TY - JOUR

T1 - Degenerate Turán densities of sparse hypergraphs

AU - Shangguan, Chong

AU - Tamo, Itzhak

N1 - Publisher Copyright:
© 2020 Elsevier Inc.

PY - 2020/7

Y1 - 2020/7

N2 - For fixed integers r>k≥2,e≥3, let fr(n,er−(e−1)k,e) be the maximum number of edges in an r-uniform hypergraph in which the union of any e distinct edges contains at least er−(e−1)k+1 vertices. A classical result of Brown, Erdős and Sós in 1973 showed that fr(n,er−(e−1)k,e)=Θ(nk). The degenerate Turán density is defined to be the limit (if it exists) [Formula presented] Extending a recent result of Glock for the special case of r=3,k=2,e=3, we show that [Formula presented] for arbitrary fixed r≥4. For the more general cases r>k≥3, we manage to show [Formula presented] where the gap between the upper and lower bounds are small for r≫k. The main difficulties in proving these results are the constructions establishing the lower bounds. The first construction is recursive and purely combinatorial, and is based on a (carefully designed) approximate induced decomposition of the complete graph, whereas the second construction is algebraic, and is proved by a newly defined matrix property which we call strongly 3-perfect hashing.

AB - For fixed integers r>k≥2,e≥3, let fr(n,er−(e−1)k,e) be the maximum number of edges in an r-uniform hypergraph in which the union of any e distinct edges contains at least er−(e−1)k+1 vertices. A classical result of Brown, Erdős and Sós in 1973 showed that fr(n,er−(e−1)k,e)=Θ(nk). The degenerate Turán density is defined to be the limit (if it exists) [Formula presented] Extending a recent result of Glock for the special case of r=3,k=2,e=3, we show that [Formula presented] for arbitrary fixed r≥4. For the more general cases r>k≥3, we manage to show [Formula presented] where the gap between the upper and lower bounds are small for r≫k. The main difficulties in proving these results are the constructions establishing the lower bounds. The first construction is recursive and purely combinatorial, and is based on a (carefully designed) approximate induced decomposition of the complete graph, whereas the second construction is algebraic, and is proved by a newly defined matrix property which we call strongly 3-perfect hashing.

KW - Algebraic construction

KW - Approximate decomposition of the complete graph

KW - Degenerate Turán density

KW - Sparse hypergraph

UR - http://www.scopus.com/inward/record.url?scp=85079371698&partnerID=8YFLogxK

U2 - 10.1016/j.jcta.2020.105228

DO - 10.1016/j.jcta.2020.105228

M3 - מאמר

AN - SCOPUS:85079371698

VL - 173

JO - Journal of Combinatorial Theory - Series A

JF - Journal of Combinatorial Theory - Series A

SN - 0097-3165

M1 - 105228

ER -