I am currently studying GMT and a topic that has popped up in the course is the use of calibrations as a tool for proving that a particular set $E$ attains the minimum for the problem
$$min \left \lbrace P(A) + \int_{A} f \right \rbrace$$
We said that, basically, if you can find a function $g$ with some properties (which is called a calibration for $E$) then $E$ is a global minimum for the above minimization problem. We also saw some examples and briefly talked about local minima, but there are some things I don't get about those. That is why I was looking for some books, notes, anything that explains this method with some examples to understand better and look at this topic more in detail, but I couldn't find anything, every book I checked doesn't talk about this at all, or if it does it's basically a remark. Do you know about anything with more explanations about this? If there were carried out examples it would be perfect.