Sklyanin twist
A 2-cocycle (comodule) twist preserves Hilbert series and AS-regularity but changes the geometry: the elliptic point scheme of the Sklyanin algebra is replaced by exactly 20 point modules together with infinitely many fat-point modules of multiplicity \(2\) (Davies). It is an "exotic elliptic algebra" in the sense of Chirvasitu–Smith. The pencil of central quadrics is preserved.
- Relations
- \((x_1 x_2 - x_2 x_1) - α (x_3 x_4 - x_4 x_3)\)
- \((x_1 x_2 + x_2 x_1) - (x_3 x_4 + x_4 x_3)\)
- \((x_1 x_3 - x_3 x_1) - β (x_4 x_2 - x_2 x_4)\)
- \((x_1 x_3 + x_3 x_1) - (x_4 x_2 + x_2 x_4)\)
- \((x_1 x_4 - x_4 x_1) + γ (x_2 x_3 - x_3 x_2)\)
- \((x_1 x_4 + x_4 x_1) + (x_2 x_3 + x_3 x_2)\)
- Parameters
- \(β, γ\) — As for the Sklyanin algebra, with \(α = -(β + γ)/(1 + βγ)\).
- Point scheme
- 20 points (dimension \(0\)).
- Centre
- still \(k[Ω_1, Ω_2]\) (GK-dimension \(2\)); not a cocycle-twist invariant.
- Normal elements
- normal locus dimension \(-1\) (degree 1), \(1\) (degree 2).
- Hochschild cohomology
- \(\mathrm{HH}^\bullet_0(A)\) \(= (1, 1, 8, 21)\)
\(\mathrm{HH}^\bullet(\operatorname{qgr} A)\) \(= (1, 0, 14, 19)\) - Kodaira–Spencer
- rank \(2\); injective: yes; surjective: no.
- Introduced
- 2016, MR3490085 . The cocycle twist of the 4-dimensional Sklyanin algebra by the Klein four-group \(G = (\mathbb{Z}/2)^2\), studied by Davies.
- References
- MR3490085 , MR3885145 , arXiv:2511.08390
Code
The presentation, ready to paste into a computer algebra system:
needsPackage "AssociativeAlgebras"
K = frac(QQ[beta, gamma]);
alpha = -(beta + gamma)/(1 + beta*gamma);
A = K<|x1,x2,x3,x4|>;
I = ideal {
(x1*x2 - x2*x1) - alpha*(x3*x4 - x4*x3),
(x1*x2 + x2*x1) - (x3*x4 + x4*x3),
(x1*x3 - x3*x1) - beta*(x4*x2 - x2*x4),
(x1*x3 + x3*x1) - (x4*x2 + x2*x4),
(x1*x4 - x4*x1) + gamma*(x2*x3 - x3*x2),
(x1*x4 + x4*x1) + (x2*x3 + x3*x2)
};
B = A/I;PolyRing := FunctionField(Rationals, ["beta", "gamma"]);;
indets := IndeterminatesOfFunctionField(PolyRing);;
beta := indets[1];;
gamma := indets[2];;
alpha := -(beta + gamma)/(1 + beta*gamma);;
kQ := FreeKAlgebra(PolyRing, 4, "x");;
x1 := kQ.x1;; x2 := kQ.x2;; x3 := kQ.x3;; x4 := kQ.x4;;
rels := [
(x1*x2 - x2*x1) - alpha*(x3*x4 - x4*x3),
(x1*x2 + x2*x1) - (x3*x4 + x4*x3),
(x1*x3 - x3*x1) - beta*(x4*x2 - x2*x4),
(x1*x3 + x3*x1) - (x4*x2 + x2*x4),
(x1*x4 - x4*x1) + gamma*(x2*x3 - x3*x2),
(x1*x4 + x4*x1) + (x2*x3 + x3*x2)
];;
A := kQ / rels;;// untested (Magma not available here)
K<beta, gamma> := RationalFunctionField(Rationals(), 2);
alpha := -(beta + gamma)/(1 + beta*gamma);
F<x1,x2,x3,x4> := FreeAlgebra(K, 4);
rels := [
(x1*x2 - x2*x1) - alpha*(x3*x4 - x4*x3),
(x1*x2 + x2*x1) - (x3*x4 + x4*x3),
(x1*x3 - x3*x1) - beta*(x4*x2 - x2*x4),
(x1*x3 + x3*x1) - (x4*x2 + x2*x4),
(x1*x4 - x4*x1) + gamma*(x2*x3 - x3*x2),
(x1*x4 + x4*x1) + (x2*x3 + x3*x2)
];
A := quo< F | rels >;