Remark. The induced law of a stopped interaction [ftip-0028]

Sutton and Barto's agent--environment interface in [sutton2018reinforcement, sec. 3.1] supplies alternating states, actions, and rewards. Kaelbling et al. separate hidden state and public observation in [kaelbling1998planning, Section 2]. A stopped interaction is determined by its initial law, transition--observation kernel, policy, and public stopping rule.

The induced interaction law uses four independently declared objects: the initial law \(\rho _0\) in Definition [ftip-001Y], the environment kernels \(K_t\) in Definition [ftip-001Z], the policy \(\pi _t\) in Definition [ftip-0023], and the stopping rule in Definition [ftip-0024]. On finite spaces, the probability of a length-\(n\) trajectory before applying its stopping indicator factors as \[ \rho _0(s_0,o_0) \prod _{t=0}^{n-1} \pi _t(a_t\mid h_t) K_t(s_{t+1},o_{t+1}\mid s_t,a_t). \] The event \(\{\tau _{\mathrm {stop}}=n\}\) selects the stopped trajectories.

This factorization supports later expectations, likelihood ratios, and off-policy reuse. It also exposes an identifiability limit: an outcome distribution by itself generally does not reveal which of the initial law, environment, policy, or stopping rule changed.