Focusing on the formal development of mathematics, this book demonstrates how to read and understand, write and construct mathematical proofs. It emphasizes active learning, and uses elementary number theory and congruence arithmetic throughout. Chapter content covers an introduction to writing in mathematics, logical reasoning, constructing proofs, set theory, mathematical induction, functions, equivalence relations, topics in number theory, and topics in set theory. For learners making the transition form calculus to more advanced mathematics.
It's nice that it's free, but fails in the same way that most textbooks fail, it won't give you the answers but instead leaves everything as "an exercise for the reader". Students need to see worked examples more than anything else. I don't understand why teachers and textbooks writers seem to feel that working through an example is somehow cheating the students out of the "learning experience" because now they know the answer.