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Finite-horizon simulation bound for one fixed policy

Statement

Let M=(S,(A(s))s∈S,r,P) and M^=(S,(A(s))s∈S,r^,P^) be finite models on a common nonempty finite state set and common nonempty finite action sets, with r,r^∈[0,1] and probability transition rows. Fix a horizon H≥1, a common initial state law μ, and one possibly time-dependent randomized Markov policy π=(πt)t=0H−1. Let dt and d^t be the state occupancies under M and M^, respectively, starting from d0=d^0=μ. Define

CHπ={(s,a):dt(s)πt(a∣s)>0 or d^t(s)πt(a∣s)>0 for some 0≤t<H}.

Suppose ∣r(s,a)−r^(s,a)∣≤ϵr and TV⁡(P(⋅∣s,a),P^(⋅∣s,a))≤ϵp for every (s,a)∈CHπ, where TV⁡(p,q)=12∑s′∣p(s′)−q(s′)∣. Then

∣JH(M,μ,π)−JH(M^,μ,π)∣≤Hϵr+(H2)ϵp.

The guarantee concerns this fixed policy and the stated reachable pairs; it does not bound the policy selected by optimizing the learned model.

Facts & Assumptions

Given: The finite models, common law, horizon, fixed policy, reachable-pair error bounds, and notation in the Statement.

[F1]

Finite learned transition and reward models: every transition row is a probability distribution on the common finite state set, and both reward means lie in [0,1].

[F2]

Finite-horizon return under a Markov policy: each model's state occupancy is propagated by the policy-weighted transition row, and its return is the sum of the policy-weighted expected rewards for times 0 through H−1.

[F3]

One-step total-variation propagation under a fixed policy: if transition-row total-variation error is at most ϵp on all pairs with positive learned state-action occupancy under a policy row, then the next-state total-variation distance is at most the current distance plus ϵp.

[F4]

Half-ℓ1 formula for total variation on a countable space: on the finite state set, total variation is one half of the sum of absolute mass differences.

[F5]

The triangle inequality: for real u,v, ∣u+v∣≤∣u∣+∣v∣.

[F6]

Triangle inequality for finite sums: for every finite initial segment, the absolute value of a sum of reals is at most the sum of their absolute values.

Proof

Given: The two finite models, common initial law, fixed policy, and error bounds from the Statement.

1.1givenF1F2F3algebra

At time 0, TV⁡(d0,d^0)=0. If 0≤t<H−1, every pair with d^t(s)πt(a∣s)>0 belongs to CHπ, so its transition rows satisfy [F3]. Applying [F3] to the policy at time t gives TV⁡(dt+1,d^t+1)≤TV⁡(dt,d^t)+ϵp. Induction yields TV⁡(dt,d^t)≤tϵp for every 0≤t<H.

2.1givenF1F2F4F5step 1.1algebra

For a fixed time t, set gt(s)=∑aπt(a∣s)r(s,a) and g^t(s)=∑aπt(a∣s)r^(s,a). These functions lie in [0,1] by [F1]. For xs=dt(s)−d^t(s), one has ∑sxs=0; by [F4] the total positive mass and total negative mass of (xs) both equal TV⁡(dt,d^t). Since 0≤gt(s)≤1, it follows that ∣∑sxsgt(s)∣≤TV⁡(dt,d^t). Also, every pair with d^t(s)πt(a∣s)>0 lies in CHπ, so ∣∑s,ad^t(s)πt(a∣s)(r(s,a)−r^(s,a))∣≤ϵr. Decomposing the difference of the two time-t expected rewards and applying [F5] gives an absolute difference at most tϵp+ϵr by step 1.1.

3.1F2F6step 2.1algebra∎

By [F2], the return difference is the sum of the H timewise expected-reward differences. The finite-sum triangle inequality [F6] and step 2.1 therefore give ∣JH(M,μ,π)−JH(M^,μ,π)∣≤∑t=0H−1(tϵp+ϵr)=Hϵr+H(H−1)2ϵp=Hϵr+(H2)ϵp. The arithmetic sum evaluates to H(H−1)/2.

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