We show the non-arithmeticity of 1st order theories of lattices of Σ n sentences modulo provable equivalence in a formal theory, of diagonalizable algebras of a wider class of arithmetic theories than has been previously known, and of the lattice of degrees of interpretability over PA. The first two results are applications of Nies’ theorem on the non-arithmeticity of the 1st order theory of the lattice of r.e. ideals on any effectively dense r.e. Boolean algebra. The theorem on degrees of (...) interpretability relies on an adaptation of techniques leading to Nies’ theorem. (shrink)

It is well known that for all recursively enumerable sets X 1 , X 2 there are disjoint recursively enumerable sets Y 1 , Y 2 such that $Y_1 \subseteq X_1, Y_2 \subseteq X_2$ and Y 1 ∪ Y 2 = X 1 ∪ X 2 . Alistair Lachlan called distributive lattices satisfying this property separated. He proved that the first-order theory of finite separated distributive lattices is decidable. We prove here that the first-order theory of all separated distributive lattices (...) is undecidable. (shrink)