Definitely frame theory in linear algebra. It is part of linear algebra, so you already mentioned it. I wasn't taught this in university and found it later. At

Definitely frame theory in linear algebra. It is part of linear algebra, so you already mentioned it. I wasn't taught this in university and found it later. At first I understood much of nothing and it looked really complicated and convoluted. After getting my head around it is just everywhere. Sometimes I wonder if we actually have another big trick.