Pullback of homology via “trace” and Poincaré duality

algebraic-topologyhomology-cohomology

Let $f: X \to Y$ be a morphism of compact Riemann surfaces (in particular smooth orientable compact manifolds of dimension 2). Denote the degree by $d$. We have a natural pushforward $H_i(f)$ in homology. I also want a pullback of homology, and I can think of two ways to do that:

  • the composition $H_i(Y) \xrightarrow{\text{Poincaré duality}} H^{2-i}(Y) \xrightarrow{\text{natural cohomology pullback}} H^{2-i}(X) \xrightarrow{\text{Poincaré duality}} H_i(X)$
  • the "trace map" induced by: For a path in $Y$, take all the $d$ primages of the start point, and lift the path with those starting points. Then add those paths (in the chain group), which should give you a cycle (?!), and hence a homology class.

My question is: Are those two maps the same?

Best Answer

Yes they are the same and what you call the trace map (and its dual in cohomology) is often called the transfer homomorphism. I will generalize to $X$ and $Y$ oriented triangulable closed $n$-manifolds but assume that $f : X \to Y$ is a degree $d$ covering map to ensure that the basepoint does have $d$ preimages. With this assumption it is easy to check that the transfer map on chains commutes with taking boundaries, so does produce a map on homology. With some more care you can also make it work for branched covers. Denote the transfer or trace map by $\DeclareMathOperator{\tr}{tr} \tr : H_*(Y) \to H_*(X)$.

Let $\alpha \in H^k(Y)$. Note that if $\sigma \in C_k(Y)$ and $\sigma'$ is any lift in $C_k(X)$ then by definition $f_*(\sigma') = \sigma$, so $f^*(\alpha)(\sigma') = \alpha(\sigma)$. From this it follows that for $h \in H_*(Y)$ (possibly assuming $* \ge k$ depending on how general your definitions are), $$\tr(h \frown \alpha) = \tr(h) \frown f^*(\alpha).$$ Taking $h = [Y]$ (the fundamental class of $Y$) you get $$\tr[Y] \frown f^*(\alpha) = \tr([Y] \frown \alpha).$$ This is exactly what you want once we verify that $\tr[Y] = [X]$. But if you start with a triangulation of $Y$ and pull back to $X$ then both $\tr[Y]$ and $[X]$ are just the sum of all the oriented top-dimensional simplices in $X$.

As a remark, one advantage of the Poincaré duality definition of this wrong-way or Umkehr map is that it works for any continuous map of oriented closed manifolds, or even more generally any proper continuous map of oriented manifolds.

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