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For a fixed finite alphabet $\Sigma$, a formal language $L$ over $\Sigma$ is regular if there exists a deterministic finite automaton (DFA) over $\Sigma$ which accepts exactly $L$. I am interested in …

The pumping lemma for regular languages can be proved by considering a finite state automaton which recognizes the language studied, picking a string with a length greater than its number of states, a …

I'm interested in the (very restricted) subset of regular languages for which there is an automaton having the following property: for every letter $a$ of the alphabet, the automaton has at most one t …

A language $L$ is closed under circular shifts if, for every word $w = a_1 ... a_n$ and circular shift $w' = a_i ... a_n a_1 ... a_{i-1}$ of $w$, then $w \in L$ iff $w' \in L$. It is equivalent to req …

Consider a non-empty language $L$ of binary strings of length $n$. I can describe $L$ with a Boolean circuit $C$ with $n$ inputs and one output such that $C(w)$ is true iff $w \in L$: this is well-kno …

Fix a regular language $L$ on the alphabet $\{a, b\}$, and consider the following problem. I am given as input: some number $m \in \mathbb{N}$ of copies of the letter $a$, and some number $n \in \ma …

Consider a pushdown automaton $A$ with stack alphabet $\Gamma$. Let $L$ be the language on $\Gamma$ of the stack configurations encountered during accepting runs of $A$. Is $L$ a context-free language …

Possible Duplicate: Is finding the minimum regular expression an NP-complete problem? Let $\Sigma$ be an alphabet. Let $P$ and $N$ (the set of positive and negative examples) be two disjoin …