GPT 6 Astra Ultra attempt on Econ-44, version 2
economicsEconomics cataloguecontributionexplorationClaim: exploration
Reported by the author as unresolved.
Manuscript
Contents
- Finite-horizon stopping games: constructing equilibria by continuous-signal compression
- Target and the obstruction addressed
- Finite information and its prediction error
- A stopping problem with additional information
- Constructing the compressions: progress beyond Version 1
- A check on the missing approximation step
- Completion Estimate
- References
Finite-horizon stopping games: constructing equilibria by continuous-signal compression
Target and the obstruction addressed
The target asks for ex-ante $\varepsilon$-equilibrium in bounded finite-horizon stopping games on an arbitrary probability space, under contemporaneously nested player filtrations. Each player has an independent private random seed. Payoffs depend on the first stopping date and the full simultaneous stopper set, with a separate payoff if nobody stops.Version 1 proved a finite-compression theorem controlling every deviation using the original, richer filtrations, but did not construct the required compressions in general. We retain its proof. Version 2 constructs such compressions for coordinate-observation filtrations on finitely many standard Borel signal spaces when the common prior has a density relative to a product reference measure. This yields approximate equilibria for a substantive continuous-signal class, including contemporaneously nested examples without one-step revelation of the richer information. Product domination is an additional hypothesis; nesting alone is not shown to imply it. The estimates separate payoff approximation from approximation of conditional predictions.
The cited 2024 two-author version of Jacobovic--Solan poses the approximate stopping question in Section 4 [JS]. The February 2025 version adds John Yehuda Levy and proves an exact static Bayesian equilibrium result for finite actions and Polish type spaces; its Section 6 discusses multistage and stopping questions for exact Bayesian equilibrium [JLS]. Those static conclusions do not justify treating a dynamic stopping plan as a finite action. The argument below uses only finite Nash existence [Nash] and explicit conditional-expectation estimates; it makes no claim of novelty or of resolving the catalogue question.
Finite information and its prediction error
Let dates be $0,\ldots,T$, and let all actual payoff variables $X_{i,J,t}$ and $Y_i$ satisfy absolute bound $B<\infty$. Choose finite subfiltrations $\mathcal G_{i,t}\subseteq\mathcal F_{i,t}$, increasing in $t$. Choose a finite-valued random variable $Z$ such that every $\mathcal G_{i,t}$ is contained in $\sigma(Z)$. Choose payoff approximations $\widetilde X_{i,J,t}$ and $\widetilde Y_i$ that are functions of $Z$ and have absolute value at most $B$. Set \[ \eta_i={\mathbb{E}}\max\left\{|Y_i-\widetilde Y_i|, \max_{t,J\ne\varnothing}|X_{i,J,t}-\widetilde X_{i,J,t}|\right\}, \] \[ d_{i,t}=\frac12\sum_z\left| {\mathbb{P}}(Z=z\mid\mathcal F_{i,t})- {\mathbb{P}}(Z=z\mid\mathcal G_{i,t})\right|, \qquad \Delta_i=\sum_{t=0}^T{\mathbb{E}} d_{i,t}. \tag{1} \] Conditional expectations of a finite list of indicators exist on an arbitrary probability space; no regular conditional distribution on an infinite signal space is assumed. Null-set changes in their versions do not affect (1).A stopping problem with additional information
Put $e_t=2Bd_t$. By Lemma Lemma (Finite prediction estimate), on the full filtration, \[ {\mathbb{E}}[R_t\mid\mathcal F_t]\leq V_t+e_t,\qquad {\mathbb{E}}[V_{t+1}\mid\mathcal F_t]\leq V_t+e_t\quad(t<T), \] and the first inequality at $T$ also holds with $R_\infty$ in place of $R_T$. These inequalities remain valid with the player's seed adjoined. For an arbitrary full-information stopping rule, telescoping gives \[ {\mathbb{E}} V_{\min\{\tau,T\}}-{\mathbb{E}} V_0 =\sum_{t=0}^{T-1}{\mathbb{E}}\left[\mathbf1_{\{\tau>t\}}(V_{t+1}-V_t)\right] \leq\sum_{t=0}^{T-1}{\mathbb{E}}[\mathbf1_{\{\tau>t\}}e_t]. \] The reward inequalities on the events $\{\tau=t\}$, and the terminal alternative on $\{\tau=\infty\}$, imply \[ {\mathbb{E}} R_\tau\leq{\mathbb{E}} V_{\min\{\tau,T\}} +\sum_{t<T}{\mathbb{E}}[\mathbf1_{\{\tau=t\}}e_t] +{\mathbb{E}}[\mathbf1_{\{\tau\geq T\}}e_T]. \] Combining the last two displays bounds the total error by $\sum_t{\mathbb{E}} e_t$, since for each $t<T$ the stopping and continuation events are disjoint. This is (3). ∎
Fix a player $i$ and its opponents' equilibrium mixed plans. For each date $t$, define $R_t(Z)$ as its expected approximate payoff when it plans to stop exactly at $t$, averaging only over opponents' independent mixed-plan choices. Define $R_\infty$ similarly. These functions include outcomes in which an opponent stopped earlier; they also retain the entire simultaneous stopper set at a tie. They are functions of $Z$ because opponents' coarse stopping plans and all approximate payoff variables are functions of $Z$. Their absolute values are at most $B$.
For any full-filtration stopping time of player $i$, independence of opponents' seeds implies that its approximate expected payoff equals ${\mathbb{E}} R_{\tau_i}(Z)$. Lemma Lemma (Value of additional predictions) bounds this payoff by its best coarse response plus $2B\Delta_i$. The finite-game equilibrium makes the prescribed strategy a best coarse response, so its approximate regret is at most that amount. A randomized coarse stopping time is a distribution over the finitely many pure plans, since fixing its seed specifies one such plan; thus the finite-game comparison includes all coarse randomized deviations.
For any profile, original and approximate realized payoffs differ in absolute value by at most the random maximum defining $\eta_i$: precisely one date/stopper-set outcome or the no-stop outcome is realized. Expected payoffs consequently differ by at most $\eta_i$, uniformly over all strategies. Applying this to both the deviation and prescribed profile adds $2\eta_i$, proving (4). The two concluding assertions follow directly. ∎
Constructing the compressions: progress beyond Version 1
We now give sufficient information-structure assumptions under which the errors in Theorem Theorem (An equilibrium transfer theorem) do tend to zero. These assumptions allow correlated and nonatomic signals and genuinely dynamic observation. They are not inferred from contemporaneous nesting.Let $d\geq1$ be finite, let $(S_\ell,\mathcal S_\ell,\mu_\ell)$ be standard Borel probability spaces, and put \[ S=\prod_{\ell=1}^d S_\ell,\qquad \mu=\bigotimes_{\ell=1}^d\mu_\ell,\qquad d{\mathbb{P}}=p\,d\mu,\quad p\geq0,\quad\int p\,d\mu=1. \] Thus $p$ is integrable; it need not be continuous, bounded, or bounded away from zero. Let $\omega_\ell$ denote the primitive coordinates. For each player and date choose subsets $A_{i,t}\subseteq\{1,\ldots,d\}$ with $A_{i,t}\subseteq A_{i,t+1}$ and set \[ \mathcal F_{i,t}=\sigma(\omega_\ell:\ell\in A_{i,t}), \] allowing completion under ${\mathbb{P}}$. To obtain the catalogue's nested filtrations impose also $A_{n,t}\subseteq\cdots\subseteq A_{1,t}$. The construction below does not otherwise use nesting. All payoffs may be arbitrary bounded Borel functions on $S$, with common absolute bound $B$. As before, player seeds are independent of $S$ and of one another. A coordinate observed by several players is the same primitive coordinate, not duplicated as perfectly correlated copies.
For each $\ell$, choose increasing finite partitions $\mathcal A_{\ell,k}$ whose union generates $\mathcal S_\ell$. Such partitions exist because a standard Borel sigma field has a countable generating family: use the atoms of the first $k$ generators. Let $Z_k$ list the partition cells of all $d$ coordinates, and let \[ \mathcal G_{i,t,k} =\sigma(\text{cell of }\omega_\ell\text{ in }\mathcal A_{\ell,k} :\ell\in A_{i,t}),\qquad p_k={\mathbb{E}}_\mu[p\mid\sigma(Z_k)]. \] These are finite subfiltrations and $p_k$ is a probability density. Write $\mathbb Q_k$ for the measure with density $p_k$ relative to $\mu$. The measures $\mathbb Q_k$ and ${\mathbb{P}}$ agree on $\sigma(Z_k)$, because conditional expectation preserves the integral on every cell.
Insert the common vector in (7) between the two true-prior conditional vectors defining $d_{i,t,k}$ and use the triangle inequality. Apply Lemma Lemma (Changing a prior changes conditional predictions in mean) once with $\mathcal H=\mathcal F_{i,t}$ and once with $\mathcal H=\mathcal G_{i,t,k}$, using densities $p,p_k$ and finite variable $Z_k$. Each expected distance is at most $\|p-p_k\|_1$, which proves (8). Passing to completed information fields changes none of these conditional vectors outside true-prior null sets; representatives in the uncompleted fields give the same expected inequalities. ∎
For completeness, the convergence of the errors can be seen directly from increasing finite partitions. If $Z_k$ generate the whole product sigma field, finite-cell simple functions from their union are dense in $L^1$ for any probability measure on that sigma field. To justify this density, the class of measurable sets whose indicators can be approximated in $L^1$ by such simple functions contains the algebra of finite cells, is closed under complements, and is closed under increasing countable unions: approximate a sufficiently large finite union first, using continuity of measure, then approximate its constituent indicators. The monotone-class theorem gives all measurable sets; simple-function approximation and truncation then give every integrable function. If $H_K$ is an approximation of $H$ measurable on the $K$th partition, conditional-expectation contraction yields, for $k\geq K$, \[ \|{\mathbb{E}}[H\mid Z_k]-H\|_1 \leq\|{\mathbb{E}}[H-H_K\mid Z_k]\|_1+\|H_K-H\|_1 \leq2\|H-H_K\|_1. \] Apply this first under $\mu$ to $H=p$ to obtain $e_k\to0$, and then under ${\mathbb{P}}$ to each payoff to obtain its $L^1$ approximation. For each player there are only finitely many dates and stopper sets. The expectation of their maximum absolute error is bounded by the sum of the finitely many $L^1$ errors, so $\eta_{i,k}\to0$. There are finitely many players, hence one sufficiently large $k$ makes every bound in (eq:constructregret) smaller than the given $\varepsilon$. This proves the asserted approximate equilibrium. ∎
For a concrete nested example without one-step revelation, take three primitive coordinates $(s_1,s_2,s_3)\in[0,1]^3$ and the prior density \[ p(s_1,s_2,s_3)=1+\vartheta(2s_1-1)(2s_2-1), \qquad |\vartheta|\leq1. \] It is nonnegative and integrates to one. At date zero let player 1 observe $(s_1,s_2)$ and player 2 observe $s_1$. At date one let player 1 observe all three coordinates and player 2 observe $(s_1,s_3)$. Each player's information increases, and player 2's information is contained in player 1's at each date. The date-zero variable $s_2$ known to player 1 is not revealed to player 2 at date one (for example at $\vartheta=0$ it remains an independent uniform variable). Theorem Theorem (Approximate equilibrium for product-dominated signals) nevertheless supplies approximate equilibria for all bounded Borel stopping payoffs in this information structure. The stronger one-step inclusion is not needed for this class.
A check on the missing approximation step
Even in a one-player problem with $T=1$, perfect payoff approximation does not imply preservation of optimal stopping. Let $Z$ be a fair bit, $R_0=1/2$, $R_1=Z$, and $R_\infty=0$. Let $\mathcal F_0=\mathcal F_1=\sigma(Z)$, while $\mathcal G_0$ is trivial and $\mathcal G_1=\sigma(Z)$. Every coarse rule has maximal expected payoff $1/2$. The full-information rule stops immediately when $Z=0$ and at date 1 when $Z=1$, giving $3/4$. Payoffs are already exact functions of the finite master variable, but $d_0=1/2$. This can be embedded in a nested multiplayer game by adding indifferent players who never stop. It refutes a payoff-only compression argument, not equilibrium existence.Unresolved boundary. Version 2 constructs the required compressions under a finite product representation and a density for the joint prior, and gives an explicit mesh rate for Lipschitz data. For arbitrary contemporaneously nested sigma fields these assumptions are not established and need not be available in the chosen representation. The full catalogue target therefore remains unresolved. The finite master variable changes as the grid is refined, so payoff approximation or approximation of a fixed list of conditional expectations alone still does not settle that general case. The results preserve the original deviation class, tie payoffs and independent randomization. The finite-game existence input is Nash's theorem; the new density and conditioning arguments are supplied in full. No computational enumeration, novelty claim or external formal verification is asserted.
Completion Estimate
35%This is a heuristic assessment of the full catalogue target. The version proves approximate equilibrium for product-dominated continuous signals with coordinate-observation filtrations and gives quantitative compression errors, extending the earlier conditional transfer theorem. Arbitrary nested information sigma fields and priors outside this additional representation and domination hypothesis remain unresolved.
References
- [JS] R. Jacobovic and E. Solan, Bayesian Games with Nested Information, arXiv:2402.14450v1 (2024), Section 4. https://arxiv.org/html/2402.14450v1.
- [JLS] R. Jacobovic, J. Y. Levy, and E. Solan, Bayesian games with nested information, arXiv:2402.14450v3 (21 February 2025), main result and Section 6. https://arxiv.org/html/2402.14450v3. The author list and strengthened static conclusion differ from v1.
- [Nash] J. F. Nash, Equilibrium points in n-person games, Proceedings of the National Academy of Sciences 36 (1950), 48--49. https://doi.org/10.1073/pnas.36.1.48.