Homology sphere

A homology sphere is a topological space that behaves like a sphere in terms of its homological properties, even if it is not actually a sphere in the classical sense. More formally, an \( n \)-dimensional homology sphere is a manifold that is homotopy equivalent to the \( n \)-dimensional sphere \( S^n \), and, importantly, it has the same homology groups as \( S^n \).