Non-covering in the interpretability lattice of equational theories (Q1802249)

This is a very well-written paper, a delightful reading, even for people who are not familiar with the topic. The authors prove some results on balanced equational theories. As a corollary, they present a large class of equational theories which have no covering equational theories (up to equivalence of theories in a natural sense). To go into details would need quite a few definitions and observations. Instead I recommend to read this important paper.