I'm now completing a new online course designed to introduce logic and much of discrete maths in a new way. If you've battled a bit with Shen and sequent calculus, then you might enjoy this course because it teaches logic through sequent calculus and should make you more confident in using it. The course is quite unique in its kind, because it focuses on logic at the intersection of three important disciplines - philosophy, mathematics and computer science and shows the connections to logic in all three areas.
So if you are a student in one of these areas, you'll learn a lot of the topology of the subject and how these areas link together. The course is derived from my discrete maths course at Stony Brook. There is an accompanying text Logic, Proof and Computation 2nd edition (extensively revised from a version I bashed out in one month years ago) coming out - priced between £10-15 for those who want it.
The topics covered include formal language theory, truth-tables, propositional and first-order logic, identity and Boolean algebra dealt from within sequent based reasoning. All the proofs are driven by a provedit! program written in Shen (no pen and paper needed). The formalisation taught actually reflects the reasoning needed to tackle more advanced proofs in mathematics and I discuss the use of indirect proof, how to pose lemmas, how to prune dead assumptions, discriminating safe inference rules from unsafe ones and recognising dead ends which need to be abandoned.
In computing, the course touches on material such as the use of the Sheffer stroke function and its reflection in NAND gates; building a simple circuit from logic gates and the relation of SAT solvers to propositional calculus.
Later, after first-order logic, we cover set theory, mentioning ZF but we use a version called AFST (abstraction free set theory) which is a fragment of set theory designed for machine-assisted proof in basic set theory and you get to produce machine-assisted proofs of some foundational theorems of set theory. We study tuples, relations and functions and based on this we look at Codd's Relational Algebra - the basis for MySQL and a host of RDBMS.
After that, we look at the Peano axioms for arithmetic and induction and we look at solving inductive problems. I use the heuristics developed by Boyer and Moore to teach you how to tackle inductive proofs and prove functional programs correct. We learn this in the context of reasoning about Shen programs. Note, the course does not require facility with Shen, but one lecture, on the automation of machine proof through tactics, introduces the ideas of Robin Milner through very simple Shen.
The later lectures cover Turing machines and the strategy for proving undecidability using a proof of the undecidability first-order logic as an example. We close on a reconstruction of Godel's Incompleteness Theorem using a Shen program as an example (much less complex than Godel's pre-computer approach of 80+ years ago).
There is a lot of philosophy embedded in the text. You learn the distinction between minimal logic and intuitionistic logic, the case for paraconsistent logic and what it is. Also anti-realism in mathematics in response to the Continuum Hypothesis. Russell's Paradox and responses to it, Cantor's proof and responses to it, logicism, conventionalism and formalism as philosophies of mathematics. Also the formalisation of English, Russell's Theory of Definite Descriptions, Davidson's work on formalising action sentences and the challenges to formalisation (paradoxes of material implication etc) from Strawson and Austin. We conclude by looking at Lucas on the implications of Godel's Incompleteness Theorem for artificial intelligence.
This is a quite unique course; nothing quite like it is taught in CS departments. Years ago, I campaigned for an M.Sc. in Combined Logic in which computational, mathematical and philosophical logic was integrated into one degree. The course is really the foundation course for such an M.Sc. So if you've ever wanted a magical mystery tour of some of the greatest thinkers of the C20 in these three disciplines, this is for you.
I'll flag this up again when the course is about to commence.
Mark