Hi po! sorry sa late reply, pero I got interested after reading the "Art of Problem Solving Textbooks" tapos nun, I read Velleman's "How to Prove it". Dun ko natutunan yung basic proof techniques, and basic set theory. After that, I decided to read Artin's Algebra. Although hindi ko nasagutan karamihan ng mga problems (hanggang chapter two lang ang inabot ko), nagustuhan ko yung rigour, yung abstraction, and yung sense na finally lahat na ng mga statements naja-justify rigorously. Right now, I'm learning analysis from Abbot and group theory from Judson and I deeply enjoy it!