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# 7 Descriptions of degree-one rational Cherednik algebras

Here we present, via generators and relations, degree-one PBW deformations of the skew group algebras S(W⊕W)#Sn and S(h⊕h)#Sn that result from Theorems 4.1, 5.9, and 6.4when n≥3. This facilitates comparison with degree-zero deformations (i.e., rational Cherednik alge-bras) and with the PBW deformations of S(W)#Sn in [8]. The classifications are summarized in Tables 2 and 3. We reiterate that the case when n= 2 can be analyzed in similar fashion, but involves some differences in the dimensions of spaces of pre-Drinfeld orbifold algebra maps and in the parameter relations required in order to satisfy the PBW conditions.

7.1 Algebras for the doubled permutation representation

First, the Lie orbifold algebra maps involving 17 parameters classified in Theorem 4.1 yield a variety controlling the Lie orbifold algebras that deformS(W⊕W)#Snin degree one. Based on representative calculations in Macaulay2 [14] we conjecture that this projective variety is of dimension seven. Some subvarieties of potential interest are indicated in Table1in Section4.

WhenκL1 ≡0 these Lie orbifold algebras specialize to rational Cherednik algebras corresponding to the parameter c and the general G-invariant skew-symmetric bilinear form κC1 involving α and β (because W is decomposable — see [6, proof of Theorem 1.3]).

Theorem 7.1 (Lie orbifold algebras for doubled permutation representation overC[t]). Let Sn (n≥3) act onV =W⊕W with basis B={x1, . . . , xn, y1, . . . , yn} by the doubled permutation representation. Fora1, . . . , a7, b1, . . . , b7, α, β, c∈Csubject to conditions (4.1),(4.2), and (4.3), define κLL1 and κCC1Cref to be the linear and constant cochains such that for 1≤i6=

j≤n,

κL(xi, xj) = (a1(xi−xj) +b1(yi−yj)), κC(xi, xj) = 0, κL(yi, yj) = (a2(xi−xj) +b2(yi−yj)), κC(yi, yj) = 0,

κL(xi, yi) = (a3xi+a4x[n]+b3yi+b4y[n]), κC(xi, yi) =α+cX

k6=i

(ik), κL(xi, yj) = (a5xi+a6xj +a7x[n]+b5yi+b6yj+b7y[n]), κC(xi, yj) =β−c(ij).

Then the quotient Hκ,t of T(V)#Sn[t]by the ideal generated by uv−vu−κL(u, v)t−κC(u, v)t2 |u, v∈ B

is a Lie orbifold algebra over C[t]. In fact, the algebras Hκ,1 are precisely the Drinfeld orbifold algebras such that κL is supported only on the identity.

Table 2. Classification of Drinfeld orbifold algebra maps for Sn acting onWW. Linear partκL Constant partκC Parameter relations Reference

κL1 0 (4.1) Theorem4.1

κC1 (4.1)–(4.2)

κC1Cref withκCref 6≡0 (4.4)–(4.17)

κLref κC3-cyc none Theorem5.9

κC3-cycCref none

κC3-cycCrefC1 (5.3)–(5.4)

κL1Lref κC3-cycCrefC1 Theorem5.10(1)–(3) Theorem5.10

? ?

0 κC1Cref none

When the indicated parameter relations are satisfied, the mapκ=κL+κC is a Drinfeld orbifold algebra map. The question marks indicate there could be further maps withκL=κL1 +κLref.

Second, forκLsupported only off the identity, Theorem 5.9shows that by comparison there is only a seven-parameter family of Drinfeld orbifold algebra maps and these are controlled by a projective variety which, according to a few representative calculations in Macaulay2 [14], appears to be four-dimensional. The resulting algebras also specialize to rational Cherednik algebras parametrized by α,β, andc when κLrefC3-cyc≡0.

Theorem 7.2 (Drinfeld orbifold algebras for doubled permutation representation over C[t]).

Let Sn (n ≥ 3) act on V = W ⊕W with basis B = {x1, . . . , xn, y1, . . . , yn} by the doubled permutation representation. Suppose a, a, b, b, c, α, β ∈C satisfy conditions (5.3) and (5.4).

Define κLL1 and κCC1CrefC3-cyc to be the cochains such that for 1≤i6=j≤n, κL(xi, xj) =κL(yi, yj) = 0,

κL(xi, yi) = P

k6=i

a−a

xi,k+ax[n]+ b−b

yi,k+by[n]

⊗(ik), κL(xi, yj) =− a−a

xi,j+ax[n]+ b−b

yi,j+by[n]

⊗(ij) and

κC(xi, yj) =β−c(ij)− a−a

b−b P

k6=i,j

(ijk)−(kji), κC(xi, xj) = b−b2 P

k6=i,j

(ijk)−(kji), κC(yi, yj) = a−a2 P

k6=i,j

(ijk)−(kji), κC(xi, yi) =α+cP

k6=i

(ik).

Then the quotient Hκ,t of T(V)#Sn[t]by the ideal generated by uv−vu−κL(u, v)t−κC(u, v)t2 |u, v∈ B

is a Drinfeld orbifold algebra over C[t]. Further, the algebras Hκ,1 are precisely the Drinfeld orbifold algebras such thatimκLg ⊆Vg for eachg∈Sn andκL is supported only off the identity.

An analogous statement may be made for algebras constructed from the family of lifts of κL1Lref described in Theorem 5.10but is omitted here.

Table 3. Classification of Drinfeld orbifold algebra maps forSn acting onhh.

Linear part κL Constant partκC Parameter relations Reference

κLref κC3-cyc 2a+ (n−2)a= 0 Theorem6.4

2b+ (n−2)b = 0 κC3-cycCref 2a+ (n−2)a= 0 2b+ (n−2)b = 0

0 κCref none Remark6.5

κC1 α+ (n−1)β= 0

κC1Cref α+ (n−1)β= 0

When the parameter relations hold, the mapκ=κL+κC is a Drinfeld orbifold algebra map.

7.2 Algebras for the doubled standard representation

By Theorem 6.3the only Lie orbifold algebras forSn acting onh⊕hby the doubled standard representation are the known rational Cherednik algebrasHnβ,c. However, by Theorem6.4there is in this case a three-parameter family of Drinfeld orbifold algebras which are not graded Hecke algebras, but which specialize whena=b= 0 to the rational Cherednik algebrasH0,cforSn. Theorem 7.3(Drinfeld orbifold algebras for doubled standard representation overC[t]). LetSn

(n≥3) act on h⊕h by the doubled standard representation. For a, b, c ∈C and x¯i and y¯i as in (3.1) define κL andκC as in Theorem 6.4. Then the quotient Hκ,t of T(h⊕h)#Sn[t]by the ideal generated by

u¯¯v−¯v¯u−κL(¯u,v)t¯ −κC(¯u,v)t¯ 2 |u,¯ v¯∈B¯

is a Drinfeld orbifold algebra ofS(h⊕h)#Sn overC[t]. Further, the algebrasHκ,1 are precisely the Drinfeld orbifold algebras such that imκLg ⊆(h⊕h)g for each g∈ Sn and κL is supported only off the identity. Specializing a=b= 0 yields the rational Cherednik algebra H0,c. Acknowledgements

We thank the referees for helpful suggestions and questions that improved the writing of the paper, particularly those that prompted us to add Proposition 6.1, improve Section 4.5, and reorganize overall. We also thank Lily Silverstein for helpful conversations related to Section4.5.

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