To decide whether a set with a binary operation forms a group, we check each property from Item 1 to Item 4 in Definition 14.1.2. If all the properties hold, the set with the operation forms a group. If even one property fails, the set with the operation does not form a group.
As the sum of two integers is an integer, addition is a binary operation on the set of integers \(\Z\text{.}\) We check whether the set of integers \(\Z\) with the operation addition (\(+\)) fulfills the properties Item 1 to Item 4 from Definition 14.1.2:
Identity: For all \(a\in\Z\) we have \(a+0=a\) and \(0+a=a\text{,}\) hence the integer 0 is the identity element with respect to addition. (Compare Example 13.3.3 (a).)
Inverses: For all \(a\in \Z\) we have \(a+(-a)=0\) and \((-a)+a=0\text{,}\) hence the inverse of \(a\) with respect to addition is the integer \(-a\text{.}\) So every integer has an inverse. (Compare Example 13.4.7 (a).)
By Example 13.4.7 (b), a natural number does not have a multiplicative inverse. So property Item 2 does not hold, and the natural numbers with multiplication do not form a commutative group. The properties Item 1, Item 3, and Item 4 do hold by part (b) of Example 13.3.3, Example 13.2.3, and Example 13.5.3, respectively.
Inverses: The inverse of \(\Tx\) is \(\Tx^{-1\star} = \Tz\text{,}\) the inverse of \(\Ty\) is \(\Ty^{-1\star} = \Ty\text{,}\) and the inverse of \(\Tz\) is \(\Tz^{-1\star} = \Tx\text{,}\) so every element of \(T\) has an inverse. (See Example 13.4.9 and Example 13.4.10.)
Similarly it follows from Example 13.3.7, Example 13.4.11, Example 13.2.5, and Example 13.5.6 that \((\Z_5,\oplus)\) where \(\Z_5=\{0,1,2,3,4\}\) and \(\oplus:\Z_5\times\Z_5\to\Z_5\) is defined by \(a\oplus b=(a+b)\fmod 5\) is a group.
So far we have considered all of the sets and operations that were given in Chapter 13. Now it is time to study more examples. A commutative group must contain at least one element, namely the identity. The following problem shows that there are commutative groups with just one element.
We consider the properties Item 1 to Item 4 from Definition 14.1.2. Since \(1 \cdot 1 = 1 \in \{1\}\) we have that \(\cdot\) is a binary operation on \(\{1\}\text{.}\)
Associativity: \(1\cdot(1\cdot 1)=1\cdot 1=1\) and \((1\cdot 1)\cdot 1=1\cdot 1=1\text{.}\) Since \(1 = 1\text{,}\) we have that \(\cdot\) is associative.
Each entry in the table is an element in \(S\text{,}\) so \(\diamond\) is a binary operation on \(S\text{.}\) We show that \((S,\diamond)\) is a commutative group by checking that the properties Item 1 to Item 4 from Definition 14.1.2 hold.
Identity: The row corresponding to the element \(\So\) matches the header row at the top of the table and the column corresponding to the element \(\So\) matches the header column on the left side of the table. Thus the identity element is \(\So\text{.}\)
Inverses: Since \(\So\) is the identity element, we begin by locating all of the places \(\So\) appears in the table. For each table entry that is \(\So\text{,}\) we trace back to the header column on the left side of the table and the header row on the top of the table. Since \(\So \diamond \So = \So\text{,}\) we conclude that \(\So\) is its own inverse, and since \((\Sno) \diamond (\Sno) = \So\text{,}\) we conclude that \(\Sno\) is also its own inverse. Finally, since \(\Si \diamond (\Sni) = \So\) and \((\Sni) \diamond \Si = \So\text{,}\) we conclude that \(\Si\) and \(\Sni\) are inverses of each other. Thus every element of \(S\) has an inverse.
Commutativity: The table is symmetric about the diagonal from the operation symbol in the top left corner of the table to the bottom right corner of the table. Thus \(\diamond\) is commutative.
