Field
In abstract algebra, a field is a set equipped with two operations, usually called addition and multiplication. These operations satisfy several properties, which we outline below.
- Closure: For any two elements \( a \) and \( b \) in a field \( F \), the sum \( a + b \) and the product \( a \cdot b \) are also in \( F \).
- Associativity: Addition and multiplication are associative, meaning that \( (a+b)+c = a+(b+c) \) and \( (a \cdot b) \cdot c = a \cdot (b \cdot c) \) for all \( a, b, \) and \( c \) in \( F \).
- Commutativity: Addition and multiplication are commutative, meaning that \( a + b = b + a \) and \( a \cdot b = b \cdot a \) for all \( a \) and \( b \) in \( F \).
- Existence of Identity Elements: There exist two distinct identity elements: an additive identity \( 0 \) and a multiplicative identity \( 1 \), such that for all \( a \) in \( F \), \( a + 0 = a \) and \( a \cdot 1 = a \).
- Existence of Inverses: Every element \( a \) in \( F \) (except for the additive identity \( 0 \)) has an additive inverse \( -a \) such that \( a + (-a) = 0 \). Additionally, every element \( a \) in \( F \) (except for the multiplicative identity \( 1 \) and the additive identity \( 0 \)) has a multiplicative inverse \( a^{-1} \) such that \( a \cdot a^{-1} = 1 \).
- Distributivity: Multiplication distributes over addition, meaning that \( a \cdot (b + c) = (a \cdot b) + (a \cdot c) \) for all \( a, b, \) and \( c \) in \( F \).
Group
A group \( G \) is a set equipped with a binary operation \( \cdot \), denoted as \( (G, \cdot) \), satisfying the following properties:
1. Closure: For any two elements \( a, b \) in \( G \), the result of \( a \cdot b \) is also in \( G \).
2. Associativity: The operation \( \cdot \) is associative, meaning that \( (a \cdot b) \cdot c = a \cdot (b \cdot c) \) for all \( a, b, c \) in \( G \).
3. Identity Element: There exists an element \( e \) in \( G \) such that for any element \( a \) in \( G \), \( a \cdot e = e \cdot a = a \). This element \( e \) is called the identity element of \( G \).
4. Inverse Element: For every element \( a \) in \( G \), there exists an element \( b \) in \( G \) such that \( a \cdot b = b \cdot a = e \), where \( e \) is the identity element of \( G \). The element \( b \) is called the inverse of \( a \) and is denoted as \( a^{-1} \).
Distinguish between field and group
- Definition:
- Field: A field is a set equipped with two operations, typically addition and multiplication, that satisfy specific properties. These properties include closure, associativity, commutativity, distributivity, existence of additive and multiplicative identities, and existence of additive inverses and multiplicative inverses (except for the additive identity).
- Group: A group is a set equipped with a single binary operation that satisfies closure, associativity, existence of an identity element, and existence of inverses for every element.
- Operations:
- Field: Fields have two operations: addition and multiplication. These operations satisfy the properties mentioned above.
- Group: Groups have a single operation, typically denoted as *, which combines two elements of the group to produce another element of the group. This operation also satisfies the group properties mentioned above.
- Inverses:
- Field: In a field, every nonzero element has a multiplicative inverse. Additionally, every element has an additive inverse.
- Group: In a group, every element has an inverse with respect to the group operation.
- Example:
- Field: The set of real numbers \(R\) forms a field under addition and multiplication. Every real number except 0 has a multiplicative inverse.
- Group: The set of integers \(Z\) forms a group under addition. The identity element is 0, and every integer has an additive inverse.