And now we're just going to do some algebra, some sigma algebra I guess you could call it.
现在我们要做一些代数，一些 sigma 代数，我想你可以称之为。
英语百科
Sigma-algebra
In mathematical analysis and in probability theory, a σ-algebra (also sigma-algebra, σ-field, sigma-field) on a set X is a collection Σ of subsets of X that includes the empty subset, is closed under complement, and is closed under union or intersection of infinitely-countably many subsets. The pair (X, Σ) is called a measurable space.