Getting Started

Unconstrained Rosenbrock Function

The Rosenbrock function is defined as

The minimum function value is at the point .

The following code calls the NLPTR subroutine to solve the optimization problem:

The NLPTR is a trust-region optimization method. The F_ROSEN module represents the Rosenbrock function, and the G_ROSEN module represents its gradient. Specifying the gradient can reduce the number of function calls by the optimization subroutine. The optimization begins at the initial point . For more information about the NLPTR subroutine and its arguments, see the section NLPTR Call. For details about the options vector, which is given by the OPTN vector in the preceding code, see the section Options Vector.

A portion of the output produced by the NLPTR subroutine is shown in Figure 14.1.

Since , you can also use least squares techniques in this situation. The following code calls the NLPLM subroutine to solve the problem. The output is shown in Figure 14.2.

The Levenberg-Marquardt least squares method, which is the method used by the NLPLM subroutine, is a modification of the trust-region method for nonlinear least squares problems. The F_ROSEN module represents the Rosenbrock function. Note that for least squares problems, the functions are specified as elements of a vector; this is different from the manner in which is specified for the other optimization techniques. No derivatives are specified in the preceding code, so the NLPLM subroutine computes finite-difference approximations. For more information about the NLPLM subroutine, see the section NLPLM Call.

Constrained Betts Function

The linearly constrained Betts function (Hock and Schittkowski; 1981) is defined as

The boundary constraints are

The linear constraint is

The following code calls the NLPCG subroutine to solve the optimization problem. The infeasible initial point is specified, and a portion of the output is shown in Figure 14.3.

The NLPCG subroutine performs conjugate gradient optimization. It requires only function and gradient calls. The F_BETTS module represents the Betts function, and since no module is defined to specify the gradient, first-order derivatives are computed by finite-difference approximations. For more information about the NLPCG subroutine, see the section NLPCG Call. For details about the constraint matrix, which is represented by the CON matrix in the preceding code, see the section Parameter Constraints.

Since the initial point is infeasible, the subroutine first computes a feasible starting point. Convergence is achieved after three iterations, and the optimal point is given to be with an optimal function value of . For more information about the printed output, see the section Printing the Optimization History.

Rosen-Suzuki Problem

The Rosen-Suzuki problem is a function of four variables with three nonlinear constraints on the variables. It is taken from problem 43 of Hock and Schittkowski (1981). The objective function is

The nonlinear constraints are

Since this problem has nonlinear constraints, only the NLPQN and NLPNMS subroutines are available to perform the optimization. The following code solves the problem with the NLPQN subroutine:

The F_HS43 module specifies the objective function, and the C_HS43 module specifies the nonlinear constraints. The OPTN vector is passed to the subroutine as the OPT input argument. See the section Options Vector for more information. The value of OPTN[10] represents the total number of nonlinear constraints, and the value of OPTN[11] represents the number of equality constraints. In the preceding code, OPTN[10]=3 and OPTN[11]=0, which indicate that there are three constraints, all of which are inequality constraints. In the subroutine calls, instead of separating missing input arguments with commas, you can specify optional arguments with keywords, as in the CALL NLPQN statement in the preceding code. For details about the CALL NLPQN statement, see the section NLPQN Call.

The initial point for the optimization procedure is , and the optimal point is , with an optimal function value of . Part of the output produced is shown in Figure 14.4.

In addition to the standard iteration history, the NLPQN subroutine includes the following information for problems with nonlinear constraints:

• CONMAX is the maximum value of all constraint violations.

• PRED is the value of the predicted function reduction used with the GTOL and FTOL2 termination criteria.

• ALFA is the step size of the quasi-Newton step.

• LFGMAX is the maximum element of the gradient of the Lagrange function.