Mathematical programs with complementarity constraints and application to hyperparameter tuning
We consider the Mathematical Program with Complementarity Constraints (MPCC). One of the main challenges in solving this problem is the systematic failure of standard Constraint Qualifications (CQs). Carefully accounting for the combinatorial nature of the complementarity constraints, tractable versions of the Mangasarian Fromovitz Constraint Qualification (MFCQ) have been designed and widely studied in the literature. This paper looks closely at two such MPCC-MFCQs and their influence on MPCC algorithms. As a key contribution, we prove the convergence of the sequential penalisation and Scholtes relaxation algorithms under a relaxed MPCC-MFCQ that is much weaker than the CQs currently used in the literature. We then form the problem of tuning hyperparameters of a nonlinear Support Vector Machine (SVM), a fundamental machine learning problem for classification, as a MPCC. For this application, we establish that the aforementioned relaxed MPCC-MFCQ holds under a very mild assumption. Moreover, we program robust implementations and comprehensive numerical experimentation on real-world data sets, where we show that the sequential penalisation method applied to the MPCC formulation for tuning SVM hyperparameters can outperform both the Scholtes relaxation technique and the state-of-the-art derivative-free methods from the machine learning literature.