Concept

Karush–Kuhn–Tucker conditions

Summary
In mathematical optimization, the Karush–Kuhn–Tucker (KKT) conditions, also known as the Kuhn–Tucker conditions, are first derivative tests (sometimes called first-order necessary conditions) for a solution in nonlinear programming to be optimal, provided that some regularity conditions are satisfied. Allowing inequality constraints, the KKT approach to nonlinear programming generalizes the method of Lagrange multipliers, which allows only equality constraints. Similar to the Lagrange approach, the constrained maximization (minimization) problem is rewritten as a Lagrange function whose optimal point is a (global) saddle point, i.e. a global maximum (minimum) over the domain of the choice variables and a global minimum (maximum) over the multipliers, which is why the Karush–Kuhn–Tucker theorem is sometimes referred to as the saddle-point theorem. The KKT conditions were originally named after Harold W. Kuhn and Albert W. Tucker, who first published the conditions in 1951. Later scholars discovered that the necessary conditions for this problem had been stated by William Karush in his master's thesis in 1939. Consider the following nonlinear optimization problem in standard form: minimize subject to where is the optimization variable chosen from a convex subset of , is the objective or utility function, are the inequality constraint functions and are the equality constraint functions. The numbers of inequalities and equalities are denoted by and respectively. Corresponding to the constrained optimization problem one can form the Lagrangian function where The Karush–Kuhn–Tucker theorem then states the following. Since the idea of this approach is to find a supporting hyperplane on the feasible set , the proof of the Karush–Kuhn–Tucker theorem makes use of the hyperplane separation theorem. The system of equations and inequalities corresponding to the KKT conditions is usually not solved directly, except in the few special cases where a closed-form solution can be derived analytically.
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