Consider an abelian group \((S,+)\).
A ring takes this and adds a multiplicative function which satisfies the distributive property.
Groups have an identity element for their function. Rings must have identity elements for both their functions.
The multiplicative function does not have to be a bijection. For example the set of integers, addition and multiplication form a ring.
A rng is a ring without the multiplicative identity (hense no ’i’).
The multiplication operation commutes.
\([a,b]=ab-ba\)
The trivial ring is a ring with just one element.
\(0\) with addition and multiplication work.
The integers with addition and multiplication form a ring.
The integers mod \(n\) with addition and multiplication form a group.
The integers \(\{1,2,3\}\) form a ring.
The characteristic of a ring is the number of times the multiplicative identity must be added to get the additive identity.
If this never happens, the characteristic is \(0\).
The integer mod \(2\) ring, the characteristic is \(2\).
A divison ring is a ring where every non-zero element has a multiplicative inverse.
The rational numbers are a divison ring.
Fields (not yet introduced) are different from division rings only in that multiplication for a field must be commutative.
A unit is an element of a ring which has a multiplicative inverse.
The ring of integers with addition and multiplication, only \(-1\) and \(1\) are units, as both have multiplicative inverses in the ring.
A subring is a subset of the ring, where the addition and multiplication operations on the subring result in elements also in the subring.
The even numbers are a subring of the integers.
An ideal is a subring where the multiplication of any element of the ideal with any element of the ring is also in the ideal.
Even numbers are an ideal of the integers.
Odd numbers are not an ideal. For example \(1\) is in the ideal, but multiplied by \(2\) gives \(2\), which is not in the ideal.