enced.  If UPLO = 'L', the  leading  N-by-N  lower
               triangular  part of the array A contains the lower
               triangular matrix, and the strictly upper triangu-
               lar  part  of A is not referenced.  If DIAG = 'U',
               the diagonal elements of A are also not referenced
               and are assumed to be 1.

     LDA       (input) INTEGER
               The leading dimension of  the  array  A.   LDA  >=
               max(1,N).

     B         (input)   DOUBLE   PRECISION   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)   DOUBLE   PRECISION   array,    dimension
               (LDX,NRHS)
               The 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  solu-
               tion  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  ele-
               ment 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)  DOUBLE  PRECISION  array,   dimension
               (3*N)

     IWORK     (workspace) INTEGER array, dimension (N)

     INFO      (output) INTEGER
               = 0:  successful exit
               < 0:  if INFO = -i, the i-th argument had an ille-
               gal value

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