Computability

Given a set A in the unit interval and the associated Lebesgue measure λ, it is a natural question whether we may (in some sense) compute the measure λ ( A ) in terms of the set A. Under the moniker measure theoretic uniformity, Tanaka and Sacks have (independently) provided a positive answer for the well-known class of hyperarithmetical sets of reals, and provided a basis theorem for such sets of positive measure. The hyperarithmetical sets are exactly the sets computable in terms of the functional 2 E , in the sense of Kleene’s S1–S9. In turn, Kleene’s 2 E essentially corresponds to arithmetical comprehension as in ACA 0 . In this paper, we generalise the aforementioned results to the ‘next level’, namely Π 1 1 -CA 0 , in the form of the Suslin functional, or the equivalent hyperjump. We also generalise the Tanaka-Sacks basis theorem to sets of positive measure that are semi-computability relative to the Suslin functional. Finally, we discuss similar generalisations for infinite time Turing machines.

We show that if 1 ⩽ W F and F ⋆ F ⩽ W F , then F ◇ ⩽ W F , where ⋆ and ◇ are the following operations in the Weihrauch lattice: ⋆ is the compositional product, which allows the use of two principles in sequence, while the diamond operator ◇ allows an arbitrary but finite number of uses of the given principle in sequence. This answers a question of Pauly.

We prove the following result: there is a family R = ⟨ R 0 , R 1 , … ⟩ of subsets of ω such that for every stable coloring c : [ ω ] 2 → k hyperarithmetical in R and every finite collection of Turing functionals, there is an infinite homogeneous set H for c such that none of the finitely many functionals map R ⊕ H to an infinite cohesive set for R. This provides a partial answer to a question in computable combinatorics, whether COH is omnisciently computably reducible to SRT 2 2 .

We establish upper bounds on bit complexity of computing solution operators for symmetric hyperbolic systems of PDEs, combining symbolic and approximate algorithms to obtain the solutions with guaranteed prescribed precision. Restricting to algebraic real inputs allows us to use the classical (“discrete”) bit complexity concept.

The term ‘Halting Problem’ arguably refers to computer science’s most celebrated impossibility result and to the core notion underlying the language-theoretic approach to security. Computer professionals often ignore the Halting Problem however. In retrospect, this is not too surprising given that several advocates of computability theory implicitly follow Christopher Strachey’s alleged 1965 proof of his Halting Problem (which is about executable – i.e., hackable – programs) rather than Martin Davis’s correct 1958 version or his 1994 account (each of which is solely about mathematical objects). For the sake of conceptual clarity, particularly for researchers pursuing a coherent science of cybersecurity, I will scrutinize Strachey’s 1965 line of reasoning – which is widespread today – both from a charitable, historical angle and from a critical, engineering perspective.

We consider how changes in transfinite machine architecture can sometimes alter substantially their capabilities. We approach the subject by answering three open problems touching on: firstly differing halting time considerations for machines with multiple as opposed to single heads, secondly space requirements, and lastly limit rules. We: 1) use admissibility theory, Σ 2 -codes and Π 3 -reflection properties in the constructible hierarchy to classify the halting times of ITTMs with multiple independent heads; the same for Ordinal Turing Machines which have On length tapes; 2) determine which admissible lengths of tapes for transfinite time machines with long tapes allow the machine to address each of their cells – a question raised by B. Rin; 3) characterise exactly the strength and behaviour of transfinitely acting Blum–Shub–Smale machines using a Liminf rule on their registers – thereby establishing there is a universal such machine. This is in contradistinction to the machine using a ‘continuity’ rule which fails to be universal.