Universality and Almost Decidability

We present and study new definitions of universal and programmable universal unary functions and consider a new simplicity criterion: almost decidability of the halting set. A set of positive integers S is almost decidable if there exists a decidable and generic i.e. a set of natural density one set whose intersection with S is decidable. Every decidable set is almost decidable, but the converse implication is false. We prove the existence of infinitely many universal functions whose halting sets are generic negligible, i.e. have density zero and not almost decidable. One result---namely, the existence of infinitely many universal functions whose halting sets are generic negligible and not almost decidable---solves an open problem in [9]. We conclude with some open problems.