Degenerate 3-dimensional Sklyanin algebras are monomial algebras

The 3-dimensional Sklyanin algebras, S(a,b,c), over a field k, form a flat family parametrized by points (a,b,c) lying in P^2-D, the complement of a set D of 12 points in the projective plane, P^2. When (a,b,c) is in D the algebras having the same defining relations as the 3-dimensional Sklyanin algebras are said to be "degenerate". Chelsea Walton showed the degenerate 3-dimensional Sklyanin algebras do not have the same properties as the non-degenerate ones. Here we prove that a degenerate Sklyanin algebra is isomorphic to the free algebra on u,v,w, modulo either the relations u^2=v^2=w^2=0 or the relations uv=vw=wu=0. These monomial algebras are Zhang twists of each other. Therefore all degenerate Sklyanin algebras have the same category of graded modules. A number of properties of the degenerate Sklyanin algebras follow from this observation. We exhibit a quiver Q and an ultramatricial algebra R such that if S is a degenerate Sklyanin algebra then the categories QGr(S), QGr(kQ), and Mod(R), are equivalent; neither Q nor R depends on S. Here QGr(-) denotes the category of graded right modules modulo the full subcategory of graded modules that are the sum of their finite dimensional submodules.

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