I think my weakest area is things like integration techniques, complex analysis, and integral transforms, but after a few google searches I can't really find what makes QFT so hard when people see it the first time. I really don't understand how to evaluate path integrals or those n-dimensional partition functions or how they're related. The infinite sums baffle me in the ising model things. Anyone have any reference for making sure my math skills are adequate before I get lost next semester? I really want to get a handle on working with fields, both quantum and classical.