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I am reading the article "Signature of links" by Kauffman and Taylor. Here they show that it is possible to calculate the nullity of a link $L\subset S^3$ by knowing the first betti number of the double cover of the exterior of the link in $S^3$. As they say in pages 354 and 355, to compute such betti number one needs to apply the Gysin sequence with $\mathbb{Z}/2\mathbb{Z}$ coefficients and then the Bockstein spectral sequence.

Why is it not possible to apply the Gysin sequence directly with $\mathbb{Q}$ coefficients? It seems to me that this would simplify the proof of the subsequent Theorem 2.6.

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  • $\begingroup$ To apply the Gysin sequence with $\mathbb{Q}$-coefficient you need an orientability condition on your bundle. Maybe they want to circumvent that? $\endgroup$
    – John Greenwood
    Commented Mar 4, 2020 at 16:14
  • $\begingroup$ I do not know. It seems to me that in the article they always have these orientability construction. Maybe since they are working with double covers it is easier to compute the $\mathbb{Z}/2\mathbb{Z}$-homology? $\endgroup$
    – Diego95
    Commented Mar 5, 2020 at 10:41

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