Generic Newton polygons for curves of given p-rank

We survey results and open questions about the $p$-ranks and Newton polygons of Jacobians of curves in positive characteristic $p$. We prove some geometric results about the $p$-rank stratification of the moduli space of (hyperelliptic) curves. For example, if $0 \leq f \leq g-1$, we prove that every component of the $p$-rank $f+1$ stratum of ${\mathcal M}_g$ contains a component of the $p$-rank $f$ stratum in its closure. We prove that the $p$-rank $f$ stratum of $\overline{\mathcal M}_g$ is connected. For all primes $p$ and all $g \geq 4$, we demonstrate the existence of a Jacobian of a smooth curve, defined over $\overline{\mathbb F}_p$, whose Newton polygon has slopes $\{0, \frac{1}{4}, \frac{3}{4}, 1\}$. We include partial results about the generic Newton polygons of curves of given genus $g$ and $p$-rank $f$.

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