國立清華大學 數學系

Abstract A set of lines through the origin in Euclidean space is called equiangular when any pair of lines from the set intersects with each other at a common angle. We study the maximum size of equiangular lines in Euclidean space and use a graph theoretic approach to prove that all the currently known constructions for maximum equiangular lines in $\mathbb R^d$ cannot be enlarged by any other line to form a larger equiangular set of lines when $d = 14, 16, 17, 18, 19$, and $20$. We give new constructions of large equiangular lines which are 248 equiangular lines in $\mathbb R^{42}$, 200 equiangular lines in $\mathbb{R}^{41}$, 168 equiangular lines in $\mathbb{R}^{40}$, 152 equiangular lines in $\mathbb R^{39}$ with angle $1/7$, and 56 equiangular lines in $\mathbb R^{18}$ with angle $1/5$. This talk is based on joint work with Yen-Chi Lin.