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Rings and Algebras Problem set #1: Solutions Sept. 15, 2011. 1. Prove that the following are equivalent for a ring R
 

Summary: Rings and Algebras Problem set #1: Solutions Sept. 15, 2011.
1. Prove that the following are equivalent for a ring R:
a) R has an identity element;
b) whenever R S for a ring S (possibly without an identity), then R is a ring direct summand of S
(i. e. there is an ideal T S such that R T = {0} and R + T = S).
Solution. a) b). Let e be the identity element of R, and consider the set T = {s - se | s S}. A straightforward
calculation shows that T is a left ideal in S. Moreover, if r R T, then r = s - se for some s S, and then
r = re = (s - se)e = se - se = 0. Hence R T = {0}. Finally, we can observe that s = se + (s - se) for each s S, and
here se R and s - se T. This means that S is the direct sum as a left S-module of R and T, hence every element in S
can be (uniquely) written in the form r + t with r R and t T. We will now show that T is also a right ideal in S. Let
s S be arbitrary and compute (s - se)(r + t) with r R and t T. We have: (s - se)(r + t) = (sr - ser) + (s - se)t.
Since e is the identity element in R and r R, we get that sr - ser = sr - sr = 0. On the other hand (s - se)t T since
T is a left ideal. Thus T is a two sided ideal in S, hence S = R T, a ring direct sum. -- b) a) We know that any
ring R can be embedded as an ideal into a ring S with identity. If R is also a direct summand of S, then the projection
of 1S in R is going to be an identity element in R.
2. Give an example of a non-commutative ring R for which the multiplicative group of invertible elements
is commutative.
Solution. Take R = K x, y , the free algebra over a (commutative field) K. Here the invertible elements are only
the non-zero elements of K and the multiplication in K is commutative. ­ Second solution: Take R = T2(Z2), the
set of upper triangular matrices over Z2. The multiplication is non-comutative (for example 0 1

  

Source: Ágoston, István - Institute of Mathematics, Eötvös Loránd University

 

Collections: Mathematics