Rigidity is undecidable

Rigidity is undecidable

We show that the problem ‘whether a finite set of regular-linear axioms defines a rigid theory’ is undecidable. 2010 Mathematical Subject Classification 03D35, 03C05, 03G30, 18C10, 18C15

System NEL is Undecidable

System NEL is a conservative extension of multiplicative exponential linear logic (extended by the rules mix and nullary mix) by a self-dual noncommutative connective called seq which has an intermediate position between the connectives par and times. In this paper, I will show that system NEL is undecidable by encoding two counter machines into NEL. Although the encoding is simple, the proof o...

Quasi-Linearizability is Undecidable

Quasi-linearizability is a quantitative relaxation of linearizability. It preserves the intuition of the standard notion of linearizability and permits more flexibility. The decidability of quasi-linearizability has been remaining open in general for a bounded number of processes. In this paper we show that the problem of whether a library is quasilinearizable with respect to a regular sequenti...

Whitehead's Problem is Undecidable

Braid is undecidable

Braid is a 2008 puzzle game centered around the ability to reverse time. We show that Braid can simulate an arbitrary computation. Our construction makes no use of Braid’s unique time mechanics, and therefore may apply to many other video games. We also show that a plausible “bounded” variant of Braid lies within 2-EXPSPACE. Our proof relies on a technical lemma about Turing machines which may ...