Greetings, I have come across the effective mass in my solid state physics lectures, and I have troubles understanding it. I have uploaded the part of the script (handwritten 4,5 pages) that I don't understand. At page 3, it is shown that the effective mass in the vicinity is negative. What does this mean? What does the divergence mean? And in general, what do you think from my short explanation of the effective mass: "The effective mass is the mass a bloch electron within a crystal potential would have if it were to be described as a free particle, on which classical physics (e.g Lorentz-force for outer fields) can be applied. In general, it is written as a tensor yielding different elements depending on the direction of the applied forces" It would be nice if someone could give an explanation. Thanks!