Approximations play a pivotal role in verifying deep neural networks (DNNs). Existing approaches typically rely on either single-neuron approximations (simpler to design but less precise) or multi-neuron approximations (higher precision but significantly more complex to construct). Between them, a notable gap exists.

This work bridges the gap. The idea is to lift single-neuron approximations into multi-neuron approximations with precision gain. To this end, we formulate the approximation transition as a novel problem, named Convex Approximation Lifting (CAL), and propose a constructive approach, Support Triangle Machine (STM), to solving it. STM is grounded in two core insights: (i) there exists a simple geometric structure, called the support triangle, along with an efficient triangle lifting technique that connects single-neuron approximations and multi-neuron approximations; and (ii) typical single-neuron approximations can be easily decomposed into multiple atomically liftable components. Specifically, given a CAL instance, STM constructs a multi-neuron approximation by iteratively processing each output coordinate. For each coordinate, it decomposes the single-neuron approximation into several linear parts, lifts each of them using the triangle lifting technique, and then synthesize an intermediate approximation, which later servers as input for the next iteration.

We theoretically prove the correctness of STM and empirically evaluate its performance on a variety of CAL problems and DNN verification tasks. Experimental results demonstrate STM's broad applicability, improved precision, and sustained efficiency. Beyond DNN verification, STM has the potential to facilitate approximation construction process in more general tasks, and we expect it to catalyze further research in related fields.