>Group Theory
Carter - Visual Group Theory
>Complex Analysis
Needham - Visual Complex Analysis
Less confident recommendations:
>Probability
https://ocw.mit.edu/courses/mathematics/18-05-introduction-t... as a very entry level introduction. After that you might still need some textbook for more depth.
>Mathematical Logic and Godel Incompleteness Theorems
Start with ForallX by Magnus. Then continue with Computability and Logic by Boolos (don't read all chapters, check out preface to see what you need for Godel Incompleteness Theorems).
>Set Theory
Hrbacek - Introduction to Set Theory
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I am currently developing an online course which teaches logic, set theory, and computability theory (this includes Godel Incompleteness Theorem). It uses the textbooks I've mentioned above. You can check it out at https://app.grasple.com/#/course/141?access_token=3HCK4oRipe.... Use "Fundamentals of formalization" and ignore other tracks.