I am not a math major. I studied cal I, II, III, ODE and almost finish PDE. I just want to gauge the difficulty of different math classes. To me ODE was the hardest even though you need that for the PDE. ODE have so many different topics it is hard to follow all the derivation. I had to restudy the whole ODE even I was the first in the class two years ago. PDE is hard, but it has a logical progression. You go through the same thing over and over just in different coordinates. Make it easier to understand even you might miss the first go around. The only difficult part of the PDE are the Bessel's function and Legendre function. How is complex analysis compare to ODE and PDE. that is another one I am interested in the future.