Relaxation-Informed Training of Neural Network Surrogate Models

2026-04-24

AI Summary

This work addresses the inefficiency of global optimization when standard neural network surrogates are embedded into mixed-integer linear programs (MILPs), a challenge stemming from the lack of control over their structural properties. The authors propose a novel differentiable regularizer that, for the first time, approximates the full gradient of the LP relaxation gap with respect to network parameters, enabling direct optimization of key structural attributes such as big-M constants, the number of unstable neurons, and the LP relaxation gap itself. Built upon ReLU networks and MILP formulations, the method leverages gradients from LP dual variables and requires no custom automatic differentiation. Experiments demonstrate up to four orders of magnitude reduction in MILP solve time on nonconvex benchmark functions and two-stage stochastic programming problems, all while preserving predictive accuracy.

Technology Category

• Search and Optimization: Mixed Discrete/Continuous Search
• Machine Learning: Optimization
• Constraint Satisfaction and Optimization: Mixed Discrete/Continuous Optimization

Application Category

• Graph Algorithms and Modeling for the Web: Graph neural networks and deep learning approaches for Web-related graphs
• Search and Retrieval-Augmented AI: Web learning to rank, online learning, and counterfactual learning for ranking
• Economics, Online Markets and Human Computation: Incentives in network design for Web infrastructures and ecosystems

Abstract

ReLU neural networks trained as surrogate models can be embedded exactly in mixed-integer linear programs (MILPs), enabling global optimization over the learned function. The tractability of the resulting MILP depends on structural properties of the network, i.e., the number of binary variables in associated formulations and the tightness of the continuous LP relaxation. These properties are determined during training, yet standard training objectives (prediction loss with classical weight regularization) offer no mechanism to directly control them. This work studies training regularizers that directly target downstream MILP tractability. Specifically, we propose simple bound-based regularizers that penalize the big-M constants of MILP formulations and/or the number of unstable neurons. Moreover, we introduce an LP relaxation gap regularizer that explicitly penalizes the per-sample gap of the continuous relaxation at training points. We derive its associated gradient and provide an implementation from LP dual variables without custom automatic differentiation tools. We show that combining the above regularizers can approximate the full total derivative of the LP gap with respect to the network parameters, capturing both direct and indirect sensitivities. Experiments on non-convex benchmark functions and a two-stage stochastic programming problem with quantile neural network surrogates demonstrate that the proposed regularizers can reduce MILP solve times by up to four orders of magnitude relative to an unregularized baseline, while maintaining competitive surrogate model accuracy.

Problem

• neural network surrogate models
• mixed-integer linear programming
• LP relaxation
• big-M constants
• MILP tractability

Innovation

• MILP embedding
• LP relaxation gap
• big-M regularization
• unstable neuron penalty
• surrogate model training