EDIT: two years later, I considered instead this at the level of the Tannaka–Krein categories. If at parameter
, then
if and only if
. This may still be true, but I tried to relax to
and this doesn’t work for
, the half-liberated orthogonal group. For every
, because
in
, quotienting to
forces the isotropy to be commutative. This is related to “uniformity” in the sense of Banica.
The following is an approach to the maximality conjecture for which asks what happens to a counterexample
when you quotient
. If
is noncommutative, you generate another counterexample
.}
Most of my attempts at using this approach were doomed to fail as I explain below.
Let be a quantum permutation group with (universal) algebra of continuous functions
generated by a fundamental magic representation
. Say that
is classical when
is commutative and genuinely quantum when
is noncommutative.
Definition 1 (Commutator and Isotropy Ideals)
Given , the commutator ideal
is given by:
The isotropy ideal is given by:
Lemma 1
The commutator ideal is equal to the ideal
Proof:
with a similar statement for .
On the other hand:
Proposition 1
The commutator and isotropy ideals are Hopf*-ideals. The quotient gives a classical permutation group
, the classical version
, and the quotient
giving an isotropy quantum subgroup. If
is classical, this quotient is the isotropy subgroup of
for the action
.
Via , the classical permutation group is a quantum subgroup
. It is conjectured that for all
it is a maximal subgroup.
Theorem 1 (Wang/Banica/Bichon)
is maximal for
.




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