# News

## A Fistful of Commits

I just checked in a whole bunch of changes to the part on first-order logic. Most of it is in preparation for a new version of the Logic II textbook Sets, Logic, Computation, which Nicole is planning to use in the Fall term.  Also important, and that’s why I put it right here at the top:

PDFs now live on builds.openlogicproject.org. The builds site has a nice index page now rather than a plain file list. If you link to a PDF on my ucalgary site, please update your link; that site will no longer be updated and will probably disappear sometime soon.

Here’s a list of changes:

• I’ve revised the completeness theorem thoroughly. (This was issue 38.) The main change is that instead of constructing a maximally consistent set, we construct a complete and consistent set. Of course, those are extensionally the same; but both the reason for why we need them and the way we construct them is directly related to completeness and only indirectly to maximal consistency: We want a set that contains exactly one of A or ¬A for every sentence so we can define a term model and prove the truth lemma. And we construct that set by systematically adding either A or ¬A (whichever one is consistent) to the original set. So it makes pedagogical sense to say that’s what we’re doing rather than confuse students with the notion of maximal consistency, prove that the Lindenbaum construction yields a maximally consistent set, and show that maximally consistent sets are complete so we can define the term model from it. Credit for the idea goes to Greg Restall, who does this in his course on Advanced Logic.  (I kind of wonder why standard textbooks mention maximally consistent sets. I’m guessing it’s because if you consider uncountable languages you have to use Zorn’s lemma to prove Lindenbaum’s theorem, and then using maximality is more natural. Is that right?) A bonus effect of this change is that a direct proof of the compactness theorem is now a tedious but relatively easy adaptation of the completeness proof; and I’ve added a section on this (leaving most of the details as exercises).
• I’ve revised the soundness proofs for sequent calculus and natural deduction, where the individual cases are now more clearly discussed. (This was issue 74 and issue 125.)
• In the process I also simplified a bunch of things, filled in some details, and corrected some errors. This includes fixing issue 110, and cleaning up the whole stuff about extensionality. There is a new section on assignments which you may need to add to your driver file unless you include fol/syn/syntax-and-semantics.tex in its entirety.
• The natural deduction system now uses Prawitz’s standard rules, i.e., the double negation elimination rule has been replaced with the classical absurdity rule, and the negation rules are now the special cases of the conditional rules with ⊥ as consequent. This was issue 144. Comparing the system to other treatments in the literature is now easier, and the chapter will integrate more seamlessly with the part on intuitionistic logic that’s in the works.
• The sequent calculus chapter now uses sequents that are made up of sequences, not sets, of formulas. This was issue 145. This is the standard way of doing it, and will make it easier to add material on substructural logics. It also makes the soundness proof a lot easier to understand.
• In both the sequent calculus and natural deduction chapters, the material on quantifiers is now separated from that on propositional connectives. Eventually it should be possible to present propositional logic separately (or only), and now you can reuse only the material on propositional logic. (This was issue 77.)
• There is a new chapter on proof systems, and the intro sections from both the natural deduction and sequent calculus chapters have moved there. So if you only want one of the proof systems, you’ll have to include the relevant intro section in the chapter on the proof system “by hand.” But if you include both, you now have an additional chapter that introduces and compares them.  (This was issue 61.)
• Sets, Logic, Computation (the textbook for Logic II) now includes both sequent calculus and natural deduction!

Note: The formatting of the rules in both systems now uses a defish (“definition-ish”) environment. If your remix uses a custom -envs.sty file, you will need to add a definition for that at the end (see open-logic-envs.sty). The textbooks for Logic II and Logic III have been updated accordingly.

## Diagrams, and a Chapter on How to do Proofs

Two major new additions: Paul Daniell has contributed a number of very nice illustrations for the “Sets” and “Functions” chapters, and I’ve turned Samara’s handout on how to do and write proofs into a new chapter in the “Methods” part.

The diagrams are done in Illustrator and I’ve fiddled with various ways of turning them into tikz code so they can be styled in LaTeX (e.g., the colors changed). We’ll have to define and document a “house style” for additional diagrams, and also the way the conversion to tikz code works.  (The .tikz files live in assets/diagrams for now).

The “Proofs” chapter is a brief intro on how to do mathematical proofs; all examples are from the “Sets” chapter, so don’t require a whole lot of mathematical background.  There are lots of examples, and some tips for what to do when you get stuck. Feedback welcome!

## New in Print: forall x (Summer 2017 edition), and Incompleteness and Computability

New on Amazon: the print version of the Summer 2017 edition of forall x: Calgary Remix, as well as the text I made for Phil 479 (Logic III) last term, Incompleteness and Computability.

The new edition of forall x includes a number of corrections submitted by Richard Lawrence, who taught from it at Berkeley in the Spring term. I’ve also noticed that if you don’t want Amazon to distribute the book to libraries and bookstores, you can make it a lot cheaper: USD 7.62 instead of USD 11.35.  Of course, the PDF is still free. (There’s now also a version for printing on letter-sized paper.) With Richard’s and Aaron’s help, the solutions manual now matches the text and has fewer errors.

The print version of Incompleteness and Computability incorporates a number of corrections and improvements suggested by my Logic III students. Compared to the version announced earlier, it also includes the two new chapters on Models of Arithmetic and on Second-order Logic. It, too, is still available free in both PDF and source code.

## forall x: YYC is now on Amazon (and how it got there)

Back when I described the process to publish a text on lulu.com, commenter penrodyn suggested to try CreateSpace, Amazon’s self-publishing platform.  So I did. You can now purchase forall x: Calgary Remix on Amazon (Canada, UK). (Apparently it already ranks #27 in the Math/Logic category!?)

It turns out it wasn’t hard to do at all. CreateSpace works pretty much the same way Lulu does. You need a PDF of the interior that matches one of the sizes offered. I picked 7.44″ x 9.69″, which is almost the same as Lulu’s Quarto size, so I could use the same PDF as for Lulu.  Getting the dimensions for the cover was a bit harder, since CreateSpace doesn’t tell you what the specific dimensions should be, and lets you compute them yourself.  You also need to include .125″ bleed on all sides. On the upside, they put the ISBN bar code on it for you.  The file to produce the CreateSpace cover is here. Before the book is ready to sell in the Amazon store, the PDF has to be reviewed–this takes about a day.

The final product is slightly lower quality than Lulu’s, and is probably comparable to Lulu’s “value” print option (in the US store called “standard”). Note, though, that only two format options are available at “value” prices for Lulu (e.g., if you want quarto, value isn’t an option). The cover print is less vibrant, the inside stock used is lighter and less bright. It also ended up being slightly more expensive (US$11.35 vs. US$10.64 at Lulu; CAD 15.47 vs CAD 12.08 at Lulu). Depending on if you catch Lulu at one of their very frequent discount deals,  you might be much better off with Lulu, especially if you (or your bookstore) orders a whole bunch. But if it’s not the only book you’re buying, Amazon probably will get it to you faster and the shipping will be free.

The price point surprised me a bit because the original comment said CreateSpace is a lot cheaper. And it in fact is, but only for yourself: I can order the book for $4.54.$11.35 was the minimum list price I could enter, and apparently I get $2.27 in royalties for every book sold. I feel bad about that, but I promise to use it only for good (like pay for the website!). [UPDATE: If you deselect “Expanded Distribution,” the minimum US list price is a lot less and you don’t have to take royalties.] Another difference is that CreateSpace only does USD, GBP, and EUR, so proof copies ship from the US and you have to pay US$. [UPDATE: Also, you should be aware that it can take a few days for the book to become available on Amazon. CreateSpace books sold on amazon.com also go up for sale on the Canadian site amazon.ca, but it can take up to 30 days.]

Lulu also lets you list your book on Amazon (and Barnes&Noble, etc.), but: you have to purchase a physical proof copy before it’s approved, the list price will be higher than what you can set it to on Lulu.com (\$8 more), and I suspect it will ship from an Amazon seller and so you won’t be able to bundle it with other Amazon orders for free shipping. But I haven’t tried that yet.

Overall, it was easy enough, the quality is decent, and if your bookstore can’t bulk order for your class, your students may prefer ordering from Amazon. If you want a really cheap but perfect-bound book, use Lulu, go with the standard/value option, and sell at cost. If you care about the quality, use Lulu’s standard/premium print option. If you want it to be available ASAP, use Lulu. If you want convenience and visibility, use CreateSpace.

## OLP at the Pacific APA/Spring ASL Meeting

The Open Logic Project will be part of the Thursday afternoon session “Inclusiveness in Logic Education” (session 7P) at the Pacific Division Meeting of the APA. The session is part of the Spring Meeting of the Association for Symbolic Logic, and is organized by Audrey Yap on behalf of the ASL Committee on Logic Education.

It is followed by a reception sponsored by the ASL!

## Proof Checker for forall x: Cambridge and Calgary

Kevin Klement has done up a prototype of his online natural deduction proof builder/checker that works with the natural deduction system of the Cambridge and Calgary versions of forall x.  The system was originally written for UMass’s Intro Logic course, based on Gary Hardegree’s online textbook. Kevin writes:

Earlier I mentioned making some online exercises for the “forall x” book. Not sure how far I’ll follow through with it, but I did go ahead and mock up a proof builder and checker, and sample exercises using them. That seems like the most fun/interesting part. I wasn’t intending to target the Calgary remix, but that’s the version I was looking at when I made it. I realized only later that the proof system and notation is different from the original, but I left it stand. Anyway, I thought I’d post a demo before taking it any further, so others can take a look, make suggestions, and help identify bugs. The user interface was the hardest part, and I’m not entirely happy with it as is, but not sure how to improve. Let me know what you think.

If you find a bug or have suggestions for improvements, please let Kevin know! The code is open source (download link at the bottom of the page).

UPDATE: The code is now on GitHub and runs on proofs.openlogicproject.org

## New Textbook on Incompleteness

I’m teaching the incompleteness theorems (and related material) this term, and of course I’m using the Open Logic Project as a text. The relevant sections are based on Jeremy Avigad’s notes, which originally were meant as a supplement to Epstein & Carnielli’s textbook Computability. I’ve spent a fair bit of time revising them, and making them independent of that text. That meant adding a bunch of material, reformatting things, adding explanations and examples, etc. I think it’s now in sufficiently good shape that I can share it. However, it’s by no means done (we’re only halfway through the semester). I’m waiting to hear what else my students want to hear about; and I’ll update the project with additional chapters from the OLP (perhaps yet to be written). The most unusual aspect of it so far is perhaps that I’m doing everything in natural deduction, including arithmetization of provability.

The most recent PDF version should be available here, but I’ll also attach the current version to this post. All the actual material (with the exception of the chapter summaries and the front matter) comes from the Open Logic Text. The code to produce the text in this version is on GitHub. As usual, suggestions more than welcome!

Incompleteness and Computability

## forall x: Calgary Remix

Aaron is teaching our intro logic course (“Logic I”) this term, and as part of a pilot project to redesign that course (which also includes partially “flipping” it), we’ve also adapted P. D. Magnus’s open textbook forall x. To be more precise, we’ve remixed a remix by Tim Button of P.D. Magnus’ textbook with some material from J. Rob Loftis’s remix plus some material from Tim Button’s Metatheory and then added our own material and revisions.  (Aaron previously wrote about how we decided on these changes.) Thanks to Creative Common open licensing, we can do this: just shamelessly take other people’s work and make it work for us, without paying them. In this case, the license is CC BY-SA, which requires only that we give credit (the “BY” part) and that we make the end product available under the same terms (the “SA”) part.

In order to keep track of our changes, I’ve put our remix as well as PDM’s original and TB’s remix on GitHub. (JRB maintains his own GitHub repository of the Lorain County remix/expanded “Open Introduction to Logic”.) is  So if you want to start your own remix and also want to use Git, you can start from any of the four version by forking the respective repository.

Here’s a rundown of the most important changes, with links to the corresponding Git commits where you can see what’s changed.

• We put some material from PDM’s original version back into TB’s version (b3b6f97).
• We added some material from JRL’s remix, mainly a discussion of soundness and completeness and many exercises (09b6d3a) and glossary entries (3f1276c).
• We added a chapter on normal forms and truth-functional completeness from TB’s Metatheory (f49b6b3) and make it independent of the rest of that book (0721158)
• We moved the chapter on natural deduction for TFL to before the FOL part.
• We made a whole bunch of smaller stylistic changes, e.g., change all I’s to we’s, shall’s to will’s, change some examples to make them less reliant on a US or British background, etc. (fd9b99e, f091b03, 2a95058), added a preface (afd969d), glossary entries for the part on FOL (367d614), as well as some changes to terminology (ffa6567).
• Changed the typography and layout to match the Sets, Logic, Computation layout and to fit on Crown Quarto paper for printing at lulu.com.

You can see a complete line-by-line comparison on GitHub.

You can download the final product if you don’t want to compile it yourself, and you can even buy a printed copy if you want!

Warning: We have not done anything with the solutions yet, so those do not match the numbering in the book and in fact might not match the exercises themselves!