Characterizing regular languages with polynomial densities
A language L is said to have a polynomial density if the function pL.(n)=¦L∩∑n¦ of L is bounded by a polynomial. We show that the function p R(n) of a regular language R is O(n k ), for some k≥0, if and only if R can be represented as a finite union of the regular expressions of the form xy 1 * z1 ...y t * zt with a nonnegative integer t≤k+1, where x,y 1,z 1,..., yt, zt are all strings in ∑*.
We prove a characterization for the (restricted) starheight-one languages. We show that a regular language is starheight one if and only if it is the image of a regular language of polynomial density under a finite substitution. We also show that the set of starheight-one languages includes all the regular languages with polynomial densities and their complements.
KeywordsRegular languages population functions of languages density functions polynomial densities starheight-one languages
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