Library HoTT.Spaces.BAut.Bool.IncoherentIdempotent

Require Import HoTT.Basics HoTT.Types.
Require Import Equiv.BiInv Idempotents.
Require Import Universes.BAut Spaces.BAut.Bool.

Local Open Scope path_scope.

An incoherent quasi-idempotent on BAut (BAut Bool).

Section IncoherentQuasiIdempotent.
Context `{Univalence}.

We use the identity map, and the nontrivial 2-central element of BAut (BAut Bool).

  Definition nontrivial_qidem_baut_baut_bool
  : IsQuasiIdempotent (preidem_idmap (BAut (BAut Bool)))
    := negb_center2_baut_baut_bool.

  Let ret := splitting_preidem_retractof_qidem (preidem_idmap (BAut (BAut Bool))).
  Let s := retract_sect ret.
  Let r := retract_retr ret.
  Let issect := retract_issect ret : r o s == idmap.

Since the space of splittings of the identity pre-idempotent is contractible, nontriviality of this 2-central element implies that not every quasi-idempotence witness of the identity is recoverable from its own splitting.

  Definition splitting_preidem_notequiv_qidem_baut_baut_bool
  : ¬ (s o r == idmap ).
  Proof.
    intros oops.
    assert (p := oops nontrivial_qidem_baut_baut_bool).
    assert (q := oops (isqidem_idmap (BAut (BAut Bool)))); clear oops.
    apply nontrivial_negb_center_baut_baut_bool.
    refine (p^ @ ap s _ @ q).
    pose (contr_splitting_preidem_idmap (BAut (BAut Bool))).
    apply path_contr.
  Defined.

Therefore, not every quasi-idempotence witness is obtainable from *any* splitting, i.e. it may not have any coherentification.

  Definition not_all_coherent_qidem_baut_baut_bool
  : ¬ (∀ q : IsQuasiIdempotent (preidem_idmap (BAut (BAut Bool))),
         { S : Splitting_PreIdempotent (preidem_idmap _) & s S = q }).
  Proof.
    intros oops.
    assert (IsEquiv s).
    { apply isequiv_biinv; split.
      - ∃ r; exact issect.
      - ∃ (fun q ⇒ (oops q ).1).
        exact (fun q ⇒ (oops q ).2). }
    apply splitting_preidem_notequiv_qidem_baut_baut_bool; intros q.
    refine (ap s (ap r (eisretr s q)^) @ _).
    refine (ap s (issect (s ^-1 q)) @ _).
    apply eisretr.
  Defined.

These results show only that not *every* quasi-idempotence witness is coherent. "Clearly" the nontrivial quasi-idempotence witness nontrivial_qidem_baut_baut_bool should be the one that is not coherent. To show this, we would probably need to show that isqidem_idmap *is* in the image of s, and this seems rather annoying to do based on our construction of splitting_preidem_retractof_qidem.

End IncoherentQuasiIdempotent.