34 research outputs found
Independence-friendly logic without Henkin quantification
We analyze the expressive resources of IF logic that do not stem from Henkin (partially-ordered) quantification. When one restricts attention to regular IF sentences, this amounts to the study of the fragment of IF logic which is individuated by the game-theoretical property of action recall (AR). We prove that the fragment of prenex AR sentences can express all existential second-order properties. We then show that the same can be achieved in the non-prenex fragment of AR, by using “signalling by disjunction” instead of Henkin or signalling patterns. We also study irregular IF logic (in which requantification of variables is allowed) and analyze its correspondence to regular IF logic. By using new methods, we prove that the game-theoretical property of knowledge memory is a first-order syntactical constraint also for irregular sentences, and we identify another new first-order fragment. Finally we discover that irregular prefixes behave quite differently in finite and infinite models. In particular, we show that, over infinite structures, every irregular prefix is equivalent to a regular one; and we present an irregular prefix which is second order on finite models but collapses to a first-order prefix on infinite models.Peer reviewe
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Mathematical Logic: Proof Theory, Constructive Mathematics
[no abstract available
Regular Representations of Uniform TC^0
The circuit complexity class DLOGTIME-uniform AC^0 is known to be a modest
subclass of DLOGTIME-uniform TC^0. The weakness of AC^0 is caused by the fact
that AC^0 is not closed under restricting AC^0-computable queries into simple
subsequences of the input. Analogously, in descriptive complexity, the logics
corresponding to DLOGTIME-uniform AC^0 do not have the relativization property
and hence they are not regular. This weakness of DLOGTIME-uniform AC^0 has been
elaborated in the line of research on the Crane Beach Conjecture. The
conjecture (which was refuted by Barrington, Immerman, Lautemann, Schweikardt
and Th{\'e}rien) was that if a language L has a neutral letter, then L can be
defined in first-order logic with the collection of all numerical built-in
relations, if and only if L can be already defined in FO with order.
In the first part of this article we consider logics in the range of AC^0 and
TC^0. First we formulate a combinatorial criterion for a cardinality quantifier
C_S implying that all languages in DLOGTIME-uniform TC^0 can be defined in
FO(C_S). For instance, this criterion is satisfied by C_S if S is the range of
some polynomial with positive integer coefficients of degree at least two. In
the second part of the paper we first adapt the key properties of abstract
logics to accommodate built-in relations. Then we define the regular interior
R-int(L) and regular closure R-cl(L), of a logic L, and show that the Crane
Beach Conjecture can be interpreted as a statement concerning the regular
interior of first-order logic with built-in relations B. We show that if B={+},
or B contains only unary relations besides the order, then R-int(FO_B)
collapses to FO with order. In contrast, our results imply that if B contains
the order and the range of a polynomial of degree at least two, then R-cl(FO_B)
includes all languages in DLOGTIME-uniform TC^0
Expressiveness and complexity of graph logic
We investigate the complexity and expressive power of the spatial logic for querying graphs introduced by Cardelli, Gardner and Ghelli (ICALP 2002).We show that the model-checking complexity of versions of this logic with and without recursion is PSPACE-complete. In terms of expressive power, the version without recursion is a fragment of the monadic second-order logic of graphs and we show that it can express complete problems at every level of the polynomial hierarchy. We also show that it can define all regular languages, when interpretation is restricted to strings. The expressive power of the logic with recursion is much greater as it can express properties that are PSPACE-complete and therefore unlikely to be definable in second-order logic
Panel on “Past and future of computer science theory”
The twenty-ninth edition of the SEBD (Italian Symposium on Advanced Database Systems), held on 5-9 September 2021 in Pizzo (Calabria Region, Italy), included a joint seminar on “Reminiscence of TIDB 1981” with invited talks given by some of the participants to the Advanced Seminar on Theoretical Issues in Databases (TIDB), which took place in the same region exactly forty years earlier. The joint seminar was concluded by a Panel on “The Past and the Future of Computer Science Theory” with the participation of four distinguished computer science theorists (Ronald Fagin, Georg Gottlob, Christos Papadimitriou and Moshe Vardi), who were interviewed by Giorgio Ausiello, Maurizio Lenzerini, Luigi Palopoli, Domenico Saccà and Francesco Scarcello. This paper reports the summaries of the four interviews