This paper describes an experiment to formalize sophisticated mathematics in the ℕaproche proof assistant which uses natural language input and a first-order internal logic. We view this as a contribution to the ongoing discussion whether formal systems for research mathematics require complex, computer-orientated type systems or whether approaches closer to traditional mathematical practices are possible. The formalization also explores the limits of the current ℕaproche system and avenues for further development.

Abstract "Image missing" is an emerging natural proof assistant that accepts input in the controlled natural language ForTheL. "Image missing" is included in the current version of the Isabelle/PIDE which allows comfortable editing and asynchronous proof-checking of ForTheL texts. The dialect of ForTheL can be typeset by "Image missing" into documents that approximate the language and appearance of ordinary mathematical texts.

Naproche is an emerging natural proof assistant that accepts input in a controlled natural language for mathematics, which we have integrated with LaTeX for ease of learning and to quickly produce high-quality typeset documents. We present a self-contained formalization of the Mutilated Checkerboard Problem in Naproche, following a proof sketch by John McCarthy. The formalization is embedded in detailed literate style comments. We also briefly describe the Naproche approach.

Abstract Recently, Gitik, Kanovei and the first author proved that for a classical Prikry forcing extension the family of the intermediate models can be parametrized by /finite. By modifying the standard Prikry tree forcing we define a Prikry-type forcing which also singularizes a measurable cardinal but which is minimal, i.e., there are no intermediate models properly between the ground model and the generic extension. The proof relies on combining the rigidity of the tree structure with indiscernibility arguments resulting from the normality of the associated measures.

Abstract We determine exact consistency strengths for various failures of the Singular Cardinals Hypothesis (SCH) in the setting of the Zermelo-Fraenkel axiom system ZF without the Axiom of Choice (AC). By the new notion of parallel Prikry forcing that we introduce, we obtain surjective failures of SCH using only one measurable cardinal, including a surjective failure of Shelah's pcf theorem about the size of the power set of ℵω. Using symmetric collapses to ℵω, , or , we show that injective failures at ℵω, , or can have relatively mild consistency strengths in terms of Mitchell orders of measurable cardinals. Injective failures of both the aforementioned theorem of Shelah and Silver's theorem that GCH cannot first fail at a singular strong limit cardinal of uncountable cofinality are also obtained. Lower bounds are shown by core model techniques and methods due to Gitik and Mitchell.

Abstract Using the Friedman-Koepke Hyperfine Structure Theory of [2]. we provide a short construction of a gap 1 morass in the constructible universe.

The notion of ordinal computability is defined by generalising standard Turing computability on tapes of length . In this paper we present a new proof of this theorem that makes use of a theory SO axiomatising the class of sets of ordinals in a model of set theory. The theory SO and the standard Zermelo–Fraenkel axiom system ZFC can be canonically interpreted in each other. The proof of the fundamental theorem is based on showing that the class of sets that are ordinal computable from ordinal parameters forms a model of SO.

Abstract We propose a simple notion of “extender” for coding large elementary embeddings of models of set theory. As an application we present a self-contained proof of the theorem by D. Martin and J. Steel that infinitely many Woodin cardinals imply the determinacy of every projective set.

Abstract By using the notion of a simplified (κ, 1)-morass, we construct κ-thin-tall, κ-thin-thick and, in a forcing extension, κ-very thin-thick superatomic Boolean algebras for every infinite regular cardinal κ.

A subset X of a structure S is called free in S if ∀x ∈ Xx ∉ S[X − {x}]; here, S[Y] is the substructure of S generated from Y by the functions of S. For κ, λ, μ cardinals, let Frμ(κ, λ) be the assertion: for every structure S with κ ⊂ S which has at most μ functions and relations there is a subset X ⊂ κ free in S of cardinality ≥ λ. We show that Frω(ωω, ω), the free-subset property for ωω, is equiconsistent with the existence of a measurable cardinal (2.2,4.4). This answers a question of Devlin [De]. In the first section of this paper we prove some combinatorial facts about Frμ(κ, λ); in particular the first cardinal κ such that Frω(κ, ω) is weakly inaccessible or of cofinality ω (1.2). The second section shows that, under Frω(ωω, ω), ωω is measurable in an inner model. For the convenience of readers not acquainted with the core model κ, we first deduce the existence of 0# (2.1) using the inner model L. Then we adapt the proof to the core model and obtain that ωω is measurable in an inner model. For the reverse direction, we essentially apply a construction of Shelah [Sh] who forced Frω(ωω, ω) over a ground model which contains an ω-sequence of measurable cardinals. We show in §4 that indeed a coherent sequence of Ramsey cardinals suffices. In §3 we obtain such a sequence as an endsegment of a Prikry sequence.