Weak duality


In applied mathematics, weak duality is a concept in optimization which states that the duality gap is always greater than or equal to 0. That means the solution to the primal problem is always greater than or equal to the solution to an associated dual problem. This is opposed to strong duality which only holds in certain cases.

Uses

Many primal-dual approximation algorithms are based on the principle of weak duality.

Weak duality theorem

The primal problem:
The dual problem,
The weak duality theorem states cTx ≤ bTy.
Namely, if is a feasible solution for the primal maximization linear program and is a feasible solution for the dual minimization linear program, then the weak duality theorem can be stated as
, where and are the coefficients of the respective objective functions.
Proof:
cTx
= xTc
≤ xTATy
≤ bTy

Generalizations

More generally, if is a feasible solution for the primal maximization problem and is a feasible solution for the dual minimization problem, then weak duality implies where and are the objective functions for the primal and dual problems respectively.