1. We determine at what occasion while the compact 3-manifolds with residually finite groups have only finitely many finite covers, and while those have only finitely many finite cyclic covers. 本文给出基本群为剩余有限的紧致3维流形何时有有限多有限复迭,何时有有限多有限循环复迭的充要条件。
2. An equilateral triangle cannot be dissected into finitely many incongruent equilateral triangles. 等边三角形不能分成有限多个不全等的等边三角形。
3. An equilateral triangle cannot be dissected into finitely many incongruent equilateral triangles. 等边三角形不能分成有限多个不 全等 的等边三角形。
4. An equilateral triangle cannot be dissected into finitely many incongruent equilateral triangles. 等边三角形不能分成有限多个不全等的等边三角形。