Sets, Logic, Computation: An Open Introduction to Metalogic (Open Logic Project Textbooks)

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I have been trying to teach myself metalogic and mathematical logic from a standing start. This book is rather too difficult for me. For example, the very first page states that {1,2,3} is the same set as {1,2,1,2} which is contrary to everything I have ever read. Therefore it is either a misprint, in which case the book needs to be read with caution, or it is a true statement, and thus needs more in the way of explanation for the benefit of beginners like me. I shall keep it, though, as a reference book for comparison with others on the same subject, of which I would recommend Geoffrey Hunter’s Metalogic

Haven't finished reading this book. So far it seems quite good. Please note that there is not an error on the second page. It does not read {1,2,1,2}. It does read {1,2,1,2,3} and this is correct as this is the same as {1,2,3} in terms of equal sets (repetitions are allowed).