Abstract: A language L over an alphabet A is said to have a neutral letter if there is a letter e in A such that inserting or deleting e's from any word in A* does not change its membership (or non-membership) in L.
The presence of a neutral letter affects the definability of a language in first-order logic. It was conjectured that it renders all numerical predicates apart from the order predicate useless, i.e., that if a language L with a neutral letter is not definable in first-order logic with linear order, then it is not definable in first-order logic with any set A of numerical predicates. We investigate this conjecture in detail, showing that it fails already for A={+,x}, or, possibly stronger, for any set A that allows counting up to the m times iterated logarithm, lg(m), for any constant m.
On the positive side, we prove the conjecture for the case of all monadic numerical predicates, for A={+}, for the fragment BC(Sigma1) of first-order logic, and for binary alphabets.
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