Singular Loci of SL2(C)-Representation Varieties of Torus Knot Groups

Source abstract

Let $T(q,p)=\langle x,y\mid x^p=y^q\rangle$, where $p,q>1$ are relatively prime, and let \[ R=\Hom(T(q,p),\mathrm{SL}_2(\mathbb C)). \] We compute the integral homology of the singular locus of $R$. We show that this locus has complex dimension $2$ and is a disjoint union of $μ=(p-1)(q-1)$ smooth affine surfaces, each homotopy equivalent to the two-dimensional sphere. Consequently, \[ H_0(\Sing R;\Z)\cong H_2(\Sing R;\Z)\cong\Z^μ, \] and all remaining homology groups vanish. Thus both nonzero Betti numbers equal the first Betti number of the Milnor fiber of the plane curve singularity $z^p+w^q=0$, namely its Milnor number $μ$.