Abstract Algebra
《Abstract Algebra》 · 第 68 页 · Section 2.5「The Lattice of Subgroups of a Group」，Example (2)
The Klein 4-group (Viergruppe), $V_4$, is the group of order $4$ with multiplication table $\begin{array}{c|cccc}\cdot&1&a&b&c\\\hline 1&1&a&b&c\\a&a&1&c&b\\b&b&c&1&a\\c&c&b&a&1\end{array}$ […] Note that $V_4$ is abelian and is not isomorphic to $Z_4$ (why?).