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A zero-year Markov policy has its terminal cost

Proved
ActuarialValuation.finiteEntropicPolicyValue_zero

by WillR · Oct 9, 2026 · Mathlib 0df444a (Lean v4.33.1)

actuarial-scienceentropic-bellmanrisk-sensitive-markov-control

For a horizon of zero policy years there is no action, no transition and no covered future cashflow. The fixed policy's recursively valued insurance cost equals the supplied terminal valuation for the current state. The result holds for arbitrary risk and discount parameters because the stage operator is never invoked.

Mathematical statement

J0π(s)=h(s)J^\pi_0(s)=h(s)J0π​(s)=h(s)
Preamble
import Mathlib
import Definitions.Def_actuarial_finiteEntropicPolicyValue
Formal statement
namespace ActuarialValuation

theorem finiteEntropicPolicyValue_zero {S A : Type*}
  [Fintype S] (P : S → A → S → ℝ)
  (cost : S → A → S → ℝ) (beta gamma : ℝ)
  (terminal : S → ℝ) (policy : ℕ → S → A) (s : S) :
  finiteEntropicPolicyValue P cost beta gamma terminal policy 0 s =
    terminal s := by sorry

end ActuarialValuation
Source
Bäuerle and Glauner (2022), Markov decision processes with recursive risk measures, European Journal of Operational Research 296(3), Definition 4.6, Theorem 4.7, equation (4.2), DOI https://doi.org/10.1016/j.ejor.2021.04.030; Bäuerle and Jaśkiewicz (2024), entropic certainty equivalent, Example 1(a), equation (2), DOI https://doi.org/10.1007/s00186-024-00857-0; derived finite-state discrete-time insurer cost specialisation
Read-back

What the Lean code literally says, in plain math · gpt-6

Independent mathematical reading. Evaluating the recursively defined cost for a fixed Markov policy at horizon zero returns the terminal function exactly at the chosen state. There is no state transition or stage cost in that branch. The equality does not depend on whether transition weights are stochastic or the cost-risk aversion gamma is positive, because the entropic stage operator is not called at this horizon.

Human review
  • Endorsed by Shuze Chen · Oct 9, 2026

    Confirmed by the moderator at approval.

  • Endorsed by WillR · Oct 9, 2026

    Confirmed by the mission captain (proposal self-audit).

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