一本正经的胡说八道啊。。。。。

What is the betti number of a hollow 2-simplex?

The Betti number of a hollow two-dimensional simplex is zero. A 2-simplex is a triangle, and if it is hollow, meaning it has no interior, then it can be thought of as the boundary of itself (since it has no interior). In algebraic topology, the Betti numbers of a topological space are a way of counting the number of holes of various dimensions, and since a hollow triangle has no holes, its Betti number is zero.
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FROM 211.161.243.*