Def. 拓扑空间 $X$ 中的自由同伦回路所在的类 $\alpha$ 称为是“外围的“(peripheral), 如果对于 $X$ 中的任意紧子集 $R$, 都存在 $\alpha$ 类中的一个回路, 与 $R$ 不相交.

Reference:

[1] S. V. Buyalo, Euclidean planes in open 3-manifolds of nonpositive curvature, Algebra i Analiz 3 (1991), no.1, 102-117; English transl. in St.-Petersburg Math. J. 3 (1992).

[2] S. V. Buyalo, Three-manifolds with Cr-structure, Some questions of differential geometry in the large, Amer. Math. Soc. Transl. (2) Vol. 176, 1996