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Let $A/\mathbb F_q$ be an abelian variety over a finite field with Weil numbers $q^{1/2}\alpha_1,\dots,q^{1/2}\alpha_n$.

  1. Consider the numbers $q^{d/2}\alpha_1,\dots,q^{d/2}\alpha_n$. These are still conjugate Weil numbers and by Honda-Serre-Tate should correspond to an abelian variety $\tilde A/\mathbb F_{q^d}$. What is the relation between $\tilde A$ and $A$?

  2. Consider the $nd$ numbers $\{q^{1/2}\alpha_i^{1/d}\}$. The Abelian variety $\tilde A'$ corresponding to these numbers is, I believe the Weil Restriction of $\tilde A$ from $\mathbb F_{q^d}$ to $\mathbb F_q$. Is there a direct relation between $\tilde A'$ and $A$?

In my particular case, I also have a $\mathbb Z[\zeta_{\ell^m}]$ action on $A$ which has dimension $\phi(\ell^m)$ for $\ell $ a a prime different from $q$, $m$ very large, $d = \ell$ and $\dim \tilde A = \ell\dim \tilde A'$. I don't know if this is relevant at all.

(Of course, Honda-Tate is only upto isogeny but I would be very happy if there was a functor upto isomorphism that picked out some canonical $\tilde A,\tilde A'$.)

Edit - An explanation of the terminology:

Weil numbers are algebraic integers that have absolute value $q^{w/2}$ under every complex embedding where $q$ is a prime power. The integer $w$ is denoted it's weight. They are interesting because the Riemann hypothesis shows that over a finite field $\mathbb F_q$, for any smooth variety $X$, the eigenvalues of the Frobenius on the etale cohomology $H^i(X,\mathbb Q_\ell)$ are Weil numbers of weight $i$.

In particular, since the cohomology of an Abelian variety is determined by $H^1$, the weight $1$ Weil numbers determine all the others.

Conversely, a theorem by Honda-(Serre-)Tate shows that any conjugacy class (under the Galois action) of Weil numbers of absolute value $q^{1/2}$ can be realized in the cohomology of an Abelian variety over $\mathbb F_q$.

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  • $\begingroup$ This does not produce an abelian variety of the same dimension as $A$. The dimension in Honda-Serre-Tate (for simple abelian varieties) is not simply the degree of the polynomial, as you can see when $q$ is a perfect square and $\alpha =1$. $\endgroup$
    – Will Sawin
    Commented Oct 23, 2019 at 23:06
  • $\begingroup$ Oh that's right! $\endgroup$
    – Asvin
    Commented Oct 23, 2019 at 23:15
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    $\begingroup$ Over $\mathbb F_{q^d}$, it's the $q^d$-adic valuation, so the integrality does change. $\endgroup$
    – Will Sawin
    Commented Oct 24, 2019 at 1:37
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    $\begingroup$ Already for the simplest case of elliptic curves over a prime field and odd $d$, the situation seems pretty complicated: for the supersingular curves you can just take extension-of-scalars to $\mathbb{F}_{q^d}$, while for the ordinary curves the corresponding abelian varieties are of dimension $d$. $\endgroup$ Commented Oct 29, 2019 at 18:50
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    $\begingroup$ it would be great if this question was readable to a let us say hypothetical person who was an algebraic geometer but didn't know what Weil numbers or Honda-Serre-Tate were. $\endgroup$ Commented Nov 4, 2019 at 4:55

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