On degeneration of tetrahedra under longest-edge n-section refinement
Abstract
For every integer n ≥ 2, we construct explicit tetrahedral counterexamples showing that repeated longest-edge (LE) n-section refinement does not, in general, preserve shape regularity, contrary to long-standing conjectural expectations. For n = 2, the construction disproves the finite-family conjecture put forward by Adler in 1983 and the non-degeneracy conjecture for tetrahedral longest-edge bisection formulated by Rivara and Levin in 1992. It also shows that the sharper higher-dimensional diameter decay suggested by Stynes in 1983,following his 1980 planar finite-similarity result, cannot hold for arbitrary tetrahedra. For n = 3, it disproves the published non-degeneracy conjecture for tetrahedral longest-edge trisection formulated in 2011 by Suárez, Abellón, Abad and Plaza on the basis of numerical experiments. The resulting infinite descendant sequences violate both the minimum and maximum angle conditions. For n ≥ 3, the selected edge is always the unique longest edge at every refinement step. The two-step recurrence used in the construction exhibits two distinct degeneration behaviors: a flat-tetrahedron regime for 2 ≤ n ≤ 5 and a skinny-tetrahedron regime for n ≥ 6. The degenerating sequences can also be realized within conforming partitions generated by the conforming LE n-section algorithm. For both the classical and the conforming LE n-section algorithms, the maximal element diameter tends to zero as the number of refinement steps tends to infinity, regardless of which longest edge is chosen. Thus diameter convergence, and even conformity, do not prevent shape degeneration. These algorithms should therefore be used with care in applications requiring uniform shape regularity.
Related
- Multiform Longest Edge Bisection of Tetrahedra via Sextuple Permutations
- Degenerating orbits of the Longest Edge Bisection process
- Closure complexity of longest-edge bisection for triangular meshes
- A refinement of the edge theorem for order and chain polytopes
- The Exact Dihedral Angle Sum Range for Circumcentral-Cevian Jaw Tetrahedra
BibTeX
@misc{korotov2026degeneration,
title = {{On degeneration of tetrahedra under longest-edge n-section refinement}},
author = {Sergey Korotov},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.29153},
eprint = {2609.29153},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.29153},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - On degeneration of tetrahedra under longest-edge n-section refinement
AU - Sergey Korotov
PY - 2026
DA - 2026/09/24
PB - arXiv
DO - 10.48550/arxiv.2609.29153
UR - https://arxiv.org/abs/2609.29153
AB - For every integer n ≥ 2, we construct explicit tetrahedral counterexamples showing that repeated longest-edge (LE) n-section refinement does not, in general, preserve shape regularity, contrary to long-standing conjectural expectations. For n = 2, the construction disproves the finite-family conjecture put forward by Adler in 1983 and the non-degeneracy conjecture for tetrahedral longest-edge bisection formulated by Rivara and Levin in 1992. It also shows that the sharper higher-dimensional diameter decay suggested by Stynes in 1983,following his 1980 planar finite-similarity result, cannot hold for arbitrary tetrahedra. For n = 3, it disproves the published non-degeneracy conjecture for tetrahedral longest-edge trisection formulated in 2011 by Suárez, Abellón, Abad and Plaza on the basis of numerical experiments. The resulting infinite descendant sequences violate both the minimum and maximum angle conditions. For n ≥ 3, the selected edge is always the unique longest edge at every refinement step. The two-step recurrence used in the construction exhibits two distinct degeneration behaviors: a flat-tetrahedron regime for 2 ≤ n ≤ 5 and a skinny-tetrahedron regime for n ≥ 6. The degenerating sequences can also be realized within conforming partitions generated by the conforming LE n-section algorithm. For both the classical and the conforming LE n-section algorithms, the maximal element diameter tends to zero as the number of refinement steps tends to infinity, regardless of which longest edge is chosen. Thus diameter convergence, and even conformity, do not prevent shape degeneration. These algorithms should therefore be used with care in applications requiring uniform shape regularity.
ER -