Reversal-bounded multipushdown machines

A note on the undecidability of contextfreeness

“…Some of the results presented hère are either well-known or can be concluded from theorems given by [6,12].…”

“…Throughout the following chapters we will assume that the language families F occurring are "effective" [12], i. e., there is a spécification of F (grammar, automaton, ...) such that one can effectively enumerate the languages in F, each language in F is recursively enumerable and there is a partial recursive function which assigns "yes" to the pair consisting of the "name" of the language L in F and the word w if and only if weL (see also [6] and L(I) : = 3-COPY(i) u H (S,).…”

Space classes, intersection of languages and bounded erasing homomorphisms

“…Since NTIME (n) = H (DCF ADCF ADCF) [6] and NTIME (0 = H t (NTIME (n)) [7] it follows that NTIME (t) = H t (DCF A DCF A DCF). This représentation can be improved if a class !F of bounding functions is closed under squaring, i. e. if ƒ G ^ then (f(n)) 2 e^ (e. g. the classes POL or EXP). Proof: Lemma 1 bases on an encoding of a tape bounded by 5 by three counters whose length is bounded by EXPs.…”

Fixed Point Languages, Equality Languages, and Representation of Recursively Enumerable Languages

“…Such representation theorems can be used to show that certain problems are undecidable for languages in the subclass [23,24], to show that the subclass is not closed under the operations [16], or to show the existence of simple AFL generators of RE [3,15]. Since each RE language is the homomorphic image of the set of computations, suitably coded as strings, of a Turing machine (accepting or generating the language), these results are usually obtained by representing the class of"computation languages" of Turing machines.…”

“…Representing the class of recursively enumerable languages (RE) through operations on its subclasses is a traditional topic within formal language theory (see, e.g., [3,[14][15][16][22][23][24]). Whenever one finds a representation of the class RE through one of its subclasses X, • such a result sheds light on both RE and X.…”