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Beilinson's version of Hodge conjecture has the following form. For any quasi-projective smooth complex variety $X$ the following map is surjective: $$H^i_{\mathcal{M}}(X, \mathbb{Q}(j))\rightarrow \text{Hom}_{\text{MHS}}(\mathbb{Q}(0), H^i(X,\mathbb{Q}(j)))=\Gamma_H(H^i(X,\mathbb{Q}(j)))$$ On the left side we have the Motivic cohomology groups and on the right hand we have the hom in the category of mixed Hodge structures between $\mathbb{Q}(0)$ and cohomology of $X$ with various Tate twsits $\mathbb{Q}(n)$. This conjecture is known to be false for quasi-projective varieties (the first counter-example is due to Uwe Jannsen). For projective varieties for $i=2j$ one recovers the Hodge conjecture. Let's assume $X$ has this property that we know its cohomology ring is generated by the first cohomology group then does Beilinson-Hodge conjecture hold for $X$?

As far as I can see with this assumption one can show that $\Gamma_H(H^{2j}(X,\mathbb{Q}(j)))$ is generated by $\Gamma_H(H^{2}(X,\mathbb{Q}(1)))$. (Here the fact that weight $k$ part of the $k$-th cohomology becomes the smallest weight in the mixed Hodge structure plays a role in the proof) and so this implies the classical Hodge conjecture. I was not able to figure out whether the assumption mentioned on the cohomology ring, necessarily imply the conjecture for $i\neq 2j$ or not.

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    $\begingroup$ Your hypothesis holds for abelian varieties, but the Hodge conjecture is not known for them, already in dimension 4. $\endgroup$
    – abx
    Commented Aug 13, 2022 at 7:14
  • $\begingroup$ @abx You are right, one needs more than just assuming the generation by $H^1$. I think for $i=2j$ if one adds another assumption $\Gamma_H(H^2(X, \mathbb{Q}(1)))=0$ then it implies Hodge. $\endgroup$
    – user127776
    Commented Aug 13, 2022 at 18:56
  • $\begingroup$ I think the second assumption you write in your comment fails for all projective schemes of positive dimension. $\endgroup$ Commented Aug 15, 2022 at 0:45
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    $\begingroup$ @JasonStarr You are right what I had in my mind was not projective varieties. I was considering algebraic closure of function fields. It is related to another question. So the idea was if singular cohomology of algebraic closure of varieties are generated by $H^1$ does it imply the Hodge conjecture? or Beilinson-Hodge? It seems the answer might be positive for Hodge (not sure about Beilinson-Hodge) but then again I don't know if my other question has any chance of having a positive answer or not. $\endgroup$
    – user127776
    Commented Aug 15, 2022 at 6:26

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