Equiangular lines via matrix projection
arxiv(2021)
摘要
In 1973, Lemmens and Seidel posed the problem of determining the maximum
number of equiangular lines in ℝ^r with angle arccos(α) and
gave a partial answer in the regime r ≤ 1/α^2 - 2. At the other
extreme where r is at least exponential in 1/α, recent breakthroughs
have led to an almost complete resolution of this problem. In this paper, we
introduce a new method for obtaining upper bounds which unifies and improves
upon previous approaches, thereby yielding bounds which bridge the gap between
the aforementioned regimes and are best possible either exactly or up to a
small multiplicative constant. Our approach relies on orthogonal projection of
matrices with respect to the Frobenius inner product and as a byproduct, it
yields the first extension of the Alon-Boppana theorem to dense graphs, with
equality for strongly regular graphs corresponding to r+12
equiangular lines in ℝ^r. Applications of our method in the complex
setting will be discussed as well.
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关键词
graphs,regular
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