Heat kernel approach for sup-norm bounds for cusp forms of integral and half integral weight

In this article, using the heat kernel approach from \cite{bouche}, we derive sup-norm bounds for cusp forms of integral and half integral weight. Let $Γ\subset \mathrm{PSL}_{2}(\mathbb{R})$ be a cocompact Fuchsian subgroup of first kind. For $k\in\frac{1}{2}\mathbb{Z}$ (or $k\in 2\mathbb{Z}$), let $S^{k}(Γ)$ denote the complex vector space of weight-$k$ cusp forms. Let $\lbrace f_{1},\ldots,f_{j_{k}} \rbrace$ denote an orthonormal basis of $S^{k}(Γ)$. In this article, we show that as $k\rightarrow \infty,$ the sup-norm for $\sum_{i=1}^{j_{k}}y^{k}|f_{i}(z)|^{2}$ is bounded by $O(k)$, where the implied constant is independent on $Γ$. Furthermore, using results from \cite{berman}, we extend these results to the case when $Γ$ is cofinite.

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