(t,s)-racks and their link invariants

@article{Ceniceros2010tsracksAT,
  title={(t,s)-racks and their link invariants},
  author={Jessica Ceniceros and Sam Nelson},
  journal={arXiv: Geometric Topology},
  year={2010}
}
A (t,s)-rack is a rack structure defined on a module over the ring $\ddot\Lambda=\mathbb{Z}[t^{\pm 1},s]/(s^2-(1-t)s)$. We identify necessary and sufficient conditions for two $(t,s)$-racks to be isomorphic. We define enhancements of the rack counting invariant using the structure of (t,s)-racks and give some computations and examples. As an application, we use these enhanced invariants to obtain obstructions to knot ordering. 
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