第十八讲: 测地线方程推导 推荐阅读:变分法的数学:Michio Masujima,《Applied mathematical methods in theoretical physics》、Hassani Sadri, 《Mathematical Methods For Students of Physics and Related Fields》Springer 2009 2nd Edition、Riley Hobson 2002 《Mathematical Methods For Physics》相关章节 进阶阅读:MTW Box21.1(变分原理的Palatini方法)