Composing Non-Conjugate Factor Graphs with Closed-Form Variational Inference
Abstract
Stacking probabilistic building blocks into deeper architectures typically breaks closed-form inference. We show that closed-form inference can be preserved. We identify five factor-graph primitives: a bilinear factor, an exponential link, a Gamma prior, a Gaussian likelihood, and an equality node, and prove that any model composed from them admits closed-form variational message passing. The construction works because each primitive preserves a small set of message families: under mean-field factorization, messages on Gaussian variables remain Gaussian and messages on precision variables remain Gamma, while the only non-conjugate interface, the exponential link, remains tractable through the Gaussian moment-generating function and the sufficient statistics of the Gamma family. We demonstrate composition at increasing depth, from static ensembles through input-dependent gating to split-branch routing, and show that stacking routing layers encodes arbitrary decision trees, establishing universal function approximation with closed-form inference. Applied to ensemble time-series forecasting, the framework yields a Bayesian mixture of experts in which gating functions are inferred rather than learned, providing calibrated uncertainty over expert selection across five benchmark datasets.
Mykola Lukashchuk
PhD student
I am a PhD candidate at the Electrical Engineering department, Eindhoven University of Technology.
Wouter Kouw
Assistant professor
I am an assistant professor working on active inference for mobile robots.
İsmail Şenöz
Chief Scientist
LazyDynamics
Ismail Senoz is a co-founder & chief scientist of Lazy Dynamics
