At What kind of topological invariants can you get from just Hopf algebras?, link [4] is not being processed:

However, it is there in the source:

  [1]: http://www-math.mit.edu/~ptingley/publications/minus-sign.pdf
[2]: https://arxiv.org/abs/math/0503040

Apparently you need to let the <hr> tag its own paragraph. I fixed it now.