# Complementarity problem

### From Wikimization

## Contents |

### Fixed point problems

Let be a set and a mapping. The **fixed point problem** defined by is the problem

### Nonlinear complementarity problems

Let be a closed convex cone in the Hilbert space and a mapping. Recall that the dual cone of is the closed convex cone where is the polar of The **nonlinear complementarity problem** defined by and is the problem

### Every nonlinear complementarity problem is equivalent to a fixed point problem

Let be a closed convex cone in the Hilbert space and a mapping. Then, the nonlinear complementarity problem is equivalent to the fixed point problem where is the identity mapping defined by

### Proof

For all denote and Then,

Suppose that is a solution of Then, with and Hence, by using Moreau's theorem, we get Therefore, is a solution of

Conversely, suppose that is a solution of Then, and by using Moreau's theorem

Hence, . Thus, . Moreau's theorem also implies that In conclusion, and Therefore, is a solution of

### An alternative proof without Moreau's theorem

#### Variational inequalities

Let be a closed convex set in the Hilbert space and a mapping. The **variational inequality** defined by and is the problem

#### Every variational inequality is equivalent to a fixed point problem

Let be a closed convex set in the Hilbert space and a mapping. Then the variational inequality is equivalent to the fixed point problem

#### Proof

is a solution of if and only if By using the characterization of the projection the latter equation is equivalent to

for all But this holds if and only if is a solution of

##### Remark

The next section shows that the equivalence of variational inequalities and fixed point problems is much stronger than the equivalence of nonlinear complementarity problems and fixed point problems, because each nonlinear complementarity problem is a variational inequality defined on a closed convex cone.

#### Every variational inequality defined on a closed convex cone is equivalent to a complementarity problem

Let be a closed convex cone in the Hilbert space and a mapping. Then, the nonlinear complementarity problem is equivalent to the variational inequality

#### Proof

Suppose that is a solution of Then, and Hence,

for all Therefore, is a solution of

Conversely, suppose that is a solution of Then, and

for all Particularly, taking and , respectively, we get Thus, for all or equivalently In conclusion, and Therefore, is a solution of

#### Concluding the alternative proof

Since is a closed convex cone, the nonlinear complementarity problem is equivalent to the variational inequality which is equivalent to the fixed point problem

## Implicit complementarity problems

Let be a closed convex cone in the Hilbert space and two mappings. Recall that the dual cone of is the closed convex cone where is the
polar
of The **implicit complementarity problem** defined by
and the ordered pair of mappings is the problem

### Every implicit complementarity problem is equivalent to a fixed point problem

Let be a closed convex cone in the Hilbert space and two mappings. Then, the implicit complementarity problem is equivalent to the fixed point problem where is the identity mapping defined by

### Proof

For all denote and Then,

Suppose that is a solution of Then, with and Hence, by using
Moreau's theorem,
we get Therefore, is a solution of

Conversely, suppose that is a solution of Then, and by using Moreau's theorem

Hence, . Thus, . Moreau's theorem also implies that In conclusion, and Therefore, is a solution of

### Remark

In particular if we obtain the result Every nonlinear complementarity problem is equivalent to a fixed point problem, but the more general result Every implicit complementarity problem is equivalent to a fixed point problem has no known connection with variational inequalities. Therefore, using Moreau's theorem is essential for proving the latter result.