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PAC-MDP sample complexity of R-MAX

Statement

Let M be a bounded-reward finite discounted MDP, write S=∣S∣ and A=max⁡s∣A(s)∣, and fix 0≤ϵ<(1−γ)−1, 0<δ<1, and an admissible heuristic with normalized cap Vmax⁡. At ϵ=0, if V∗(s)=0 at every state then N0=0; no finite zero-tolerance extension holds uniformly over this class: already at γ=0, every fixed finite legal sample threshold m and value-iteration tolerance β admits an admissible one-state example for R-MAX with N0=∞ with positive probability. This does not assert failure for every fixed heuristic or model. The parameter formulas below are evaluated only when ϵ>0. Assume now ϵ>0. If ϵ≥Vmax⁡, then Nϵ=0. Otherwise put η=ϵ/4,b=η(1−γ)64Vmax⁡,m=⌈8(S+ln⁡(16SA/δ))b2⌉,β=η(1−γ)16, and run R-MAX with sample threshold m and value-iteration tolerance β. Let Eη={(s,a):U(s,a)≥V∗(s)−η} and ζ=(m+1)∣Eη∣. Define q=η/(4Vmax⁡), and define H and M by H={1,γ=0,max⁡{1,⌈ln⁡(1/[η(1−γ)])/(−ln⁡γ)⌉},0<γ<1,M=⌈2(ζ+1)+2ln⁡(2/δ)q⌉. Then P(Nϵ≤HM)≥1−δ. For the uninformative heuristic U(s,a)=Vmax⁡=(1−γ)−1, the bound is O~(S2A/[ϵ3(1−γ)6]), where O~ suppresses logarithmic factors.

For every ϵ>0, a bound uniform over all MDPs and admissible heuristics with these S,A,γ is obtained by setting Vˉ=11−γ,bˉ=ϵ(1−γ)2256,mˉ=⌈8(S+ln⁡(16SA/δ))bˉ2⌉,qˉ=ϵ(1−γ)16,ζˉ=(mˉ+1)SA, and Hˉ={1,γ=0,max⁡{1,⌈ln⁡(1/[(ϵ/4)(1−γ)])/(−ln⁡γ)⌉},0<γ<1,Mˉ=⌈2(ζˉ+1)+2ln⁡(2/δ)qˉ⌉. Then P(Nϵ≤HˉMˉ)≥1−δ. This bound depends only on S,A,ϵ,δ,γ.

Facts & Assumptions

Given: The MDP, heuristic, parameters, R-MAX algorithm, and bad-step count in the Statement.

[F1]

Adaptive empirical-model accuracy for R-MAX: with the stated m and β, the simultaneous optimism and model/policy accuracy conditions hold with probability at least 1−δ/2, and updates plus escapes total at most (m+1)∣Eη∣.

[F2]

Escape-event reduction for optimistic greedy learning: if those conditions hold with failure probability at most ρ, then taking B=W=Vmax⁡ and Qc=(1−γ)−1 gives N4η≤HM with failure probability at most 2ρ for the displayed H,M.

[F3]

Admissible upper heuristic for a discounted MDP: 0≤V∗(s)≤Vmax⁡ and U(s,a)≤Vmax⁡.

[F4]

The zero-tolerance boundary of PAC-MDP exploration: at zero tolerance the all-zero-optimal-value case has no bad steps; fixed finite legal R-MAX parameters admit an admissible one-state model with infinitely many zero-bad steps, with failure probability exceeding any prescribed confidence tolerance.

Proof

1.1givenF4cases

If ϵ=0, apply [F4] to obtain both stated endpoint branches. No finite sample-size or positive bonus is defined by substituting zero into the later formulas. For the remainder assume ϵ>0.

1.2givenF1F2F3algebra

If ϵ≥Vmax⁡, then V∗(s)−ϵ≤0 for every state and every continuation value is nonnegative, so no time is counted. Assume now ϵ<Vmax⁡. The stated η satisfies 0<η<Vmax⁡/4 and η(1−γ)<1. Apply [F1] with confidence parameter δ/2; its sample threshold is exactly the displayed m. On its event, R-MAX has the hypotheses of [F2] with ζ=(m+1)∣Eη∣.

1.3givenF1F2algebra

For the uninformative heuristic, ∣Eη∣≤SA and Vmax⁡=(1−γ)−1. The displayed b gives m=O((S+ln⁡(SA/δ))Vmax⁡2/[η2(1−γ)2]). Also H=O((1−γ)−1ln⁡(1/[η(1−γ)])), since −ln⁡γ≥1−γ for 0<γ<1, and M=O((Vmax⁡/η)(ζ+ln⁡(2/δ))). Using ζ≤(m+1)SA and η=ϵ/4 yields O~(S2A/[ϵ3(1−γ)6]).

2.1step 1.2F2algebra

Take ρ=δ/2 in [F2]. Since 4η=ϵ, the theorem gives P(Nϵ≤HM)≥1−δ.

3.1givenF1F2F3step 2.1algebra∎

Put Vˉ=(1−γ)−1. Since Vmax⁡≤Vˉ and ∣Eη∣≤SA, the instance parameters satisfy b≥bˉ, m≤mˉ, ζ≤ζˉ, and q≥qˉ, while the truncation horizon is H=Hˉ. If ϵ<Vmax⁡, then M≤Mˉ and step 2.1 yields the uniform bound. If ϵ≥Vmax⁡, then Nϵ=0. Thus the displayed HˉMˉ is a finite PAC-MDP bound in the sense of the Definition.

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