Nonlinear Constraints

Nonlinear inequality constraints have the form c(x) ≤ 0, where c is a vector of constraints, one component for each constraint. Similarly, nonlinear equality constraints are of the form ceq(x) = 0.

Note

Nonlinear constraint functions must return both c and ceq, the inequality and equality constraint functions, even if they do not both exist. Return an empty entry [] for a nonexistent constraint.

For example, suppose that you have the following inequalities as constraints:

$\begin{matrix} \frac{x_{1}^{2}}{9} + \frac{x_{2}^{2}}{4} \leq 1, \\ x_{2} \geq x_{1}^{2} - 1. \end{matrix}$

Write these constraints in a function file as follows:

function [c,ceq]=ellipseparabola(x)
c(1) = (x(1)^2)/9 + (x(2)^2)/4 - 1;
c(2) = x(1)^2 - x(2) - 1;
ceq = [];
end

ellipseparabola returns an empty entry [] for ceq, the nonlinear equality function. Also, both inequalities were put into ≤ 0 form.

Including Gradients in Constraint Functions

If you provide gradients for c and ceq, your solver can run faster and give more reliable results.

Providing a gradient has another advantage. A solver can reach a point x such that x is feasible, but finite differences around x always lead to an infeasible point. In this case, a solver can fail or halt prematurely. Providing a gradient allows a solver to proceed.

To include gradient information, write a conditionalized function as follows:

function [c,ceq,gradc,gradceq]=ellipseparabola(x)
c(1) = x(1)^2/9 + x(2)^2/4 - 1;
c(2) = x(1)^2 - x(2) - 1;
ceq = [];

if nargout > 2
    gradc = [2*x(1)/9, 2*x(1); ...
             x(2)/2, -1];
    gradceq = [];
end

See Writing Scalar Objective Functions for information on conditionalized functions. The gradient matrix has the form

gradc_{i, j} = [∂c(j)/∂x_i].

The first column of the gradient matrix is associated with c(1), and the second column is associated with c(2). This is the transpose of the form of Jacobians.

To have a solver use gradients of nonlinear constraints, indicate that they exist by using optimoptions:

options = optimoptions(@fmincon,'SpecifyConstraintGradient',true);

Make sure to pass the options structure to your solver:

[x,fval] = fmincon(@myobj,x0,A,b,Aeq,beq,lb,ub, ...
           @ellipseparabola,options)

If you have a Symbolic Math Toolbox™ license, you can calculate gradients and Hessians automatically, as described in Symbolic Math Toolbox Calculates Gradients and Hessians.

Anonymous Nonlinear Constraint Functions

For information on anonymous objective functions, see Anonymous Function Objectives.

Nonlinear constraint functions must return two outputs. The first output corresponds to nonlinear inequalities, and the second corresponds to nonlinear equalities.

Anonymous functions return just one output. So how can you write an anonymous function as a nonlinear constraint?

The deal function distributes multiple outputs. For example, suppose your nonlinear inequalities are

$\begin{matrix} \frac{x_{1}^{2}}{9} + \frac{x_{2}^{2}}{4} \leq 1, \\ x_{2} \geq x_{1}^{2} - 1. \end{matrix}$

Suppose that your nonlinear equality is

x₂ = tanh(x₁).

Write a nonlinear constraint function as follows:

c = @(x)[x(1)^2/9 + x(2)^2/4 - 1;
        x(1)^2 - x(2) - 1];
ceq = @(x)tanh(x(1)) - x(2);
nonlinfcn = @(x)deal(c(x),ceq(x));

To minimize the function cosh(x₁) + sinh(x₂) subject to the constraints in nonlinfcn, use fmincon:

obj = @(x)cosh(x(1))+sinh(x(2));
opts = optimoptions(@fmincon,'Algorithm','sqp');
z = fmincon(obj,[0;0],[],[],[],[],[],[],nonlinfcn,opts)

Local minimum found that satisfies the constraints.

Optimization completed because the objective function is 
non-decreasing in feasible directions, to within the default 
value of the function tolerance, and constraints are satisfied 
to within the default value of the constraint tolerance.

z =
   -0.6530
   -0.5737

To check how well the resulting point z satisfies the constraints, use nonlinfcn:

[cout,ceqout] = nonlinfcn(z)

cout =
   -0.8704
         0

ceqout =
     0

z indeed satisfies all the constraints to within the default value of the ConstraintTolerance constraint tolerance, 1e-6.

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