My first reaction was number theory. Once upon a time, number theory was called the purest mathematics and the most primitive mathematics. People think that number theory is the mystery of studying numbers and has nothing to do with practical application. Therefore, people have found many uses, such as cryptography and physics. It happened that a few years ago, I read a book on the application of number theory in physics by an academician named Chen Nanxian. Mobius (number theory is the principle of exclusion) is committed to solving the free energy of crystal phonons.