# Niebur-Poincar\'e Series and Traces of Singular Moduli

## TL;DR

This paper computes Fourier coefficients of modular forms related to singular moduli, extending algebraicity results and providing explicit series expressions for twisted traces, with applications to higher Green's functions at CM-points.

## Contribution

It introduces a method to study twisted traces of singular moduli in weight 2 using Niebur-Poincaré series, extending previous algebraicity results and computing regularized inner products.

## Key findings

- Explicit series expressions for twisted traces of singular moduli.
- Extension of algebraicity results to weight 2 case.
- Computation of regularized inner products related to Green's functions.

## Abstract

We compute the Fourier coefficients of analogues of Kohnen and Zagier's modular forms $f_{k,D}$ of weight $2$ and negative discriminant. These functions can also be written as twisted traces of certain weight $2$ Poincar\'e series with evaluations of Niebur-Poincar\'e series as Fourier coefficients. This allows us to study twisted traces of singular moduli in an integral weight setting. In particular, we recover explicit series expressions for twisted traces of singular moduli and extend algebraicity results by Bengoechea to the weight $2$ case. We also compute regularized inner products of these functions, which in the higher weight case have been related to evaluations of higher Green's functions at CM-points.