29 research outputs found
Higher-Order Pushdown Systems with Data
We propose a new extension of higher-order pushdown automata, which allows to
use an infinite alphabet. The new automata recognize languages of data words
(instead of normal words), which beside each its letter from a finite alphabet
have a data value from an infinite alphabet. Those data values can be loaded to
the stack of the automaton, and later compared with some farther data values on
the input. Our main purpose for introducing these automata is that they may
help in analyzing normal automata (without data). As an example, we give a
proof that deterministic automata with collapse can recognize more languages
than deterministic automata without collapse. This proof is simpler than in the
no-data case. We also state a hypothesis how the new automaton model can be
related to the original model of higher-order pushdown automata.Comment: In Proceedings GandALF 2012, arXiv:1210.202
A Type System Describing Unboundedness
We consider nondeterministic higher-order recursion schemes as recognizers of languages of finite words or finite trees. We propose a type system that allows to solve the simultaneous-unboundedness problem (SUP) for schemes, which asks, given a set of letters A and a scheme G, whether it is the case that for every number n the scheme accepts a word (a tree) in which every letter from A appears at least n times. Using this type system we prove that SUP is (m-1)-EXPTIME-complete for word-recognizing schemes of order m, and m-EXPTIME-complete for tree-recognizing schemes of order m. Moreover, we establish the reflection property for SUP: out of an input scheme G one can create its enhanced version that recognizes the same language but is aware of the answer to SUP
The MSO+U theory of (N, <) is undecidable
We consider the logic MSO+U, which is monadic second-order logic extended
with the unbounding quantifier. The unbounding quantifier is used to say that a
property of finite sets holds for sets of arbitrarily large size. We prove that
the logic is undecidable on infinite words, i.e. the MSO+U theory of (N,<) is
undecidable. This settles an open problem about the logic, and improves a
previous undecidability result, which used infinite trees and additional axioms
from set theory.Comment: 9 pages, with 2 figure
Cost Automata, Safe Schemes, and Downward Closures
Higher-order recursion schemes are an expressive formalism used to define
languages of possibly infinite ranked trees. They extend regular and
context-free grammars, and are equivalent to simply typed -calculus
and collapsible pushdown automata. In this work we prove, under a syntactical
constraint called safety, decidability of the model-checking problem for
recursion schemes against properties defined by alternating B-automata, an
extension of alternating parity automata for infinite trees with a boundedness
acceptance condition. We then exploit this result to show how to compute
downward closures of languages of finite trees recognized by safe recursion
schemes.Comment: accepted at ICALP'2
A Recursive Approach to Solving Parity Games in Quasipolynomial Time
Zielonka's classic recursive algorithm for solving parity games is perhaps the simplest among the many existing parity game algorithms. However, its complexity is exponential, while currently the state-of-the-art algorithms have quasipolynomial complexity. Here, we present a modification of Zielonka's classic algorithm that brings its complexity down to , for parity games of size with priorities, in line with previous quasipolynomial-time solutions.</jats:p