Mixed neural network Gaussian processes

This paper makes two contributions. Firstly, it introduces mixed compositional kernels and mixed neural network Gaussian processes (NGGPs). Mixed compositional kernels are generated by composition of probability generating functions (PGFs). A mixed NNGP is a Gaussian process (GP) with a mixed compositional kernel, arising in the infinite-width limit of multilayer perceptrons (MLPs) that have a different activation function for each layer. Secondly, $\theta$ activation functions for neural networks and $\theta$ compositional kernels are introduced by building upon the theory of branching processes, and more specifically upon $\theta$ PGFs. While $\theta$ compositional kernels are recursive, they are expressed in closed form. It is shown that $\theta$ compositional kernels have non-degenerate asymptotic properties under certain conditions. Thus, GPs with $\theta$ compositional kernels do not require non-explicit recursive kernel evaluations and have controllable infinite-depth asymptotic properties. An open research question is whether GPs with $\theta$ compositional kernels are limits of infinitely-wide MLPs with $\theta$ activation functions.