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Normalizable linear orders and generic computations in finite models
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Normalizable linear orders and generic computations in finite models
Alexei P. Stolboushkin1 and Michael A. Taitslin3
| (1) |
UCLA Department of Mathematics, Los Angeles, CA 90095, USA
, US |
| (2) |
Fourth Dimension Software, 555 Twin Dolphin Dr., Redwood City, CA 94065-2102, USA. e-mail: aps@4ds.com; aps@math.ucla.edu
, US |
| (3) |
Department of Computer Science, Tver State University, Tver, Russia 170000. e-mail: mat@tversu.ac.ru
, RU |
Abstract. Numerous results about capturing complexity classes of queries by means of logical languages work for ordered structures
only, and deal with non-generic, or order-dependent, queries. Recent attempts to improve the situation by characterizing wide
classes of finite models where linear order is definable by certain simple means have not been very promising, as certain
commonly believed conjectures were recently refuted (Dawar's Conjecture). We take on another approach that has to do with
normalization of a given order (rather than with defining a linear order from scratch). To this end, we show that normalizability
of linear order is a strictly weaker condition than definability (say, in the least fixpoint logic), and still allows for
extending Immerman-Vardi-style results to generic queries. It seems to be the weakest such condition. We then conjecture that
linear order is normalizable in the least fixpoint logic for any finitely axiomatizable class of rigid structures. Truth of
this conjecture, which is a strengthened version of Stolboushkin's conjecture, would have the same practical implications
as Dawar's Conjecture. Finally, we suggest a series of reductions of the two conjectures to specialized classes of graphs,
which we believe should simplify further work.
Received: 13 July 1996
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