Non-finite axiomatizability of Dynamic Topological Logic
  Dynamic topological logic (DTL) is a polymodal logic designed for reasoning about {\em dynamic topological systems. These are pairs (X,f), where X is a topological space and f:X->X is continuous. DTL uses a language L which combines the topological S4 modality [] with temporal operators from linear temporal logic.   Recently, I gave a sound and complete axiomatization DTL* for an extension of the logic to the language L*, where <> is allowed to act on finite sets of formulas and is interpreted as a tangled closure operator. No complete axiomatization is known over L, although one proof system, which we shall call $\mathsf{KM}$, was conjectured to be complete by Kremer and Mints.   In this paper we show that, given any language L' between L and L*, the set of valid formulas of L' is not finitely axiomatizable. It follows, in particular, that KM is incomplete. 
David Fern\'andez-Duque
math.LO cs.LO