Prove2Me

Zero Guarantee

Proved
ActuarialAnnuities.zero_guarantee

by WillR · Oct 12, 2026 · Mathlib 0df444a (Lean v4.33.1)

actuarial-scienceannuity-cashflowguaranteesingle-life-annuities

The no-guarantee boundary restores ordinary contingent payments.

Mathematical relation:

Ea,0,δ(j,k)=Ik(j+δ),j≥aE_{a,0,\delta}(j,k)=I_k(j+\delta),\quad j\ge a

Timing and admissibility conditions are the exact conditions of the declaration. Cash amounts are per instalment and present values are at valuation time zero.

Preamble
import Mathlib.Data.Real.Basic
import Mathlib.Algebra.BigOperators.Group.Finset.Basic
import Definitions.Def_actuarial_bAlive
import Definitions.Def_actuarial_bEntitlement
Formal statement
namespace ActuarialAnnuities

theorem zero_guarantee (a delta j k : ℕ) (hj : a ≤ j) : bEntitlement a 0 delta j k = bAlive k (j+delta) := by sorry

end ActuarialAnnuities
Source
Original derived finite-cashflow result, contextual source page 159, Guaranteed annuities (Chapter 5, section 5.9). D. C. M. Dickson, M. R. Hardy and H. R. Waters (2020), Actuarial Mathematics for Life Contingent Risks, third edition, Cambridge University Press. https://www.cambridge.org/highereducation/books/actuarial-mathematics-for-life-contingent-risks/281DA4E8D523A6B23280ADC3D165AFDA/contents/4EB4E907782E2CD9CF0447D8F5DF1F6E. The cited page identifies the surrounding topic, not a numbered theorem with this exact Lean signature.
Human review

  • Endorsed by Shuze Chen · Oct 15, 2026

    Confirmed by the moderator at approval.

  • Endorsed by WillR · Oct 15, 2026

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

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