Learn complex variable functions first, then ordinary differential equations. Because differential equations must be discussed in complex domain. Real variable function is generally studied in junior year, and the prerequisite courses are complex variable function and mathematical analysis. I don't know the content of random process, so I am a junior in general. I haven't learned the partial differential equation, so I must put it behind the ordinary differential equation. I remember that a Russian partial differential textbook published by higher education also requires the foundation of real variable function. Mathematical and physical equations are also an introductory course to solve partial differential equations, and they also integrate fractions, higher order, ordinary differential, complex variables and so on. It is advisable to study the partial differential before considering it (just a suggestion). See Jong Li, Higher Education Press for complex function, which is characterized by simplicity. If you are good at mathematical analysis and have studied manifold calculus, you can look at Gong Sheng's Introduction to Complex Analysis published by China University of Science and Technology. Ordinary Differential Equation was written by Ding Tongren, Wang Gaoxiong, both of which are good, and the latter is easier to understand. In addition, Pontryagin is also very distinctive. You can understand it with a little knowledge of advanced algebra, which can be used as a supplement to domestic textbooks. Peking University has a good real variable function. I don't remember who the author is. You can search. I'm not a math major. Random and partial derivatives are not involved in undergraduate courses, so I can't evaluate these two textbooks.