Def:
Subring :
Let R be a ring. A non-empty subset S of R is said to be a subring of R if S itself is a ring under the same binary operations as in R.
Any subring S of R is said to be a proper subring of R if S ¹R . Also, {0} is known as
trivial subring.
Result :
A non-empty subset S of a ring R is a subring of R if and
only if (i) a - b Î S for all a, b Î S i.e. S is an additive
subgroup of R. (ii) ab Î S for all a, b Î S.
Left ideal :
A non – empty subset S of a ring R is
called a left ideal of R if
(i)
a- b Î
S for all a, b ÎS
(ii)
(ii) ra ÎS for all a Î
S , r ÎR
.
Right ideal :
A non – empty subset S of a ring R is called a right ideal of R if
(i) a- b ÎS
for all a, b Î S
(ii) ar ÎS for all a ÎS , r ÎR .
Remarks :
1.
In a commutative ring every left ideal or right
ideal is a both-sided ideal.
2.
An ideal A of a ring R is called a proper ideal if
A ¹R
. Further , zero ideal {0} is also
called trivial ideal or zero ideal.
3.
Every left ideal, right ideal and ideal is
always a subring.
4.
Every subring need not be an ideal.
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