arXiv subjects

Tatsunari Watanabe

Publications and source records attributed to Tatsunari Watanabe.

10 recordsLinked to original sources

arXiv2026

On the universal curve with unordered marked points in positive characteristic

We study the relative pro-$\ell$ and continuous relative completions of the algebraic fundamental groups of universal curves over the moduli stack of curves with unordered marked points in positive characteristic. Using specialization and homotopy exact sequences, we compare the ordered and unordered settings and prove that the natural projection from the relative completion of the universal curve over the unordered moduli stack admits no section in positive characteristic. This yields a non-splitting result for the corresponding projection on algebraic fundamental groups. The present paper is a sequel to our earlier work in characteristic zero.

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arXiv2025

On the Universal Curves with Unordered Marked Points

Over any field of characteristic $0$, we prove that the homotopy exact sequence of algebraic fundamental groups for the universal curve with unordered marked points does not split. The same nonsplitting holds for the universal hyperelliptic curve. Our approach extends Chen's topological result to the profinite setting and relies on the use of relative and continuous relative completions to detect the nonexistence of algebraic sections.

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arXiv2025

On the Birman exact sequence of the subgroups of the mapping class group of genus three

We prove that for any finite index subgroup of the mapping class group containing the Johnson subgroup, the profinite Birman exact sequence does not split in genus $g\ge 3$, extending prior results of Hain and the second author for $g\ge 4$. For the Torelli group, we prove that the graded Lie algebra version of the Birman exact sequence admits no section with symplectic equivariance, extending Hain's result from $g\ge 4$ to $g=3$. These results are deduced by our main tool, relative completion, with the help of Hodge theory and representation theory of symplectic groups, along with explicit structural obstructions coming from hyperelliptic mapping class groups.

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arXiv2023

Remarks on the sections of universal hyperelliptic curves

In this paper, we study the obstruction for the sections of the universal hyperelliptic curves of genus $g\geq 3$. The obstruction of our interest comes from the relative completion of the hyperelliptic mapping class groups and the Lie algebra of the unipotent completion of the fundamental group of the configuration space of a compact oriented surface. Using the obstruction, we prove that the Birman exact sequence for the hyperelliptic mapping class groups does not split for $g\geq 3$.

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arXiv2023

Non-abelian cohomology of universal curves in positive characteristic

In this paper, we will compute the non-abelian cohomology of the universal complete curve in positive characteristic. This extends Hain's result on the non-abelian cohomology of generic curves in characteristic zero to positive characteristics. Furthermore, we will prove that the exact sequence of etale fundamental groups of the universal n-punctured curve in positive characteristic does not split.

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arXiv2023

Remarks on Rational Points of Universal Curves

In this notes, we will give some remarks on the results in Rational points of universal curves by Hain. In particular, we consider the universal curves $\mathcal{M}_{g,n+1}\to \mathcal{M}_{g,n}$ and the sections of their algebraic fundamental groups.

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arXiv2023

Remarks on Collino cycles and hyperelliptic Johnson homomorphisms

A Collino cycle is a higher cycle on the Jacobian of a hyperelliptic curve. The universal family of Collino cycles naturally gives rise to a normal function, whose induced monodromy relates to the hyperelliptic Johnson homomorphism. Colombo computed this monodromy explicitly and made this relation precise. We recast this in the perspective of relative completion. In particular, we use Colombo's result to construct Collino classes, which are cohomology classes of hyperelliptic mapping class groups with coefficients in a certain symplectic representation. We also determine the dimension of their span in the case of the level two hyperelliptic mapping class group.

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arXiv2016

On the completion of the mapping class group of genus two

In this paper, we will study the Lie algebra of the prounipotent radical of the relative completion of the mapping class group of genus two. In particular, we will partially determine a minimal presentation of the Lie algebra by determining the generators and bounding the degree of the relations.

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arXiv2016

On the sections of universal hyperelliptic curves

In this paper, we will give an algebraic proof for determining the sections for the universal pointed hyperelliptic curves, when $g\geq 3$ and the image of the $\ell$-adic cyclotomic character $G_k\to \Z^\times$ is infinite. Furthermore, we will study the nonabelian phenomena associated to the universal hyperelliptic curves. For example, we will show that the section conjecture holds for the universal hyperelliptic curve without marked points and the unipotent analogue of the conjecture holds for the pointed case. This work is an extension of Hain's original work to the hyperelliptic case.

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arXiv2016

Rational points of universal curves in positive characteristics

For the moduli stack $\mathcal{M}_{g,n/\mathbb{F}_p}$ of smooth curves over $\text{Spec}~\mathbb{F}_p$ with the function field $K$, we show that if $g\geq3$, then the only $K$-rational points of the generic curve over $K$ are its $n$ tautological points. Furthermore, we show that if $g\geq4$ and $n=0$, then Grothendieck's Section Conjecture holds for the generic curve over $K$. This is an extension of Hain's work in characteristic $0$ to positive characteristics.

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