*To*: cl-isabelle-users at lists.cam.ac.uk*Subject*: Re: [isabelle] Infinity - infinity = infinity*From*: Tobias Nipkow <nipkow at in.tum.de>*Date*: Fri, 2 Dec 2016 18:45:44 +0100*In-reply-to*: <45b1de25-cef0-bf73-faee-6918320f3e8f@in.tum.de>*References*: <6592A55E-9169-4BBE-8D8E-5A2F8E7718C4@inria.fr> <2956700a-dc8e-f201-a811-c5c3e3c3a87a@in.tum.de> <1480695285.29055.11.camel@in.tum.de> <36b8ca52-c7d9-e330-ad5e-b68b384425b6@in.tum.de> <45b1de25-cef0-bf73-faee-6918320f3e8f@in.tum.de>*User-agent*: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:45.0) Gecko/20100101 Thunderbird/45.5.1

On 02/12/2016 18:09, Manuel Eberl wrote:

Do correct me if I am wrong, but in my naÃvetÃ, I would have thought that one should always be able to accommodate a different definition of "â - â" in quantifier elimination, at worst by adding an extra case for "a = â â b = â".

Tobias

Is that not the case? Manuel On 02/12/16 17:39, Tobias Nipkow wrote:On 02/12/2016 17:14, Johannes HÃlzl wrote:Another idea from Tobias (and I think also Andreas) is to add a special simpproc which does case-distinction on enat/ereal/ennreal and calls linarith.Something like this was actually implemented in Coq Asankhaya Sharma, Shengyi Wang, Andreea Costea, Aquinas Hobor, and Wei-Ngan Chin: Certified Reasoning with Infinity. FM 2015. and shows that it can be made independent of the particular choice of what "â - â" is. Hence we should not lose quantifier elimination with a different choice. TobiasI would assume the simpproc is quite slow but can be disable by default and just be activated by the user. - Johannes Am Freitag, den 02.12.2016, 16:57 +0100 schrieb Tobias Nipkow:Jasmin, there is a reason why I would not do this: Aless Lasaruk and Thomas Sturm. Effective Quantifier Elimination for Presburger Arithmetic with Infinity This paper shows that our current enat has quantifier elimination (although we have not inplemented it, and it would be some work, but not infeasible). In their system, "â - â = â". Unless we know that your proposed modification still has quantifier elimination, I would be reluctant to give up that strong property. Tobias On 02/12/2016 16:01, Jasmin Blanchette wrote:Dear all, As noted before on this mailing list, automation for "enat" ("Library/Extended_Nat.thy") is quite poor. Often, the only way to proceed is to perform case distinctions on all "enat" and use auto on the emerging subgoals. My impression is that many type classes are not available because of the definition of subtraction. Because "â - â = â" (where "â" is the infinity symbol), we lack one of the two properties required by "cancel_comm_monoid_add": 1. âa b. a + b - a = b 2. âa b c. a - b - c = a - (b + c) and we lack the third property required by "comm_semiring_1_cancel": 3. âa b c. a * (b - c) = a * b - a * c Counterexample for 1: a = â, b = 0. Counterexample for 3: a = â, b = c = 1. These omissions affect further layers in the type class hierarchy -- e.g. we cannot use "ordered_cancel_comm_monoid_diff", even though some of its theorems (e.g. "add_diff_assoc2") turn out to hold. My proposal is to change the definition of subtraction so that "â - â = 0" and to instantiate the missing type classes. I believe this would make "enat" much less painful to use, and mathematically I'm not so convinced that "â - â = â" is such a great idea anyway. Indeed, I have recently implemented ordinals below Î_0 in Isabelle and was able to have much better automation than with "enat", and there we have Ï - Ï = 0. "enat" occurs in about 70 ".thy" files in Isabelle and the AFP, so this change (including the type class instantiations) seems quite manageable. We (= Mathias and I) would wait until after the 2016-1 release to avoid any interference. Any opinions for or against? Jasmin

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**References**:**[isabelle] Infinity - infinity = infinity***From:*Jasmin Blanchette

**Re: [isabelle] Infinity - infinity = infinity***From:*Tobias Nipkow

**Re: [isabelle] Infinity - infinity = infinity***From:*Johannes Hölzl

**Re: [isabelle] Infinity - infinity = infinity***From:*Tobias Nipkow

**Re: [isabelle] Infinity - infinity = infinity***From:*Manuel Eberl

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