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[2607:f8b0:4864:20::336]) by gmr-mx.google.com with ESMTPS id a26si644694ioq.2.2019.09.18.13.36.05 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 18 Sep 2019 13:36:05 -0700 (PDT) Received-SPF: pass (google.com: domain of evancavallo@gmail.com designates 2607:f8b0:4864:20::336 as permitted sender) client-ip=2607:f8b0:4864:20::336; Received: by mail-ot1-x336.google.com with SMTP id s22so1077597otr.6 for ; Wed, 18 Sep 2019 13:36:05 -0700 (PDT) X-Received: by 2002:a9d:5f8d:: with SMTP id g13mr4403636oti.268.1568838965023; Wed, 18 Sep 2019 13:36:05 -0700 (PDT) MIME-Version: 1.0 References: <1DF8E802-2959-4BEF-A85A-3C6E5E7B9595@wesleyan.edu> In-Reply-To: From: Evan Cavallo Date: Wed, 18 Sep 2019 16:35:53 -0400 Message-ID: Subject: Re: [HoTT] Recovering an equivalence from univalence in cubical type theory To: Michael Shulman Cc: "Licata, Dan" , "HomotopyTypeTheory@googlegroups.com" Content-Type: multipart/alternative; boundary="0000000000003856ec0592d9c95c" X-Original-Sender: evancavallo@gmail.com X-Original-Authentication-Results: gmr-mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=hrH2XKC1; spf=pass (google.com: domain of evancavallo@gmail.com designates 2607:f8b0:4864:20::336 as permitted sender) smtp.mailfrom=evancavallo@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com Precedence: list Mailing-list: list HomotopyTypeTheory@googlegroups.com; contact HomotopyTypeTheory+owners@googlegroups.com List-ID: X-Google-Group-Id: 1041266174716 List-Post: , List-Help: , List-Archive: , --0000000000003856ec0592d9c95c Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable The definition of univalence for BCH cubical sets in https://arxiv.org/abs/1710.10941 does satisfy the property that fst(coe(ua(e))) =3D fst(e) : A -> B exactly. They note (Remark 7) that the definition can also be adjusted so that transporting backwards along the equivalence gives the inverse function one can extract from e. Roughly, the problem Dan describes in ABCFHL---that the B in Glue [alpha -> T] B can also depend on the direction of coercion x---can be avoided in the BCH analogue of Glue by simply demanding that B is degenerate in x. The restricted operation is still sufficient to prove univalence. We can't make sense of this in a structural cubical set model because the property of being degenerate in a given variable isn't stable under diagonal substitution. Evan 2019=E5=B9=B49=E6=9C=8818=E6=97=A5(=E6=B0=B4) 15:23 Michael Shulman : > Thanks, that's very interesting! > > The reason I ask is that I was wondering to what extent the type "A=3DB" > can be regarded as "a coherent definition of equivalence" alongside > half-adjoint equivalences, maps with contractible fibers, etc. Of > course in some sense it is (even in Book HoTT), since it's equivalent > to Equiv(A,B); but the question is how practical it is -- for > instance, is it reasonable when doing synthetic homotopy theory to > state all equivalences as equalities? > > In practice, the way we often construct equivalences is to make them > out of a quasi-inverse pair, and all the standard definitions of > equivalence have the nice property that they remember the two > functions in the quasi-inverse pair judgmentally. My experience with > the HoTT/Coq library is that this property is very useful, which is > one reason we state equivalences as equivalences rather than making > use of univalence to state them as equalities (another reason is that > it avoids "univalence-redexes" all over the theory). Half-adjoint > equivalences have the additional nice property that they remember one > of the homotopies judgmentally, and if you're willing to prove the > coherence 2-path by hand then they can be made to remember both of the > homotopies; this seems to be much less useful than I thought it would > be when we made the choice to use half-adjoint equivalences in the > HoTT/Coq library, but it has proven useful at least once. > > So I was wondering to what extent equality of types in cubical type > theory has properties like this. It sounds from what you say like the > answer is "not much". This makes the lack of regularity seem like a > rather more serious problem than I had previously thought. > > On Wed, Sep 18, 2019 at 9:15 AM Licata, Dan wrote: > > > > In ABCFHL, even the function fst(coe(ua(e))) : A -> B is only path-equa= l > to fst(e) : A -> B. If I recall correctly, the issue is that composition > in the Glue type that you use to implement ua doesn=E2=80=99t judgemental= ly give > you f; instead there is some morally-the-identity-composition (that woul= d > cancel with regularity) that gets stuck in. This is because the general > algorithm for composition in the glue type has to coerce in the =E2=80=9C= base=E2=80=9D of > the glue type (B in Glue [alpha -> T] B), which in the case of ua(e) =3D = Glue > [x =3D 0 -> (A,e), x=3D1 -> (B,id)] B is degenerate in x, but in general = might > not be. > > > > I don=E2=80=99t recall any cubical type theories solving this, but I do= n=E2=80=99t > remember the details of all of the other variations that have been explor= ed > well enough to say for sure. > > > > > On Sep 18, 2019, at 11:42 AM, Michael Shulman > wrote: > > > > > > Let Equiv(A,B) denote the type of half-adjoint equivalences, so that > > > an e:Equiv(A,B) consists of five data: a function A -> B, a function = B > > > -> A, two homotopies, and a coherence 2-path. Using univalence, we > > > can make e into an identification ua(e) : A=3DB, and then back into a= n > > > equivalence coe(ua(e)) : Equiv(A,B), which is typally equal to e. > > > > > > Now we might wonder whether coe(ua(e)) might be in fact *judgmentally= * > > > equal to e; or at least whether this might be true of some, if not > > > all, of its five components. In Book HoTT this is clearly not the > > > case, since univalence is posited as an axiom about which we know > > > nothing else. But what about cubical type theories? Can any of the > > > components of an equivalence e be recovered, up to judgmental > > > equality, from coe(ua(e))? (My guess would be that at least the > > > function A -> B, and probably also the function B -> A, can be > > > recovered, but perhaps not more.) > > > > > > -- > > > You received this message because you are subscribed to the Google > Groups "Homotopy Type Theory" group. > > > To unsubscribe from this group and stop receiving emails from it, sen= d > an email to HomotopyTypeTheory+unsubscribe@googlegroups.com. > > > To view this discussion on the web visit > https://groups.google.com/d/msgid/HomotopyTypeTheory/CAOvivQzzSXNHs%2BzbP= QTyHEuU53aHXJ0sPe4pr%2Byf0ahLGvUpVA%40mail.gmail.com > . > > > > -- > > You received this message because you are subscribed to the Google > Groups "Homotopy Type Theory" group. > > To unsubscribe from this group and stop receiving emails from it, send > an email to HomotopyTypeTheory+unsubscribe@googlegroups.com. > > To view this discussion on the web visit > https://groups.google.com/d/msgid/HomotopyTypeTheory/1DF8E802-2959-4BEF-A= 85A-3C6E5E7B9595%40wesleyan.edu > . > > -- > You received this message because you are subscribed to the Google Groups > "Homotopy Type Theory" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to HomotopyTypeTheory+unsubscribe@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/HomotopyTypeTheory/CAOvivQz_wY2CVp9Y5ca= EehVotR7w7D%3DoP6jpoUxm463pNdNHuA%40mail.gmail.com > . > --=20 You received this message because you are subscribed to the Google Groups "= Homotopy Type Theory" group. To unsubscribe from this group and stop receiving emails from it, send an e= mail to HomotopyTypeTheory+unsubscribe@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/= HomotopyTypeTheory/CAFcn59ZLBcQMwJUMnnDKq3ewf582x4z4aqce4xR16xzcQeRxgA%40ma= il.gmail.com. --0000000000003856ec0592d9c95c Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
The definition of univalence for BCH cubical sets in = https://arxiv.org/abs/1710.109= 41 does satisfy the property that fst(coe(ua(e))) =3D fst(e) : A -> = B exactly. They note (Remark 7) that the definition can also be adjusted so= that transporting backwards along the equivalence gives the inverse functi= on one can extract from e.

Roughly, the problem Da= n describes in ABCFHL---that the B in Glue [alpha -> T] B can also depen= d on the direction of coercion x---can be avoided in the BCH analogue of Gl= ue by simply demanding that B is degenerate in x. The restricted operation = is still sufficient to prove univalence. We can't make sense of this in= a structural cubical set model because the property of being degenerate in= a given variable isn't stable under diagonal substitution.

Evan

2019=E5=B9=B49=E6=9C=8818=E6=97=A5(=E6=B0= =B4) 15:23 Michael Shulman <shul= man@sandiego.edu>:
Thanks, that's very interesting!

The reason I ask is that I was wondering to what extent the type "A=3D= B"
can be regarded as "a coherent definition of equivalence" alongsi= de
half-adjoint equivalences, maps with contractible fibers, etc.=C2=A0 Of
course in some sense it is (even in Book HoTT), since it's equivalent to Equiv(A,B); but the question is how practical it is -- for
instance, is it reasonable when doing synthetic homotopy theory to
state all equivalences as equalities?

In practice, the way we often construct equivalences is to make them
out of a quasi-inverse pair, and all the standard definitions of
equivalence have the nice property that they remember the two
functions in the quasi-inverse pair judgmentally.=C2=A0 My experience with<= br> the HoTT/Coq library is that this property is very useful, which is
one reason we state equivalences as equivalences rather than making
use of univalence to state them as equalities (another reason is that
it avoids "univalence-redexes" all over the theory).=C2=A0 Half-a= djoint
equivalences have the additional nice property that they remember one
of the homotopies judgmentally, and if you're willing to prove the
coherence 2-path by hand then they can be made to remember both of the
homotopies; this seems to be much less useful than I thought it would
be when we made the choice to use half-adjoint equivalences in the
HoTT/Coq library, but it has proven useful at least once.

So I was wondering to what extent equality of types in cubical type
theory has properties like this.=C2=A0 It sounds from what you say like the=
answer is "not much".=C2=A0 This makes the lack of regularity see= m like a
rather more serious problem than I had previously thought.

On Wed, Sep 18, 2019 at 9:15 AM Licata, Dan <dlicata@wesleyan.edu> wrote:
>
> In ABCFHL, even the function fst(coe(ua(e))) : A -> B is only path-= equal to fst(e) : A -> B.=C2=A0 If I recall correctly, the issue is that= composition in the Glue type that you use to implement ua doesn=E2=80=99t = judgementally give you f; instead there is some morally-the-identity-compos= ition=C2=A0 (that would cancel with regularity) that gets stuck in.=C2=A0 T= his is because the general algorithm for composition in the glue type has t= o coerce in the =E2=80=9Cbase=E2=80=9D of the glue type (B in Glue [alpha -= > T] B), which in the case of ua(e) =3D Glue [x =3D 0 -> (A,e), x=3D1= -> (B,id)] B is degenerate in x, but in general might not be.
>
> I don=E2=80=99t recall any cubical type theories solving this, but I d= on=E2=80=99t remember the details of all of the other variations that have = been explored well enough to say for sure.
>
> > On Sep 18, 2019, at 11:42 AM, Michael Shulman <shulman@sandiego.edu> wro= te:
> >
> > Let Equiv(A,B) denote the type of half-adjoint equivalences, so t= hat
> > an e:Equiv(A,B) consists of five data: a function A -> B, a fu= nction B
> > -> A, two homotopies, and a coherence 2-path.=C2=A0 Using univ= alence, we
> > can make e into an identification ua(e) : A=3DB, and then back in= to an
> > equivalence coe(ua(e)) : Equiv(A,B), which is typally equal to e.=
> >
> > Now we might wonder whether coe(ua(e)) might be in fact *judgment= ally*
> > equal to e; or at least whether this might be true of some, if no= t
> > all, of its five components.=C2=A0 In Book HoTT this is clearly n= ot the
> > case, since univalence is posited as an axiom about which we know=
> > nothing else.=C2=A0 But what about cubical type theories?=C2=A0 C= an any of the
> > components of an equivalence e be recovered, up to judgmental
> > equality, from coe(ua(e))?=C2=A0 (My guess would be that at least= the
> > function A -> B, and probably also the function B -> A, can= be
> > recovered, but perhaps not more.)
> >
> > --
> > You received this message because you are subscribed to the Googl= e Groups "Homotopy Type Theory" group.
> > To unsubscribe from this group and stop receiving emails from it,= send an email to HomotopyTypeTheory+unsubscribe@googlegroups.c= om.
> > To view this discussion on the web visit https://groups.google.com/d/msgid/HomotopyTypeTheory/CAOvivQzzSXNHs%2BzbPQ= TyHEuU53aHXJ0sPe4pr%2Byf0ahLGvUpVA%40mail.gmail.com.
>
> --
> You received this message because you are subscribed to the Google Gro= ups "Homotopy Type Theory" group.
> To unsubscribe from this group and stop receiving emails from it, send= an email to HomotopyTypeTheory+unsubscribe@googlegroups.com.
> To view this discussion on the web visit https://groups.google.com= /d/msgid/HomotopyTypeTheory/1DF8E802-2959-4BEF-A85A-3C6E5E7B9595%40wesleyan= .edu.

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