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Describe exact Hausdorff-limit geodesic configuration for non-principal congruence groups

Describe the exact Hausdorff-limit set \overline Z_{\infty,\Gamma} of zeros in the fundamental domain as the even weight k→∞ for non-principal congruence subgroups Γ (for example Γ_0(N)), expressing the resulting finite geodesic configuration explicitly in terms of N alone.

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Background

The paper proves that for any congruence subgroup Γ the zeros of the chosen Eisenstein series converge, in Hausdorff distance, to a finite union of geodesic segments, and computes this configuration explicitly for Γ(N) (with different shapes depending on whether 4|N).

For certain conjugates (e.g., Γ0(N)) they identify specific circular segments in the limit. However, an exact closed-form description for standard non-principal groups such as Γ_0(N) remains open.

References

We list some open problems. Describe the exact limit geodesic configuration, or the limit of the zero set as the weight tends to infinity, for non-principal congruence groups, such as \Gamma_0(N), in terms of N alone.

Geodesic clustering of zeros of Eisenstein series for congruence groups (2509.16108 - Santana et al., 19 Sep 2025) in Section: Open problems (final section)