Let $\pi:\mathcal{A}\rightarrow C$ be a semi-abelian scheme, i.e. $\mathcal{A}$ is a smooth separated commutative group scheme over $C$ via $\pi$ with geometrically connected fibres, such that each fibre $\mathcal{A}\_v$, where $v\in C$, is an extension of an abelian variety $\mathcal{B}\_v$ by a torus $T\_v$ over the residue field $\kappa(v)$, or equivalently it is a semi-abelian variety.


I would like to ask if the set $\{v\in C:\mathcal{A}\_v\textrm{ is an abelian variety}\}$ is open. In particular where $C$ is a smooth, projective, geometrically integral curve over a perfect field of characteristic $p$, and the set contains the generic point of $C$.


I would like to use the constructibility results from EGA, but all of them relies on the condition that "the morphism is of finite presentation", which I can't show. To be clear, it is actually the property of "**quasi-compactness**" of $\pi$ which I can't show, with quasi-compactness then I can show finite presentation hence the above set is constructible, and it contains the generic point so it contains a dense open subset. On a curve its complement is a finite set of closed points, so itself must be open.




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Update: I have found a lemma for it, which is [[SGA3, Expose VI B](https://webusers.imj-prg.fr/%7Epatrick.polo/SGA3/Exp6B-23mai11.pdf), Corollary 5.5], it says that:



> 
> Let $G$ be a group scheme over $S$, if $G$ is locally of finite presentation and universally open over $S$ with connected fibres, then $G$ is separated and of finite presentation over $S$.
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> 
> 


The universally openness can be replaced by flatness because "locally of finite presentation" + "flat" imply "universally open", see [tag 01UA](https://stacks.math.columbia.edu/tag/01UA).



Given a semi-abelian scheme, is the set of points such that the fibres are abelian varities open?
I am posting my comments as an answer. 


Let $k$ be a field. Let $$(G,m:G\times\_{\text{Spec}\ k}G \to G)$$ be a locally finitely presented group scheme over $\text{Spec}\ k$. For every open $U$, denote by $m\_U$ the restriction of $m$, $$m\_U:U\times\_{\text{Spec}\ k}U \to G.$$


**Lemma 1.** If $G$ is connected, then for every nonempty open $U$, the morphism $m\_U$ is surjective. In particular, $G$ is quasi-compact.


**Proof.** The identity component of every group scheme is geometrically irreducible. Since $G$ is connected, it is geometrically irreducible. Thus, for an algebraically closed field extension $L/k$, for every $L$-point $g$ of $G$, the base change open $U\_L\subset G\_L$, its inverse $U\_L^{-1}$, and its translate $g\cdot U\_L^{-1}$ are each dense opens in $G\_L$. Therefore, the intersection of $g\cdot U\_L^{-1}$ and $U\_L$ is nonempty. So there exists $g\cdot h^{-1}\in g\cdot U\_L$ and $h'\in U\_L$ that are equal, i.e., $g=h\cdot h'$ for $(h,h')\in U\_L \times\_{\text{Spec}\ L}U\_L.$ **QED**


**Lemma 2.** For every flat, locally finitely presented morphism of schemes, every point of the domain has an open affine neighborhood that is fppf over an open affine neighborhood of the image point in the target.


**Proof.** Without loss of generality, assume that the target and the domain are affine. Thus the morphism corresponds to ring homomorphism, $$\phi:B\to A.$$ Since the morphism is flat and locally finitely presented, the image is open in $\text{Spec}(B)$. A basis for the topology on $\text{Spec}(B)$ consists of distinguished opens $\text{Spec}(B\_f)$. Thus, the image open equals the union of all distinguished opens $\text{Spec}(B\_f)$ that it contains. For each such, the following ring homomorphism gives an fppf morphism of schemes, $$\phi\_f:B\_f \to A\_f.$$ **QED** 


Let $S$ be a scheme. Let $$(\pi\_S:G\_S\to S, m\_S:G\_S\times\_S G\_S \to G\_S, i\_S:G\_S\to G\_S,e\_S:S\to G\_S),$$ be a flat, locally finitely presented group scheme over $S$.


**Proposition 3.** If the fibers of $\pi\_S$ are connected, then $\pi\_S$ is finitely presented.


**Proof.** It suffices to prove this locally on the target. For every point $s$ of $S$, for the image point $e\_S(s)$, by Lemma 2 there exists an open affine neighborhood $V$ of $s$ in $S$ and an open affine neighborhood $U$ of $e\_S(s)$ in $G\_S$ such that the morphism $U\to V$ is fppf.


By Lemma 1, the following morphism is surjective on fibers over geometric points of $V$, $$m\_U: U\times\_V U \to V \times\_S G\_S.$$ Since $U$ and $V$ are affine, also $U\times\_V U$ is affine. Thus, $U\times\_V U$ is quasi-compact. Since $V\times\_S G\_S$ is the image of a surjective morphism from a quasi-compact scheme, also $V\times\_S G\_S$ is a quasi-compact scheme. **QED**


