# Epi-constructivism: Decidable sets of computable numbers as foundational objects for mathematics

It is well known that the R, the set of real numbers, is an abstract set, where almost all its elements cannot be described in any finite language. We investigate possible approaches to what might be called an epi-constructionist approach to mathematics. While most constructive mathematics is concerned with constructive proofs, the agenda here is that the objects that we study, specifically the class of numbers that we study, should be an enumerable set of finite symbol strings. These might also be called decidable constructive real numbers, that is our class of numbers should be a computable sets of explicitly represented computable numbers. There have been various investigations of the computable numbers going back to Turing. Most are however not expressed constructively, rather computable is a property assigned to some of the abstract real numbers. Other definitions define constructive real numbers without reference to the abstract R, but the construction is undecidable, i.e., we cannot determine if a given construction represents a computable real number or not. For example, we may define a real as a computable convergent sequence of rationals, but cannot in general decide if a given computable sequence is convergent. This paper explores several specific classes of decidable constructive real numbers that could form foundational objects for what we might call an epi-constructionist mathematics.

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