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We prove some new theorems in additive number theory, using novel techniques from automata theory and formal languages. As an example of our method, we prove that every natural number > 2 5 is the sum of at most three natural numbers whose base- 2 representation has an equal number of 0 's and 1 's. [ABSTRACT FROM AUTHOR]

We consider a set of eight natural operations on formal languages (Kleene closure, positive closure, complement, prefix, suffix, factor, subword, and reversal), and compositions of them. Ifxandyare compositions, we sayxis equivalent toyif they have the same effect on all languagesL. We prove that the number of equivalence classes of these eight operations is finite. This implies that the orbit of any languageLunder the elements of the monoid is finite and bounded, independent ofL. This generalizes previous results about complement, Kleene closure, and positive closure. We also estimate the number of distinct languages generated by various subsets of these operations. [ABSTRACT FROM AUTHOR]

A filtration of a formal language L by a sequence s maps L to the set of words formed by taking the letters of words of L indexed only by s. We consider the languages resulting from filtering by all arithmetic progressions. If L is regular, it is easy to see that only finitely many distinct languages result; we give bounds on the number of distinct languages in terms of the state complexity of L. By contrast, there exist CFL's that give infinitely many distinct languages as a result. We use our technique to show that two related operations, including diag (which extracts the diagonal of words of square length arranged in a square array), preserve regularity but do not preserve context-freeness. [ABSTRACT FROM AUTHOR]

A famous theorem of Kuratowski states that, in a topological space, at most 14 distinct sets can be produced by repeatedly applying the operations of closure and complement to a given set. We re-examine this theorem in the setting of formal languages, where by "closure" we mean either Kleene closure or positive closure. We classify languages according to the structure of the algebras they generate under iterations of complement and closure. There are precisely 9 such algebras in the case of positive closure, and 12 in the case of Kleene closure. We study how the properties of being open and closed are preserved under concatenation. We investigate analogues, in formal languages, of the separation axioms in topological spaces; one of our main results is that there is a clopen partition separating two words if and only if the words do not commute. We can decide in quadratic time if the language specified by a DFA is closed, but if the language is specified by an NFA, the problem is PSPACE-complete. [ABSTRACT FROM AUTHOR]