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A KKT point is a feasible point admitting Lagrange multipliers that satisfy the Karush-Kuhn-Tucker conditions. Some authors use the name for the full primal and multiplier pair; specify the multipliers when this distinction matters. For a convex optimization problem with a differentiable convex function as its objective function, convex inequality functions and affine equality constraints, the KKT conditions certify a global minimizer. Their necessity requires an appropriate constraint qualification.

With inequalities written as $c_i\ge 0$, use the optimization Lagrangian $L(x,\lambda )=f(x)-{\sum }_i{\lambda }_i{c}_i(x)$. The Karush-Kuhn-Tucker conditions at $(x_{\ast },{\lambda }_{\ast })$ areFor any feasible point $z$, the first-order inequalities for a convex function and a concave function giveThe last equality uses complementary slackness. Thus $x_{\ast }$ is a global minimizer. Slater's condition is useful for necessity and existence of Lagrange multipliers, but is not required for this sufficiency argument once a KKT point is supplied.

A precise conditional convergence statement is: with the smoothness and convexity assumptions of the question, a strict feasible point, ${\mu }_k\downarrow 0$, and barrier stationarity residualsevery convergent sequence of strictly feasible points $x^k\to x_{\ast }$ has a global minimizer as its limit. The Lagrange multiplier estimates are bounded and have a subsequence converging to multipliers satisfying the KKT conditions. The theorem does not assert convergence of the primal sequence without further assumptions.

First, continuity gives $c_i(x_{\ast })\ge 0$. For an inactive optimization constraint, $c_i(x_{\ast })>0$, soFor the active constraints, the hint permits assuming ${\lambda }_i^k\to {\lambda }_i^{\ast }$. Combining this with the inactive limits gives ${\lambda }_{\ast }\ge 0$, and passage to the limit in the residual equation proves stationarity. Since ${\lambda }_i^k{c}_i(x^k)={\mu }_k\to 0$, complementary slackness also holds. All KKT conditions follow, and part (b) proves global optimality.

The hint's multiplier assumption can in fact be weakened to selecting a subsequence. To see why the estimates are bounded, choose a strict feasible point $z$ and put $\delta ={\min }_i{c}_i(z)>0$. Concavity givesThe right side is bounded because $x^k\to x_{\ast }$ and $r_k\to 0$. This is the bounded barrier multipliers under Slater's condition argument. A convergent multiplier subsequence then gives the same KKT point, which suffices to prove that the assumed primal limit is globally optimal.