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Kashiwara strings in the three-dimensional sl2-module

Example

Let g=sl2, so I={1}, a11=2, d1=1, q1=q, P=Zω with ⟨h1,ω⟩=1 and α1=2ω. Let V=L1(2) be the three-dimensional simple Uq(sl2)-module of Divided-power commutation and the simple Uqi(sl2) string modules(iii), with highest weight vector v2=w2 of weight 2ω, and put v1:=F1v2 and v0:=F1(2)v2=1[2]1F12v2; then (v2,v1,v0) is a Q(q)-basis of V with weights 2ω,0,−2ω, and F1v2=v1, F1v1=[2]1v0, F1v0=0.

(a) String decompositions. v2=v2 (the n=0 component is v2), v1=F1(1)v2 (component u1=v2), v0=F1(2)v2 (component u2=v2). Hence e~1v2=0,e~1v1=v2,e~1v0=v1,f~1v2=v1,f~1v1=v0,f~1v0=0. In particular F1v1=[2]1v0≠0 while f~1v1=v0, and F1v0=0 while f~1v0=0: the Kashiwara operator f~1 agrees with F1 on Ker⁡E1=Q(q)v2 but not on the interior of the string, and e~1 agrees with E1 on lowest weight vectors.

(b) Crystal. L(2ω)=A0v2⊕A0v1⊕A0v0 is a crystal lattice of V, and B(2ω)={b2,b1,b0} with b2=v2+qL(2ω), b1=v1+qL(2ω), b0=v0+qL(2ω) is a crystal basis with crystal graph b2→1b1→1b0. The invariants are wt⁡(b2)=2ω, wt⁡(b1)=0, wt⁡(b0)=−2ω, ε1(b2)=0, ε1(b1)=1, ε1(b0)=2, φ1(b2)=2, φ1(b1)=1, φ1(b0)=0, and φ1(b)=ε1(b)+⟨h1,wt⁡(b)⟩ holds for all three (for example 2=0+⟨h1,2ω⟩ and 0=2+⟨h1,−2ω⟩), in accordance with The crystal graph, εi, φi and the weight function.

(c) Partial inverses. e~1f~1=id⁡V−pbot and f~1e~1=id⁡V−p0, where p0 is the projection v2↦v2, v1,v0↦0 onto Ker⁡E1; evaluated at v2 this shows the failure at the top of the string: f~1e~1v2=0≠v2. No choice principle is used. Here pbot projects onto Q(q)v0, so e~1f~1v0=0≠v0 fails at the bottom as well as f~1e~1v2=0≠v2 fails at the top.

Facts & Assumptions

Given: The datum of sl2, the module V=L1(2) with basis v2,v1,v0 and the operators of The Kashiwara operators e~i and f~i of an integrable module.

[F1]

In V=L1(2) use descending weight labels v2 (highest), v1:=F1v2 and v0:=F1(2)v2. Then K1v2=q2v2, K1v1=v1, K1v0=q−2v0; F1v2=v1, F1v1=[2]1v0, F1v0=0; and E1v2=0, E1v1=[2]1v2, E1v0=v1. These are the supplier's divided-string formulas with its position index t replaced by 2−t (Divided-power commutation and the simple Uqi(sl2) string modules, part (iii)).

[F2]

A vector u∈Mλ has a unique admissible string decomposition u=∑nF1(n)un with un∈Hλ+nα1=ker⁡E1∩Mλ+nα1; the operators act by e~1(∑nF1(n)un)=∑n≥1F1(n−1)un and f~1(∑nF1(n)un)=∑n≥0F1(n+1)un (Uniqueness of the i-string decomposition of an integrable module, The Kashiwara operators e~i and f~i of an integrable module).

[F3]

dim⁡Vμ=1 for μ=2ω,0,−2ω and E1v2=0, F1v2=v1, F1v1=[2]1v0 with [2]1=q+q−1≠0 (Crystal lattices, crystal bases and their crystals, [F1], The Kashiwara lattices L(λ) and L(∞) and their generating sets).

Verification

technique · direct
1.1F1F2F3

The string components. V2ω=Q(q)v2 with E1v2=0, so the decomposition of v2 is v2=F1(0)v2 with top component v2∈H2ω. Since v1=F1v2=F1(1)v2 and V0=Q(q)v1, the decomposition of v1 is v1=F1(1)v2 with top component v2∈H2ω; likewise v0=F1(2)v2 is the decomposition of v0 with top component v2∈H2ω.

2.1F1F2step 1.1

The six formulas. Reading the definitions off the decompositions of step 1.1: e~1v2=0, e~1v1=F1(0)v2=v2, e~1v0=F1(1)v2=v1 and f~1v2=F1(1)v2=v1, f~1v1=F1(2)v2=v0, f~1v0=F1(3)v2=0. In particular F1v1=[2]1v0 while f~1v1=v0, and E1v0=v1=e~1v0 and F1v0=0=f~1v0 show the bottom agreements; the interior difference is exactly the factor [2]1≠1.

3.1F1F3step 2.1

Lattice and residue basis. Since v2,v1,v0 have distinct weights, L(2ω)=A0v2⊕A0v1⊕A0v0 and it generates V over Q(q); step 2.1 shows that e~1 and f~1 preserve it, so it is a crystal lattice. The classes b2,b1,b0 of the three basis vectors are Q-independent in L/qL because L is free with those basis vectors and the crystal-limit map is an isomorphism, so they form a Q-basis; the operator action of step 2.1 lies in {b2,b1,b0}∪{0}, and the only nonzero arrows are b2→b1 and b1→b0, whose inverses under e~1 hold, giving the partial-inverse axiom.

4.1step 3.1algebra

The invariants. From the chain b2→b1→b0 the values ε1(b2)=0, ε1(b1)=1, ε1(b0)=2 and φ1(b2)=2, φ1(b1)=1, φ1(b0)=0 are immediate, and the weights are those of step 3.1; the equalities φ1(b)=ε1(b)+⟨h1,wt(b)⟩ read 2=0+2, 1=1+0, 0=2−2.

5.1step 2.1∎

Partial inverses. Evaluating step 2.1 gives e~1f~1v2=v2, e~1f~1v1=v1, e~1f~1v0=0=v0−pbotv0, and f~1e~1v2=0=v2−p0v2, f~1e~1v1=v1=v1−p0v1, f~1e~1v0=v0=v0−p0v0; by linearity these are the stated identities, and no choice is used because everything is computed on the three displayed vectors.

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