Important to note that the maths used in AOCP is briefly covered at the start of Vol I. This was expanded upon in the form of Concrete Mathematics. If you want

Important to note that the maths used in AOCP is briefly covered at the start of Vol I. This was expanded upon in the form of Concrete Mathematics. If you want just the maths and at a slightly slower pace, go with Concrete Mathematics. If that's a struggle, I found the first part of Discrete Mathematics and its applications by Kenneth Rosen was enough for me to make progress in Concrete Mathematics. I also found learning a bit of linear algebra and calculus was a mind changer. I expect more fruits by my continuing of those studies.