The Links-Gould Invariant

David De Wit

Published 1999-09-13Version 1

The ``Links-Gould invariant'' is a two-variable Laurent polynomial invariant of oriented (1,1) tangles, which is derived from the representation of the braid generator associated with the one-parameter family of four dimensional representations with highest weights (0,0|a) of the quantum superalgebra U_q[gl(2|1)]. We use an abstract tensor state model to evaluate the invariant, as per the construction of the bracket polynomial state model used by Louis Kauffman to derive the Jones polynomial. This model facilitates both computation and theoretical exploration. Our family of representations has a two-variable quantum R matrix (unique up to orthogonal transformations). Choosing this R matrix to yield the representation of the braid generator ensures that our polynomial invariant will also have two variables. We construct this R matrix from first principles. We have evaluated the invariant for several critical link examples and numerous other links of special forms. Throughout, the assistance of Mathematica has been invoked. We observe that the Links-Gould invariant is distinct from the two-variable HOMFLY polynomial in that it detects the chirality of some links where the HOMFLY fails. Notably, it does not distinguish inverses, which is not surprising as we are able to demonstrate that no invariant of this type should be able to distinguish between inverses. It also does not distinguish between mutants.