Group Theory and Symmetry. Part II: Groups
The article discusses the group theory of symmetry elements. It defines that a group is a set of elements in which a combination of two elements would generate a third element of the same group. It offers a set of criteria for the set of elements to satisfy in order to be considered a group, including the requirement of the third element to be of the same group, the group must follow the requirement of associativity, and an inverse element for each element.
Ball, David W., "Group Theory and Symmetry. Part II: Groups" (2010). Chemistry Faculty Publications. 147.