On 2-categories of extensions

This is essentially an illustration for the general technology of homotopical enhancements developed recently in arxiv:2409.17489. We take the derived category of an abelian category, and we look at the full subcategory spanned by complexes of length 2. This has a natural refinement to a 2-category that we call “the 2-category of extensions”. However, just using the triangulated structure on the derived category is not enough to obtain this refinement. In this short note, we first construct the 2-category of extensions by hand – that is, using abelian category techniques – and then show how it can be recovered very easily and naturally in the enhanced formalism of arxiv:2409.17489.