Abstract:
Belyi's theorem states that a smooth complex projective curve can be defined
over $\overline{\mathbb{Q}}$ if and only if there exists a finite map $f
\colon X \rightarrow \mathbb{P}^1$ branched over no more than three points.
Last year we have discussed higher-dimensional analogues of Belyi's theorem
(after G. González-Diez and A. Javanpeykar), in these results the finite
map $f$ is replaced by a Lefschetz pencil on $X$.
But there exists a different approach — we can realise $X$ as a cover of $
\mathbb{P}^n$, and characterise when $X$ can be defined over $
\overline{\mathbb{Q}}$ in terms of the branching divisor on $\mathbb{P}^n$.
Following K. H. Paranjape, we will discuss such a Belyi-type result for
projective surfaces.