Hi, I'm in the early stage of my ph.d program in electrical engineering and I have realized I need to know more math (I'm in circuits). Problem is that I have no idea where to even begin. In my undergraduate, I took the basic math courses (calculus , DEQ, linear algebra) but these aren't really helping me get very far. Any recommendation on courses / textbook would be appreciated. I am planning to study a subject alone. For other fellow elec.engineers, the math that I am mostly interested in are the ones we use in PLL, odcillator, ADC, filter, and noise.