Wednesday, 6 January 2010

ag.algebraic geometry - Why is the Brauer Loop Scheme Not a Variety?

I am trying to grapple with the basics of scheme theory. Is the scheme defined by Spec[C[x,y,z]/(xy,yx,zx)] a variety? What do the points look like?



I suspect it represents points satisfying xy = yz = zx = 0, so it should have three irreducible components {x = y = 0}, {y = z = 0} and {z = x = 0}. The motivation for this example comes from statistical mechanics and it has quite a bit more content:



Consider the space of 3x3 matrices (entries in C) with the following deformation of the matrix product: $P circ Q = sum_{i leq j leq k, cyc} P_{ij} P_{jk} $. Here we summing over j such that i, j, k appear in cyclic order mod 3. It appears in a set of slides on The Combinatorics of the Brauer Loop Scheme.



The paper then proceeds to define a scheme using equations in matrices. In the space of matrices with 0's along the diagonal, we consider the matrices with $M circ M = 0$. In coordinates, the matrix product therefore looks like:
$ left( begin{array}{ccc} 0 & b_{12} & b_{13} \\
b_{21} & 0 & b_{23} \\
b_{31} & b_{32} & 0 end{array} right) circ left(
begin{array}{ccc} 0 & b_{12} & b_{13} \\
b_{21} & 0 & b_{23} \\
b_{31} & b_{32} & 0 end{array} right) = left( begin{array}{lll}
0 & 0 & b_{12}b_{23} \\
b_{23}b_{31} & 0 & 0 \\
0 & b_{31}b_{12} & 0 end{array} right)$
As all the entries on the right side vanish, this defines three equations in six unknowns. (Actually, only the "clockwise" matrix entries seem to be involved.)

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