
doi: 10.1007/pl00001594
With an identification of finite models and binary strings, a logic \(\mathcal L\) captures a complexity class \(\mathcal C\) iff every language in \(\mathcal C\) is the class of finite models of some sentence over \(\mathcal L\) and, conversely, every such model class is in \(\mathcal C\) [see for example \textit{H.-D. Ebbinghaus} and \textit{J. Flum}, Finite model theory (1995; Zbl 0841.03014)]. The authors show that existential monadic second-order logic with addition captures NLIN, i.e., the class of languages which can be recognized in linear time by nondeterministic RAMs. Here in addition the first-order part of the sentence which characterizes a given language can be restricted to the form \(\forall^{\ast}\exists^{\ast}\). This result answers a question raised by James F. Lynch who showed a similar result for the class NTIME(n) of languages which can be recognized in linear time by nondeterministic Turing machines: every language in NTIME(n) is the class of finite models of a sentence as above [see \textit{J. F. Lynch}, Math. Syst. Theory 15, 127-144 (1982; Zbl 0484.03020) and Comput. Complexity 2, No. 1, 40-66 (1992; Zbl 0752.68040)]. The authors remark that their logical characterization of the class NLIN has been refined in an article by \textit{F. Olive} published in Lect. Notes Comput. Sci. 1414, 360-372 (1998).
descriptive complexity theory, nondeterminism, Complexity of computation (including implicit computational complexity), computational complexity, [INFO.INFO-DS]Computer Science [cs]/Data Structures and Algorithms [cs.DS], monadic second-order logic, Model theory of finite structures, Modes of computation (nondeterministic, parallel, interactive, probabilistic, etc.), [INFO.INFO-DS] Computer Science [cs]/Data Structures and Algorithms [cs.DS], unary functions, Interpolation, preservation, definability, finite model theory, linear time, random access machine, NP-complete problem, Complexity classes (hierarchies, relations among complexity classes, etc.), Higher-order logic; type theory
descriptive complexity theory, nondeterminism, Complexity of computation (including implicit computational complexity), computational complexity, [INFO.INFO-DS]Computer Science [cs]/Data Structures and Algorithms [cs.DS], monadic second-order logic, Model theory of finite structures, Modes of computation (nondeterministic, parallel, interactive, probabilistic, etc.), [INFO.INFO-DS] Computer Science [cs]/Data Structures and Algorithms [cs.DS], unary functions, Interpolation, preservation, definability, finite model theory, linear time, random access machine, NP-complete problem, Complexity classes (hierarchies, relations among complexity classes, etc.), Higher-order logic; type theory
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