Absolute continuity, supports and idempotent splitting in categorical probability
This paper addresses the categorical decomposition of probabilistic structures in Markov categories, establishing a rigorous categorical foundation for absolute continuity, support sets, and idempotent splittings. Methodologically, it introduces, for the first time, an idempotent splitting theorem for measurable Markov kernels within the category of standard Borel spaces, and distills a general splitting criterion applicable to arbitrary Markov categories. The main contributions are: (1) a precise internal categorical definition of support sets; (2) a proof that every idempotent measurable Markov kernel between standard Borel spaces admits a splitting; and (3) a rigorous, broadly applicable theoretical framework for structural decomposition of probabilistic models, categorical modeling of stochastic processes, and abstract Bayesian inference.