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The q=2 torus boundary
Example
Assume the Axiom of Choice, used through Tits deformation. For the multiplicative group is trivial, so the diagonal torus is trivial and there is exactly one character of for every : the regular case is empty for (for the unique coordinate is vacuously pairwise distinct), for all , and every principal series is spherical, . The general theorems remain valid: , the constituents are indexed by partitions with multiplicities , and the finite Hecke algebra at is semisimple by The finite spherical Hecke algebra is semisimple with nondegenerate trace form, so Tits deformation still identifies it with . Its generators also satisfy , with distinct roots and . The boundary phenomenon is the collapse of the parametrising torus and of the Weyl action, not a failure of the constituent description.
Facts & Assumptions
Given: The prime power , the group with Borel and diagonal torus , its character group , the Weyl group with its action on , and the spherical principal series .
For the group is trivial, so is trivial and ; the Weyl action is trivial and for the unique character, while the regular case consists of characters whose coordinates are pairwise distinct (Diagonal torus characters and the Weyl action).
, the permutation module on the complete flags (The spherical principal series is the flag permutation module).
, so for the unique character this dimension is (The Weyl stabiliser controls the principal series endomorphisms).
The constituents of the spherical principal series are indexed by the partitions with multiplicities , the numbers of standard tableaux (The constituents of the spherical principal series of GL_n).
The finite Hecke algebra is semisimple, with quadratic relation ; at this is , whose two roots and are distinct (The finite spherical Hecke algebra is semisimple with nondegenerate trace form, The rank-one quadratic relation in the finite Hecke algebra).
Assume AC; Tits deformation identifies with for every prime power , hence also for (The finite Hecke algebra is non-canonically isomorphic to the group algebra of S_n, The Axiom of Choice).
Verification
For the group is trivial, so the diagonal torus is the trivial group and its character group has the single element ; the Weyl action fixes it, so , and every principal series is . A regular character would need pairwise distinct coordinates, impossible in a one-element group when ; for the single coordinate is vacuously pairwise distinct.
Since the unique character is fixed by , [F3] gives , and [F4] gives the constituent indexing by partitions with multiplicities .
By [F5] the Hecke algebra is semisimple with quadratic relation at , and the two roots are distinct; by [F6] Tits deformation identifies it with in this case as in every other. Hence the collapse of to a point and of the Weyl action to the trivial action is a genuine boundary phenomenon of the parametrising torus, while the endomorphism algebra, the constituent multiplicities and the Tits isomorphism retain their general form.
Steps 1.1-1.3 establish the two boundary statements: the torus and the regular characters collapse, whereas the endomorphism algebra, the partition parametrisation with multiplicities and the Tits isomorphism to remain valid. AC is carried only from the Tits-deformation supplier [F6], as declared.
Depends on
- Diagonal torus characters and the Weyl action
- The Weyl stabiliser controls the principal series endomorphisms
- Regular finite principal series are irreducible
- The constituents of the spherical principal series of GL_n
- The finite spherical Hecke algebra is semisimple with nondegenerate trace form
- The spherical principal series is the flag permutation module
- The rank-one quadratic relation in the finite Hecke algebra
- The finite Hecke algebra is non-canonically isomorphic to the group algebra of S_n
- The Axiom of Choice
Used by
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Sources
- Ivan Losev, Lecture 8: Representations of GL_n(F_q) - Sections 2.1 and 2.3 (the spherical case and Tits deformation), PDF pp. 3-5 (standard reference, not scraped)
- Jay Taylor, Finite Reductive Groups - Theorem 5.18 and Example 5.22, printed pp. 45-46 (standard reference, not scraped)
- Olivier Dudas and Jean Michel, Lectures on Finite Reductive Groups and Their Representations - Section 11.1 and Remark 11.6, printed pp. 45-47 (standard reference, not scraped)