Dualizations of Approximations, \(\aleph _1\)-Projectivity, and Vopěnka’s Principles

The approximation classes of modules that arise as components of cotorsion pairs are tied up by Salce’s duality. Here we consider general approximation classes of modules and investigate possibilities of dualization in dependence on closure properties of these classes. While some proofs are easily dualized, other dualizations require large cardinal principles, and some fail in ZFC, with counterexamples provided by classes of \(\aleph _1\) ${\aleph }_1$ -projective modules over non-perfect rings. For example, we show that the statement “each covering class of modules closed under homomorphic images is of the form \({\mathrm{Gen\,}}(M)\) $\mathrm{Gen}(M)$ for a module M” is equivalent to Vopěnka’s Principle.