From: Martin Escardo <escardo...@googlemail.com>
To: "HomotopyT...@googlegroups.com" <HomotopyT...@googlegroups.com>
Subject: Does MLTT have "or"?
Date: Tue, 2 May 2017 10:09:15 +0100 [thread overview]
Message-ID: <c0286cfb-2394-8e33-715b-d996dea6ab82@googlemail.com> (raw)
Last week in a meeting I had a technical discussion with somebody, who
suggested to post the question here.
(1) Prove (internally) or disprove (as a meta-theorem, probably with a
counter-model) the following in (intensional) Martin-Loef Type Theory:
* The poset of subsingletons (or propositions or truth values) has
binary joins (or disjunction).
(We know it has binary meets and Heyting implication, which amounts to
saying it is a Heyting semilattice. Is it a lattice?)
(2) The question is whether given any two propositions P and Q we can
find a proposition R with P->R and Q->R such that for any proposition
R' with P->R' and Q->R' we have R->R'. (R is the least upper bound of
P and Q.)
(3) Of course, if MLTT had propositional truncations ||-||, then the
answer would be R := ||P+Q||. But we can ask this question for MLTT
before we postulate propositional truncations as in (1)-(2).
(4) What is a model of intensional MLTT with a universe such that
||-|| doesn't exist?
More precisely, define, internally in intensional MLTT,
hasTruncation(X:U) := Σ(X':U),
isProp(X')
× (X→X')
× Π(P:U), (isProp(P) × (X→P)) → (X'→P).
Is there a model, with universes, the falsifies this?
Preferably, we want models that falsify this but validate
function extensionality (and perhaps also propositional
extensionality).
Best,
Martin
next reply other threads:[~2017-05-02 16:45 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-05-02 9:09 Martin Escardo [this message]
2017-05-02 19:04 ` [HoTT] " Michael Shulman
2017-05-03 6:47 ` Martin Escardo
2017-05-12 18:10 ` Martin Escardo
2017-05-12 18:41 ` Martin Escardo
2017-05-13 21:46 ` Michael Shulman
2017-05-14 9:54 ` stre...
2017-05-16 6:20 ` Michael Shulman
2017-05-03 10:55 ` Thomas Streicher
2017-05-03 14:25 ` Martin Escardo
2017-05-03 14:48 ` Thomas Streicher
2017-05-03 15:04 ` Martin Escardo
2017-05-03 12:17 ` Andrew Polonsky
2017-05-03 12:24 ` [HoTT] " Martin Escardo
2017-05-03 12:24 ` Michael Shulman
2017-05-06 13:51 ` Andrew Swan
2017-05-07 13:49 ` [HoTT] " Martin Escardo
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