## Tuesday, February 12, 2019

### Revisiting Rings, Fields And Ideals

We now return to looking again at the mathematical entities known as rings, fields and ideals.

1.      Definitions:

i)                    Binary operation:

Let S be an abstract set. By binary operation on S, denoted by o  one means a mapping on a function from S x S  to S. In other words, a binary operation assigns  to each pair of elements in S: (a,b) an element a o b in S.

ii)                   Ring

By a ring (S, +, · ) one means an abstract set  S  which has two binary operations defined on it. The operation + is called addition, and the operation · is called multiplication. Moreover, the set (S, +, ·) is required to satisfy the following axioms:

Let a, b and c Î  S

A1):  a + (b + c) = (a + b) + c

A2)    an element  0 Î  S with the property that 0 + a = a

A3)  For each element a Î  S there exists an element
(-a)  Î  S  such that  a + (-a) = 0

A4)   a + b = b + a

A5)  a · (b · c)     = (a · b)  · c

iii)                Commutative Ring

By a commutative ring (S, + · ) one means a ring (S, + · )  which satisfies, in addition to the ring axioms the axiom:

A6)   For all  a, b  Î  S  then a · b  = b · a

And there also exist the properties:

a)Closure: If a,b Î S, then the sum a+b and the product a·b are uniquely defined and belong to S.

b)Associative laws: For all a,b,c  Î S,

a+(b+c) = (a+b)+c and a·(b·c) = (a·b)·c.

c)Commutative laws: For all a,b  Î S,    a+b = b+a and a·b = b·a.
d)Distributive laws: For all a,b,c Î  S,
a·(b+c) = a·b + a·c and (a+b)·c = a·c + b·c

e)Additive identity: The set R contains an additive identity element, denoted by 0, such that for all     a Î  S,

a+0 = a and 0+a = a.

f)Additive inverses: For each a Î  S, the equations:

a+x = 0 and x+a = 0

have a solution x  Î  S, called the additive inverse of a, and denoted by  (-a).

The commutative ring S  is called a commutative ring with identity if it contains an element 1, assumed to be different from 0, such that for all a Î  S,

a·1 = a and 1·a = a.

In this case, 1 is called a multiplicative identity element

iv)Commutative Ring with unity

By a commutative ring with unity one means a commutative ring (S, + · ) which satisfies the axiom:

A7)  There exists an element 1  Î  S  with the property:

1  · a  = a for all a  Î  S

v)Integral domain

By an integral domain  ring (S, + · )  one means a commutative ring with unity which satisfies the following axiom:

If a, b  Î  S    and a · b  = 0 then either a = 0 or b = 0.

vi)Fields

By a field (S, + · )  one means a commutative ring with unity which satisfies the additional axiom:

A7) For every non-zero element a  Î  S    there exists an element a - 1  Î  S  such that:    a · a - 1 =   1.  The element    a - 1   is called the reciprocal or multiplicative inverse of a.

vii)                    Ordered Fields

An ordered field is a field (S, + · )  which contains a subset P (called the positive cone in P) which has the following properties:

P is a proper subset of S.

O.1) P is closed under addition.

O.2)  P is closed under multiplication

O.3) If  a  Î  S   then one and only one of the following three conditions must hold:

a = 0,    a  Î  P, or    - a  Î  P

viii)Order relation on an ordered field

Let   (F , + · )  be an ordered field and let P represent the positive cone in F. We define a relation ‘<’  on F, called ‘less than’ by the formula:

<   =    {(a, b) :   a, b  Î  F, b – a Î  P }

Or, equivalently:

a  < b if an only if   b – a Î  P

We also define the relation ‘greater than’ on F by
>  as follows:

s > b if and only if b < a

Definition. The field K is said to be an extension field of the field  (F + · )   if  (F + · )    is a subset of K which is a field under the operations of K.

Definition. Let K  be an extension field of F and let u  Î K. If there exists a nonzero polynomial f(x)  Î  F[x] such that f(u)=0, then u is said to be algebraic over F. If there does not exist such a polynomial, then u is said to be
transcendental over K.

Proposition. Let K be an extension field of F , and let u Î K be algebraic over F. Then there exists a unique irreducible polynomial p(x)  Î  F[x] such that p(u)=0. It is characterized as the monic polynomial of minimal degree that has u as a root.

Furthermore, if f(x) is any polynomial in F[x] with f(u)=0, then p(x) | f(x).

Definition. Let F   be a field and let f(x) = a0 + a1 x + · · · + an xn be a polynomial in F [x] of degree n> 0. An extension field K   of F   is called a splitting field for f(x) over F if there exist elements r1, r2, . . . , rn   Î  F such that

(i)                 f(x) = an (x-r1) (x-r2) · · · (x-rn), and

(ii)               K  = F (r1,r2,...,rn).

In the above situation we usually say that f(x) splits over the field F. The elements r1, r2, . . . , rn are roots of f(x), and so K  is obtained by adjoining to F  a complete set of roots of f(x).

Definition: Subfield of a field.  Let (F + · )   be a field. A subset  T      F is  called a subfield if  T is closed under the operations of   +  and  · , and T is a field under those operations.

Definition: Ideal:   Let (S,  + · )    be a commutative ring, by an ideal I in S one means the following:

i)I is a subring of S,

ii)If  a   Î  I ,  b   Î  S then a, b    Î  I

Definition: Congruence modulo I.

Let S be a commutative ring. Let I be an ideal in S. We write  a    b modulo I for the two elements a, b    Î  S if
a   Î [ b ]. Hence,  b   Î [ a ].

Definition:  S is a commutative ring and I is an ideal. Then let S/ I denote  a quotient ring and the set of congruence classes modulo I.

Perhaps the most critical aspect of ideals to note is that: “an ideal is to a ring as a normal subgroup is to a group”.  Also, in a similar manner to groups, one can have left and right ideals.

Let   a   Î  S,   a commutative ring. Then the set (a) defined by: (a) = {ax: ,  x   Î  S }  is an ideal.

Now, let S be a commutative ring and I be an ideal in S. For each element a   Î  S let [a] be the subset of S defined by:

[a]  = I  + a = {a + j:  j   Î  S}

Further, let s be a subring of a ring S satisfying:

[a]  s       s   and s[a]     s  for all  a   Î  S is an ideal (or two-sided ideal of S). Also, a subring s of S satisfying
[a]  s       s    for all  a   Î  S is a left ideal of S. One satisfying  s [a]      s    for all  a   Î  S is a right ideal of S.

Problems for Math Mavens:

1)      Let Z be the integers. Then prove for each class:

[a] mod n there exists an integer  r   Î  Z, for:         0 < r < n,   r   Î  [n]

2)     Let Z be the integers.  The ideal:

(5)  =   {5 j:  j Î  Z }

Show all the congruence classes with respect to this ideal.

Hint:  [a] =  {a + j: j Î [(5)}  = {a +  5j: j Î  Z }

3)      Let S be a commutative ring, and let I be an ideal in S. If A, B are subset of S, then use set notation to define: i) A ·  B,   A + B

4)     Take S as the set of integers, Z. Let the ideal I = (2) so that SI =  Z 2     Thence or otherwise, find:

a)          b)        c) S/ I  =  Z 5

5)      Show every ring S has two ideals: S itself and {0}.