Solution to Abstract Algebra by Dummit & Foote 3rd edition Chapter 1.6 Exercise 1.6.9
Prove that $D_{24}$ and $S_4$ are not isomorphic.
Solution: We know from Exercise 1.2.3 that $D_{24}$ has 13 elements of order two; namely $r^6$ and $sr^k$ for $0 \leq k < 12$. However, $S_4$ has 9 elements of order two; namely the 2-cycles and products of two 2-cycles.
