arXiv subjects
Publications and source records attributed to Ernst-Ulrich Gekeler.
We give closed formulas for the first few expansion coefficients of the basic modular forms for \(\mathrm{GL}(r, \mathbb{F}_{q}[T])\). Here the rank \(r\) is larger or equal to \(3\), and the forms in question include the coefficient forms \(g_{1}, \dots, g_{r}\) and the Eisenstein series \(E_{q^{i}-1}\) (\(i \in \mathbb{N}\)).
We determine the action of the Hecke operators \(T_{\mathfrak{p},i}\) on the coefficient forms \(g_{1}, \dots, g_{r-1}, g_{r} = Δ\), and \(h\), which together generate the ring of modular forms for \(\mathrm{GL}(r, \mathbf{F}_{q}[T])\). All these are eigenforms with powers of \(π\) as eigenvalues, where \(π\) is the monic generator of the prime ideal \(\mathfrak{p}\) of \(\mathbb{F}_{q}[T]\). We further describe the growth of the \(t\)-expansion coefficients of the discriminant function \(Δ\). It is such that the product expansion of \(Δ\) as well as the \(t\)-expansion of each modular form converges on the natural fundamental domain for \(\mathrm{GL}(r, \mathbf{F}_{q}[T])\).
We present a new notion of distribution and derived distribution of rank $r \in \mathbb{N}$ for a global function field $K$ with a distinguished place $\infty$. It allows to describe the relations between division points, isogenies, and discriminants both for a fixed Drinfeld module of rank $r$ for the above data, or for the corresponding modular forms. We introduce and study three basic distributions with values in $\mathbb{Q}$, in the group $μ(\overline{K})$ of roots of unity in the algebraic closure $\overline{K}$ of $K$, and in the group $U^{(1)}(C_{\infty})$ of $1$-units of the completed algebraic closure $C_{\infty}$ of $K_{\infty}$, respectively. There result product formulas for division points and discriminants that encompass known results (e.g. analogues of Wallis' formula for $(2πi)^{2}$ in the rank-$1$ case, of Jacobi's formula $Δ= (2πi)^{12} q \prod (1-q^{n})^{24}$ in the rank-$2$ case, and similar boundary expansions for $r > 2$) and several new ones: the definition of a canonical discriminant for the most general case of Drinfeld modules and the description of the sizes of division and discriminant forms. In the now classical case where $(K, \infty) = (\mathbb{F}_{q}(T), \infty)$ and $r = 1$, $2$ or $3$, we give explicit values for the logarithms of such forms.
We study expansions of Drinfeld modular forms of rank \(r \geq 2\) along the boundary of moduli varieties. Product formulas for the discriminant forms \(Δ_{\mathfrak{n}}\) are developed, which are analogous with Jacobi's formula for the classical elliptic discriminant. The vanishing orders are described through values at \(s=1-r\) of partial zeta functions of the underlying Drinfeld coefficient ring \(A\). We show linear independence properties for Eisenstein series, which allow to split spaces of modular forms into the subspaces of cusp forms and of Eisenstein series, and give various characterizations of the boundary condition for modular forms.
The coefficient forms \( {}_{a} \ell_{k} \) and the para-Eisenstein series \(α_{k}\) are simplicial Drinfeld modular forms. We study the attached simplicial complexes \(\mathcal{BT}^{r}( {}_{a} \ell_{k})\) and \(\mathcal{BT}^{r}(α_{k})\), which are full subcomplexes of the Bruhat-Tits building \(\mathcal{BT}^{r}\) of \( \mathrm{PGL}(r, K_{\infty})\). They are connected (if the rank \(r\) is larger than 2), strongly equidimensional of codimension 1 in \(\mathcal{BT}^{r}\), boundaryless, and satisfy a symmetry property under the non-trivial involution of the Dynkin diagram \(A_{r-1}\).
The zeroes of Goss polynomials $G_{k, Λ}(X)$ for $Λ= A \defeq \mathbb{F}_{q}[T]$ and similar lattices $Λ$ are studied. Generically, the zero distribution follows a simple pattern governed by the $q$-adic expansion of $k-1$. However, if $q=p^{f}$ with $f \geq 2$ is a proper power of the prime $p$, irregularities of the $q$-adic sum-of-digits function may lead to deviations from this pattern, to irregular zeroes, and an abundance of trivial zeroes of $G_{k, Λ}$ compared to the generic formula. These phenomena are related to properties of the Sheats compositions of natural numbers $n$ divisible by $q-1$. Among other things, we give a necessary and sufficient condition for the existence of irregular zeroes of $G_{k,A}$ and a formula for the vanishing order of $G_{k,A}$ at $x=0$.
\begin{document} \begin This paper continues work of the earlier articles with the same title. For two classes of modular forms $f$: \begin{itemize} \item para-Eisenstein series $α_{k}$ and \item coefficient forms ${}_a \ell_{k}$, where $k \in \mathbb{N}$ and $a$ is a non-constant element of $\mathbb{F}_{q}[T]$, \end{itemize} the growth behavior on the fundamental domain and the zero loci $Ω(f)$ as well as their images $\mathcal{BT}(f)$ in the Bruhat-Tits building $\mathcal{BT}$ are studied. We obtain a complete description for $f = α_{k}$ and for those of the forms ${}_{a}\ell_{k}$ where $k \leq °a$. It turns out that in these cases, $α_{k}$ and ${}_{a}\ell_{k}$ are strongly related, e.g., $\mathcal{BT}({}_{a}\ell_{k}) = \mathcal{BT}(α_{k})$, and that $\mathcal{BT}(α_{k})$ is the set of $\mathbb{Q}$-points of a full subcomplex of $\mathcal{BT}$ with nice properties. As a case study, we present in detail the outcome for the forms $α_{2}$ in rank 3. \end{abstract} \maketitle \end{document}
Let $u$ be a nowhere vanishing holomorphic function on the Drinfeld space $Ω^{r}$ of dimension $r-1$, where $r \geq 2$. The logarithm $\log_{q}\lvert u \rvert$ of its absolute value may be regarded as an affine function on the attached Bruhat-Tits building $\mathcal{BT}^{r}$. Generalizing a construction of van der Put in case $r=2$, we relate the group $\mathcal{O}(Ω^{r})^{*}$ of such $u$ with the group $\mathbf{H}(\mathcal{BT}^{r}, \mathbb{Z})$ of integer-valued harmonic 1-cochains on $\mathcal{BT}^{r}$. This also gives rise to a natural $\mathbb{Z}$-structure on the first ($\ell$-adic or de Rham) cohomology of $Ω^{r}$.
Generalizing the results of Maurischat in \cite{Maurischatxx}, we show that the field $K_{\infty}(Λ)$ of periods of a Drinfeld module $ϕ$ of rank $r$ defined over $K_{\infty} = \mathds{F}_{q}((T^{-1}))$ may be arbitrarily large over $K_{\infty}$. We also show that, in contrast, the residue class degree $f( K_{\infty}(Λ) | K_{\infty})$ remains bounded by a constant that depends only on $r$.
We construct and study a natural compactification $\overline{M}^r(N)$ of the moduli scheme $M^r(N)$ for rank-$r$ Drinfeld $\F_q[T]$-modules with a structure of level $N \in \F_q[T]$. Namely, $\overline{M}^r(N) = {\rm Proj}\,{\bf Eis}(N)$, the projective variety associated with the graded ring ${\bf Eis}(N)$ generated by the Eisenstein series of rank $r$ and level $N$. We use this to define the ring ${\bf Mod}(N)$ of all modular forms of rank $r$ and level $N$. It equals the integral closure of ${\bf Eis}(N)$ in their common quotient field $\widetilde{\MF}_r(N)$. Modular forms are characterized as those holomorphic functions on the Drinfeld space $\Om^r$ with the right transformation behavior under the congruence subgroup $\Ga(N)$ of $\Ga = {\rm GL}(r,\F_q[T])$ ("weak modular forms") which, along with all their conjugates under $\Ga/\Ga(N)$, are bounded on the natural fundamental domain $\BF$ for $\Ga$ on $\Om^r$.
Continuing the work of \cite{7} and \cite{8}, we derive an analogue of the classical "$k/12$-formula" for Drinfeld modular forms of rank $r \geq 2$. Here the vanishing order $ν_ω(f)$ of one modular form at some point $ω$ of the complex upper half-plane is replaced by the intersection multiplicity $ν_{\bo}(f_1,\ldots,f_{r-1})$ of $r-1$ independent Drinfeld modular forms at some point $\bo$ of the Drinfeld symmetric space $\OM^r$. We apply the formula to determine the common zeroes of $r-1$ consecutive Eisenstein series $E_{q^{i}-1}$, where $n-r<i<n$ for some $n \geq r$.
We show that the absolute value $|f|$ of an invertible holomorphic function $f$ on the Drinfeld symmetric space $\OM^r$ $(r \geq 2)$ is constant on fibers of the building map to the Bruhat-Tits building $\MB\MT$. Its logarithm $\log|f|$ is an affine map on the realization of $\MB\MT$. These results are used to study the vanishing loci of modular forms (coefficient forms, Eisenstein series, para-Eisenstein series) and to determine their images in $\MB\MT$.
This work is a survey of relations between Drinfeld modules and higher dimensional fields of positive characteristic. The main new result stated is the expression of vanishing orders of certain modular forms through partial zeta values.