Sequential algorithms and strongly stable functions

  • Authors:
  • Paul-André Melliès

  • Affiliations:
  • Equipe Preuves Programmes Systèmes, Université Paris VII, Paris Cedex, France

  • Venue:
  • Theoretical Computer Science - Game theory meets theoretical computer science
  • Year:
  • 2005

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Abstract

Intuitionistic proofs and PCF programs may be interpreted as functions between domains, or as strategies on games. The two kinds of interpretation are inherently different: static vs. dynamic, extensional vs. intentional. It is thus extremely instructive to compare and to connect them. In this article, we investigate the extensional content of the sequential algorithm hierarchy [-]SDS introduced by Berry and Curien. We equip every sequential game [T]SDS of the hierarchy with a realizability relation between plays and extensions. In this way, the sequential game [T]SDS becomes a directed acyclic graph, instead of a tree. This enables to define a hypergraph [T]HC on the extensions (or terminal leaves) of the game [T]SDS. We establish that the resulting hierarchy [-]HC coincides with the strongly stable hierarchy introduced by Bucciarelli and Ehrhard. We deduce from this a gametheoretic proof of Ehrhard's collapse theorem, which states that the strongly stable hierarchy coincides with the extensional collapse of the sequential algorithm hierarchy.