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Abelian varieties are projective algebraic varieties endowed with an Abelian group structure. Over the complex numbers, they can be described as quotients of a vector space by a lattice of full rank. They are analogs in higher dimensions of elliptic curves, and play an important role in algebraic geometry and number theory.

5 votes
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Can the simplicity of abelian varieities be implied by the reduction

Here is a slightly different proof. We have the following facts: (1) If $B, C$ are abelian varieties over $K$, then the Néron model of $B\times C$ is the product of the Néron models (this is a sim …
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5 votes
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geometrical reducedness of the identity connected component (reference request)

You can find a proof here following hints of Raynaud. The main idea is that the prime-to-$p$ torsion of $G^0$ is dense in $G^0_\mathrm{red}$ and its Zariski closure (with reduced structure) is geometr …
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6 votes
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dual isogeny for abelian varieties over a general field

You can find the statement in the general case (any isogeny over arbitrary field) and the proof in van der Geer and B. Moonen's book (draft) on abelian varieties. More precisely it is in Chapter 5, Pr …
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16 votes
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Can we always find a curve which doesn't have semi-stable reduction

Yes. Take $f\in O_K$ a uniformizing element of some prime $\mathfrak p$. Consider the hyperelliptic curve defined by the equation $$y^2=x^{2g+1}+f.$$ Then this curve doesn't have semi-stable reducti …
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5 votes

Torsion of an abelian variety under reduction.

To complete Pete and Milne's answers when A is not an abelian scheme (for example, when it is the Néron model of an abelian variety over ${\mathbb Q}_p$ with not necessary good reduction), then for an …
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11 votes

Are there Néron models over higher dimensional base schemes?

This is not answer to the OP, but to the remark of Pete on singular base scheme $S$. Here is an example explainning why one should assume regularity even in dimension $1$. Let $T$ be a smooth curve …
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