From: "Martín Hötzel Escardó" <"escardo..."@gmail.com>
To: Homotopy Type Theory <HomotopyT...@googlegroups.com>
Subject: Re: [HoTT] Univalence <-> equivalence induction
Date: Fri, 18 May 2018 14:03:30 -0700 (PDT) [thread overview]
Message-ID: <55fca524-24be-4f98-b9a4-fa4846d9c51c@googlegroups.com> (raw)
In-Reply-To: <CAOvivQwFXZkCzDt+h=RM9=7+ku3SE3mFYXkf+ovH6tyQ907fAw@mail.gmail.com>
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On Friday, 18 May 2018 17:41:00 UTC+2, Michael Shulman wrote:
>
> I certainly knew that univalence is equivalent to
> equivalence-induction *with* computation rule, which I think is what
> is in Egbert's notes. But I don't think I knew that you can do
> without the computation rule.
Yes, this is the difference - or are you doing the same, Egbert?
Also, I should have said that I needed to adapt Peter's argument slightly -
unfortunately, I couldn't use his result off-the-shelf. The main difference
is that Peter works with a global identity system on all types (of a
universe), whereas I work with an identity system on a single type, namely
a universe. As a result, I can't define the type of left-cancellable maps
using the notion of equality given by the identity system. Instead, I
define it using the native (Martin-Loef) identity type, and with this
little modification, Peter's argument goes through for the situation
considered here.
Can you give a link to the "some years
> ago" discussion claiming it strictly weaker?
>
I will try to dig it up tomorrow.
Martin
>
> On Fri, May 18, 2018 at 6:04 AM, Egbert Rijke <e.m...@gmail.com
> <javascript:>> wrote:
> > Hi Martin,
> >
> > I think it was known. I taught this in my intro to HoTT class this
> semester:
> >
> > http://www.andrew.cmu.edu/user/erijke/hott/univalence.pdf
> >
> > Best wishes,
> > Egbert
> >
> > On Fri, May 18, 2018 at 2:36 AM, Martín Hötzel Escardó
> > <escar...@gmail.com <javascript:>> wrote:
> >>
> >> Equivalence induction says that in order to prove something for all
> >> equivalences, it is enough to prove it for all identity equivalences
> for all
> >> types.
> >>
> >> This follows from univalence. But also, conversely, univalence follows
> >> from it:
> >>
> >> http://www.cs.bham.ac.uk/~mhe/agda-new/UF-Univalence.html#JEq
> >>
> >> Is this known? Some years ago it was claimed in this list that
> equivalence
> >> induction would be strictly weaker than univalence.
> >>
> >> To prove the above, I apply a technique I learned from Peter Lumsdaine,
> >> that given an abstract identity system (Id, refl , J) with no given
> >> "computation rule" for J, produces another identity system (Id, refl ,
> J' ,
> >> J'-comp) with
> >> a "propositional computation rule" J'-comp for J'.
> >>
> >> http://www.cs.bham.ac.uk/~mhe/agda-new/Lumsdaine.html
> >>
> >> Martin
> >>
> >> --
> >> You received this message because you are subscribed to the Google
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> >> "Homotopy Type Theory" group.
> >> To unsubscribe from this group and stop receiving emails from it, send
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> >> email to HomotopyTypeThe...@googlegroups.com <javascript:>.
>
> >> For more options, visit https://groups.google.com/d/optout.
> >
> >
> >
> >
> > --
> > egbertrijke.com
> >
> > --
> > You received this message because you are subscribed to the Google
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next prev parent reply other threads:[~2018-05-18 21:03 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-18 6:36 Martín Hötzel Escardó
2018-05-18 13:04 ` [HoTT] " Egbert Rijke
2018-05-18 15:40 ` Michael Shulman
2018-05-18 21:03 ` Martín Hötzel Escardó [this message]
2018-05-19 18:09 ` Nicolai Kraus
2018-05-19 19:38 ` Thierry Coquand
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