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Let $X$ and $Y$ be complex projective varieties. Let's assume we have a finite flat morphism $f:X\rightarrow Y$ of degree $k$. We know that it is possible to pullback and also pushforward algebraic cycles along $f$. These pullback and pushforward of cycles also induces morphisms on Chow groups. I was wondering whether this also induces regular morphisms on Chow varieties or not. More precisely let $C_{r,d}(Y)$ and $C_{r,d}(X)$ be the Chow variety of degree $d$ and $r$ dimensional cycles on $X$ and $Y$ respectively. Note that degree of cycles depends on the projective embedding.

The two questions below were answered positively:

1) Does pullback induce a regular morphism of Chow varieties from $C_{r,d}(Y)$ to $C_{r, kd'}(X)$? Note that $d'$ is not necessarily same as $d$ since the degrees can change depending on the embedding, I am tempted to assume that there is some embedding for the pair $X$ and $Y$ such that one can assume $d=d'$ but I decided to not to assume that.

2) Does pushforward induce a regular morphism of Chow varieties like $C_{r, d}(X) \rightarrow C_{r, d'}(Y)$? again the $d'$ can be something other than $d$ depending on the embedding of varieties.

Let's consider the push-forward map $f_*:C_{r, d}(X) \rightarrow C_{r, d'}(Y)$. Is $f_*$ a flat morphism? Furthermore you can assume $f$ is a quotient map under some finite group action. If not, does it become flat after restricting to the Chow variety of irreducible cycles?

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    $\begingroup$ Yes, there are pushforwards and pullbacks on the level of Chow schemes. The basic idea is explained in Mumford's "Lectures on curves on an algebraic surface." $\endgroup$ Commented Aug 25, 2022 at 0:31
  • $\begingroup$ @JasonStarr Thanks I'll take a look. So it seems it becomes easier by working with Chow scheme rather than variety right? The fact the variety structure can be extended to scheme structure makes it slightly easier since I think in that case it just follows from the functor of points view and the regular pushforward/pullback of cycles. $\endgroup$
    – user127776
    Commented Aug 25, 2022 at 0:44
  • $\begingroup$ The only reason that I wrote "Chow scheme" is because the argument also works for the Chow scheme, and not merely for the Chow variety. There is a functor of points view for the Chow variety as well, cf., the first chapter of "Rational curves on algebraic varieties" by J'anos Koll'ar. $\endgroup$ Commented Aug 25, 2022 at 10:53
  • $\begingroup$ The morphism $f_*$ sometimes is flat, but usually it is not. Consider cyclic branched covers of the projective plane $\mathbb{P}^2$ and the parameter space of lines on the cover. Every line on the cover maps to a line in the projective plane, but typically a line in the projective plane "lifts" to a line in the cover if and only if it satisfies some "contact condition" with respect to the branch divisor in $\mathbb{P}^2$. Thus, for a double cover of the plane branched over a plane conic (i.e., for a quadric surface), only the tangent lines to the conic lift. $\endgroup$ Commented Aug 26, 2022 at 10:55
  • $\begingroup$ If you like at a refined pushforward map to the space of cycles in the target satisfying the specified contact conditions, that often is flat. This is one of the ingredients for relating “relative” Gromov-Witten invariants to Gromov-Witten theory of cyclic covers. $\endgroup$ Commented Aug 26, 2022 at 11:11

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