We show that the decision problem of determining whether a given (abstract simplicial) k-complex has a geometric embedding in ℝ^d is complete for the Existential Theory of the Reals for all d ≥ 3 and k ∈ {d-1,d}. Consequently, the problem is polynomial time equivalent to determining whether a polynomial equation system has a real solution and other important problems from various fields related to packing, Nash equilibria, minimum convex covers, the Art Gallery Problem, continuous constraint satisfaction problems, and training neural networks. Moreover, this implies NP-hardness and constitutes the first hardness result for the algorithmic problem of geometric embedding (abstract simplicial) complexes. This complements recent breakthroughs for the computational complexity of piece-wise linear embeddability.
@InProceedings{abrahamsen_et_al:LIPIcs.SoCG.2023.1, author = {Abrahamsen, Mikkel and Kleist, Linda and Miltzow, Tillmann}, title = {{Geometric Embeddability of Complexes Is \exists\mathbb{R}-Complete}}, booktitle = {39th International Symposium on Computational Geometry (SoCG 2023)}, pages = {1:1--1:19}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-273-0}, ISSN = {1868-8969}, year = {2023}, volume = {258}, editor = {Chambers, Erin W. and Gudmundsson, Joachim}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2023.1}, URN = {urn:nbn:de:0030-drops-178518}, doi = {10.4230/LIPIcs.SoCG.2023.1}, annote = {Keywords: simplicial complex, geometric embedding, linear embedding, hypergraph, recognition, existential theory of the reals} }
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