The pair <S, +> is said to be a group if:
If we use the symbol * as the operator in the group, we usually call the identity 1 and write the inverse of a as a-1.
A group <S, +> is said to be commutative if for every a and b in S, a + b = b + a. We also call such a group an abelian group.
Suppose S has n elements. Then we say the group <S, +> is of order n.
Let <S, +> be a group of order n. We abbreviate the sum of i a's, a + a + ... + a, as ia. (Likewise, we abbreviate the product of i a's, a * a * ... * a, as ai.) <S, +> is said to be cyclic if there is a generator k in S such that nk = 0 and for every positive integer i < n, ik ≠ 0. In this case, it is easily seen that for each a in S, there is exactly one positive integer i ≤ n such that ik = a.
Let <S, +> and <S', +> be groups such that S' is a subset of S. Then <S', +> is said to be a subgroup of <S, +>.
The triple <S, +, *> is said to be a ring if:
If there is an element 1 in S such that for every a in S, 1 * a = a * 1 = a, then the ring <S, +, *> is said to be a ring with unit element.
If for every a and b in S, a * b = b * a, then the ring <S, +, *> is said to be a commutative ring.
Let 0 be the additive identity (i.e., the identity of <S, +>) of the ring <S, +, *>. If <S-{0}, *> is an abelian group, then <S, +, *> is said to be a field.