SUBROUTINE OPTIF0(NR,N,X,FCN,XPLS,FPLS,GPLS,ITRMCD,A,WRK)
      IMPLICIT DOUBLE PRECISION (A-H,O-Z)
C
C PURPOSE
C -------
C PROVIDE SIMPLEST INTERFACE TO MINIMIZATION PACKAGE.
C USER HAS NO CONTROL OVER OPTIONS.
C
C OPTIF0 is designed to solve the unconstrained nonlinear optimization
C problem.  This problem involves finding the minimums of a twice
C continuously differentiable real-valued function f of n variables
C from a given start point x(0).
C
C
C PARAMETERS
C ----------
C NR        --> Row dimension of matrix.
C               A positive integer specifying the row dimension of the
C               matrices A and WRK in the user's calling program.  NR
C               must satisfy the relation NR >= N (see N).  The provision
C               of this variable allows the user the flexibility of
C               solving several problems of different order N one after
C               the other.
C
C N         --> Dimension of problem.  A positive integer specifying
C               the order or dimension of the problem.  The program will
C               abort if N <= 0.  The program is inefficient for the
C               one-dimensional case (N=1); the user is advised to find
C               more efficient software.  The package will abort for
C               N=1 unless the parameter MSG is appropriately set; in
C               this instance the interface OPTIF9 must be utilized.
C
C X(N)      --> Initial estimate of minimum
C
C FCN       --> The name of a user supplied subroutine that evaluates
C               the optimization function at an arbitrary vector X.
C               The subroutine must be declared EXTERNAL in the user's
C               calling program and must conform to the usage:
C                    CALL FCN (N,X,F)
C               where X is a vector of length N.  The subroutine must
C               not alter the values of X or N.  On return F is the
C               value of the optimization function at X.
C
C XPLS(N)     <--  Local minimum
C
C FPLS        <--  Funtion value at local minimum XPLS
C
C GPLS(N)     <--  Gradient at local minimum XPLS
C ITRMCD      <--  Termination code
C               = 0  Erroneous input detected
C               = 1  Relative gradient is close to zero.  Current
C                    iterate is probably solution.
C               = 2  Successive iterates within tolerance.  Current
C                    iterate is probably solution.
C               = 3  Last global step failed to locate a point lower
C                    than XPLS.  Either XPLS is an approximate local
C                    minimum of the function, the function is too
C                    non-linear for this algorithm, or STEPTL is too
C                    large.
C               = 4  Iteration limit exceeded.
C               = 5  Maximum step size STEPMX exceeded five consecutive
C                    times.  Either the functon is unbounded below,
C                    becomes asymtotic to a finite value from above
C                    in some direction, or STEPMX is too small.
C
C
C A(N,N)    --> Workspace. A real matrix array which is used to store
C               the Hessian and its Cholesky decomposition.
C WRK(N,9)  --> Workspace
C
C For a more detailed description of the parameters, see:
C
C  AN UNCONSTRAINED NON-LINEAR OPTIMIZATION SOLVER: A USER'S GUIDE
C
C available through Richard Jackson, Operations Research Division
C National Bureau of Standards, Washington, DC  (301)921-3855.