isotopy

They came to think about algebraic objects associated with knots/links that are invariant under isotopy c .J.W.Alexander came up with the first good such quantity, the Alexander polynomiald .

我认为剩下人里面绝大多数对于一个中国人只身在国外奋斗所遭遇困难、辛酸、挫折、落、压抑、他人，功和兴奋没有了解，更遑论理解。