The Lemmens-Seidel conjecture for base size $5$

Abstract

In 2020, Lin and Yu claimed to prove the so-called Lemmens--Seidel conjecture for base size $5$. However, their proof has a gap. In this paper, we prove the conjecture for base size $5$ using the pillar method. We also show that the sets of $57$ equiangular lines with common angle $\arccos (1/5)$ in dimension $18$ found by Greaves et~al. \ in 2021 are indeed counterexamples to one of Lin and Yu's claims.We prove this by answering the question posed by Greaves et~al. \ in 2021 in the negative.They asked whether these sets are contained in the unique set of $276$ equiangular lines with common angle $\arccos (1/5)$ in dimension $23$. Furthermore, we show that these sets are strongly maximal. This gives a negative answer to the question posed by Cao et~al.\ in 2021. They asked whether the unique set of $276$ equiangular lines with common angle $\arccos (1/5)$ in dimension $23$ is the unique strongly maximal set of equiangular lines with common angle $\arccos (1/5)$.

11 pages