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Daniel Loughran
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The answer to question 1 is yes. Examples almost like what you are after can be found here:

Does isomorphic generic fibre imply isomorphic special fibre for smooth morphisms?

Though admittedly the examples there are non-minimal as the generic fibre is isomorphic to $\mathbb{P}^2$ blown up in two points. But it is possible to come up with minimal examples as $\mathbb{P}^2$$\mathbb{P}^1 \times \mathbb{P}^1$ can be specialised to both $\mathbb{P}^2$$\mathbb{P}^1 \times \mathbb{P}^1$ and a Hirzebruch surface; I can try to find the details if you would like.

The answer to question 2 is however no. The smooth proper base change theorem implies that the $\ell$-adic cohomology of the special fibres is isomorphic. Therefore the Weil conjectures implies that they have the same number of points modulo $p$.

The answer to question 1 is yes. Examples almost like what you are after can be found here:

Does isomorphic generic fibre imply isomorphic special fibre for smooth morphisms?

Though admittedly the examples there are non-minimal as the generic fibre is isomorphic to $\mathbb{P}^2$ blown up in two points. But it is possible to come up with minimal examples as $\mathbb{P}^2$ can be specialised to both $\mathbb{P}^2$ and a Hirzebruch surface; I can try to find the details if you would like.

The answer to question 2 is however no. The smooth proper base change theorem implies that the $\ell$-adic cohomology of the special fibres is isomorphic. Therefore the Weil conjectures implies that they have the same number of points modulo $p$.

The answer to question 1 is yes. Examples almost like what you are after can be found here:

Does isomorphic generic fibre imply isomorphic special fibre for smooth morphisms?

Though admittedly the examples there are non-minimal as the generic fibre is isomorphic to $\mathbb{P}^2$ blown up in two points. But it is possible to come up with minimal examples as $\mathbb{P}^1 \times \mathbb{P}^1$ can be specialised to both $\mathbb{P}^1 \times \mathbb{P}^1$ and a Hirzebruch surface; I can try to find the details if you would like.

The answer to question 2 is however no. The smooth proper base change theorem implies that the $\ell$-adic cohomology of the special fibres is isomorphic. Therefore the Weil conjectures implies that they have the same number of points modulo $p$.

Source Link
Daniel Loughran
  • 21.3k
  • 3
  • 46
  • 136

The answer to question 1 is yes. Examples almost like what you are after can be found here:

Does isomorphic generic fibre imply isomorphic special fibre for smooth morphisms?

Though admittedly the examples there are non-minimal as the generic fibre is isomorphic to $\mathbb{P}^2$ blown up in two points. But it is possible to come up with minimal examples as $\mathbb{P}^2$ can be specialised to both $\mathbb{P}^2$ and a Hirzebruch surface; I can try to find the details if you would like.

The answer to question 2 is however no. The smooth proper base change theorem implies that the $\ell$-adic cohomology of the special fibres is isomorphic. Therefore the Weil conjectures implies that they have the same number of points modulo $p$.