Quotient Complexities of Atoms in Regular Ideal Languages

Abstract: A (left) quotient of a language $L$ by a word $w$ is the language $w^{-1}L={x\mid wx\in L}$. The quotient complexity of a regular language $L$ is the number of quotients of $L$; it is equal to the state complexity of $L$, which is the number of states in a minimal deterministic finite automaton accepting $L$. An atom of $L$ is an equivalence class of the relation in which two words are equivalent if for each quotient, they either are both in the quotient or both not in it; hence it is a non-empty intersection of complemented and uncomplemented quotients of $L$. A right (respectively, left and two-sided) ideal is a language $L$ over an alphabet $\Sigma$ that satisfies $L=L\Sigma<sup>*$ (respectively, $L=\Sigma<sup>*L$ and $L=\Sigma<sup><em>L\Sigma^</em>$). We compute the maximal number of atoms and the maximal quotient complexities of atoms of right, left and two-sided regular ideals.