Associativity2

Multiplication of 2×2 matrices over the real numbers is associative
Solution to Abstract Algebra by Dummit & Foote 3rd edition Chapter 1.4 Exercise 1.4.9 Prove that...
Direct product of groups is essentially associative
Solution to Abstract Algebra by Dummit & Foote 3rd edition Chapter 1.6 Exercise 1.6.12 Let $A$, ...