Fedor Sandomirskiy

a game theorist @PrincetonEcon

[Fedor Sandomirskiy @fsandomi]

The 2025 Vita Kreps Prize goes to Jiarui Xie (Tsinghua) for “Games under the Tiered Deferred Acceptance Mechanism,” written as a junior in 2023–24. The committee received 30 exceptionally high-quality submissions. Congratulations, Jiarui!
#GameTheory #EconTwitter

[Fedor Sandomirskiy @fsandomi]

Please spread the word about the Vita Kreps International Student Prize in Game Theory!
Details: https://t.co/EaSicAVwxH
#GameTheory #EconTwitter #EconTwitter

[Fedor Sandomirskiy @fsandomi]

@NickArnosti @giorgiomartini The mechanism of Marek and Peter is equivalent to RSD: for each agent-house pair, the probability that this agent receives this object is the same. Their paper makes an important point that this equivalence cannot be replaced by equality when stating the conjecture

[Fedor Sandomirskiy @fsandomi]

@NickArnosti What you get by taking quotient space, as Yannai suggested, is known as the Lebesgue space L_1 https://t.co/1UOjAtSn4J
For compactness, see https://t.co/iHJdEvLhWw

[Fedor Sandomirskiy @fsandomi]

@NickArnosti Another observation: For n=4, truthfulness can be relaxed to upper-invariance & lower-invariance. In other words, you can drop swap-monotonicity in the decomposition of truthfulness by https://t.co/FFmkMH9YRN

[Fedor Sandomirskiy @fsandomi]

@NickArnosti Two papers about NP-hardness, maybe there are follow-ups:
https://t.co/Q9TJWew7uz
https://t.co/yt4diqgXdz

[Fedor Sandomirskiy @fsandomi]

@NickArnosti Absolute continuity is implied by ex-post efficiency. For n=4, RSD happens to be the unique mechanism even if the efficiency requirement is weakened to absolute continuity. I wonder if this generalizes beyond n=4.

[Fedor Sandomirskiy @fsandomi]

@NickArnosti A workaround could be to replace ex-post efficiency with an alternative notion tailored for allocation-matrix representation. One candidate is absolute continuity w.r.t. RSD: an agent cannot get an object with positive probability if she gets it with zero probability for RSD.

[Fedor Sandomirskiy @fsandomi]

@AramGrigoryan_ Now I see, thanks! The basic conjecture is for the number of agents equal to that of objects. More importantly, each agent must receive the same number of objects. Without this assumption, picking an agent at random and allocating all the objects to her satisfies the axioms.

[Fedor Sandomirskiy @fsandomi]

@NickArnosti I once wrote a program that generates a human-readable proof for n=4 agents and objects. Still struggle to publish it as people say it is hard to follow because of the length. Just 70000 pages. https://t.co/DBy2urWHnf

[Fedor Sandomirskiy @fsandomi]

@AramGrigoryan_ @NickArnosti Aram, could you clarify how rule 2 works? Maybe it coincides with RSD by itself?

By the way, for 3 agents and objects, it is known that RSD is the unique mechanism satisfying the axioms (Bogomolnaia, Moulin 2001). For 4, this also seems to be true (see below)