Cusp restrictions and Bunke--Naumann invariants with level structure
Abstract
Restriction to the full cusp divisor of a modular curve defines a secondary invariant whose rational indeterminacy comes from a single global modular form. For every integral weakly holomorphic level-one modular form h of weight divisible by four, we prove that the imported value [h/2] vanishes at every nontrivial Γ0(N) level. The construction localizes coefficients before completion and rationalizes only after passing to homotopy groups. At odd prime level, we identify the full cusp spectrum as a product of two real Tate factors and construct a single holomorphic weight-two correction. This correction yields an actual integral global homotopy class and a rational global source class satisfying the equality required for joint annihilation. The equality transports to every odd composite level, while even levels follow by inverting two. At level three, the Mahowald--Rezk homotopy calculation leaves only the periodic ν family in stems 8K+3. A cusp-residue homomorphism detects its secondary value, which has exact order two throughout the periodic family. As an application, the Bunke--Naumann secondary invariants of products in bidegrees (8k+1,8k'+2) vanish after passage to every nontrivial level.
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BibTeX
@misc{li2026cusp,
title = {{Cusp restrictions and Bunke--Naumann invariants with level structure}},
author = {Yuqi Li and Hao-Yu Sun},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.11924},
eprint = {2609.11924},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.11924},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Cusp restrictions and Bunke--Naumann invariants with level structure
AU - Yuqi Li
AU - Hao-Yu Sun
PY - 2026
DA - 2026/09/10
PB - arXiv
DO - 10.48550/arxiv.2609.11924
UR - https://arxiv.org/abs/2609.11924
AB - Restriction to the full cusp divisor of a modular curve defines a secondary invariant whose rational indeterminacy comes from a single global modular form. For every integral weakly holomorphic level-one modular form h of weight divisible by four, we prove that the imported value [h/2] vanishes at every nontrivial Γ_0(N) level. The construction localizes coefficients before completion and rationalizes only after passing to homotopy groups. At odd prime level, we identify the full cusp spectrum as a product of two real Tate factors and construct a single holomorphic weight-two correction. This correction yields an actual integral global homotopy class and a rational global source class satisfying the equality required for joint annihilation. The equality transports to every odd composite level, while even levels follow by inverting two. At level three, the Mahowald--Rezk homotopy calculation leaves only the periodic ν family in stems 8K+3. A cusp-residue homomorphism detects its secondary value, which has exact order two throughout the periodic family. As an application, the Bunke--Naumann secondary invariants of products in bidegrees (8k+1,8k'+2) vanish after passage to every nontrivial level.
ER -