To understand the energy of a dilute gas of interacting bosons has been a fundamental challenge in mathematical physics for many decades. This is one of the most simple, yet still a very difficult, quantum mechanical many-particle systems. Recently several breakthroughs have been made on this problem. In the talk I will review the classical, non-rigorous Bogoliubov calculation, which gives a good guide to the modern approaches, and point out where new ideas have to be added in order to reach a mathematical proof. The focus will be on the first correction term to the energy, the so-called Lee-Huang-Yang term.
Based on joint work with Jan Philip Solovej (Copenhagen).