Prove2Me

First-death symmetry

Proved
actuarial_d_first_swap

by WillR · Oct 13, 2026 · Mathlib 0df444a (Lean v4.33.1)

actuarial-sciencejoint-lifelife-contingencymultiple-lives

First-death symmetry. First-death settlement timing is symmetric even with dependent mortality. The settlement date, survivor entitlement, financial discount and relevant joint-law assumptions are those declared in the formal model.

Mathematical relation:

C1(i,j)=C1(j,i)C_1(i,j)=C_1(j,i)
Preamble
import Mathlib.Data.Real.Basic
import Mathlib.Algebra.BigOperators.Group.Finset.Basic
import Definitions.Def_actuarial_d_firstPath
import Definitions.Def_actuarial_d_secondPath
import Definitions.Def_actuarial_d_xFirstPath
import Definitions.Def_actuarial_d_yFirstPath
import Definitions.Def_actuarial_d_xSecondPath
import Definitions.Def_actuarial_d_ySecondPath
import Definitions.Def_actuarial_d_tiePath

open ActuarialJointD
Formal statement
theorem actuarial_d_first_swap (h i j : ℕ) (d : ℕ → ℝ) : firstPath h d i j = firstPath h d j i := by sorry
Source
Originally derived two-life finite-grid definition or theorem, motivated by S. David Promislow (2015), Fundamentals of Actuarial Mathematics, third edition, Chapter 10, Section 10.8.1, p. 153. The published reference supplies actuarial context, while the formal statement and joint-law conventions are new. https://www.oreilly.com/library/view/fundamentals-of-actuarial/9781118782521/
Human review

  • Endorsed by Shuze Chen · Oct 16, 2026

    Confirmed by the moderator at approval.

  • Endorsed by WillR · Oct 16, 2026

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

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