  Neural networks with wide layers have attracted significant attention due to
their equivalence to Gaussian processes, enabling perfect fitting of training
data while maintaining generalization performance, known as benign overfitting.
However, existing results mainly focus on shallow or finite-depth networks,
necessitating a comprehensive analysis of wide neural networks with
infinite-depth layers, such as neural ordinary differential equations (ODEs)
and deep equilibrium models (DEQs). In this paper, we specifically investigate
the deep equilibrium model (DEQ), an infinite-depth neural network with shared
weight matrices across layers. Our analysis reveals that as the width of DEQ
layers approaches infinity, it converges to a Gaussian process, establishing
what is known as the Neural Network and Gaussian Process (NNGP) correspondence.
Remarkably, this convergence holds even when the limits of depth and width are
interchanged, which is not observed in typical infinite-depth Multilayer
Perceptron (MLP) networks. Furthermore, we demonstrate that the associated
Gaussian vector remains non-degenerate for any pairwise distinct input data,
ensuring a strictly positive smallest eigenvalue of the corresponding kernel
matrix using the NNGP kernel. These findings serve as fundamental elements for
studying the training and generalization of DEQs, laying the groundwork for
future research in this area.
