fixed point theorem不动点定理
existence theorem[计]存在性定理
词根：theory
theory 理论；原理；学说；推测
1.If you want, in a way, it's this theorem.
可以这样想，这就是定理告诉我们的。
2."It's a very interesting theorem," says Brualdi.
“这是一个非常有趣的定理，”Brualdi说。
3.So, let's prove this theorem.
下面来证明这个定理。
4.The theorem that alternativity can replace associativity is true.
交替性可以代替关联性的定理是正确的。
5.In the plane that is called Green's theorem.
平面上的就是格林公式。
6.OK, and actually that's how we prove the theorem.
实际上，这也是我们证明的思路。