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I am reading Dummit and Foote's book: Abstract Algebra ... ... and am currently focused on Section 7.4 Properties of Ideals ... ...
I have a basic question regarding the generation of a two sided principal ideal in the noncommutative case ...
In Section 7.4 on pages 251-252 Dummit and Foote write the following:
In the above text we read:" ... ... If ##R## is not commutative, however, the set ##\{ ras \ | \ r, s \in R \}## is not necessarily the two-sided ideal generated by ##a## since it need not be closed under addition (in this case the ideal generated by ##a## is the ideal ##RaR##, which consists of all finite sums of elements of the form ##ras, r,s \in R##). ... ... "
I must confess I do not understand or follow this argument ... I hope someone can clarify (slowly and clearly
) what it means ... ...Specifically ... ... why, exactly, is the set ##\{ ras \ | \ r, s \in R \}## not necessarily the two-sided ideal generated by ##a##?The reason given is "since it need not be closed under addition" ... I definitely do not follow this statement ... surely an ideal is closed under addition! ...... ... and why, exactly does the two-sided ideal generated by a consist of all finite sums of elements of the form ##ras, r,s \in R## ... ... ?
Hope someone can help ... ...
Peter================================================================================To give readers the background and context to the above text from Dummit and Foote, I am providing the introductory page of Section 7.4 as follows ... ...
I have a basic question regarding the generation of a two sided principal ideal in the noncommutative case ...
In Section 7.4 on pages 251-252 Dummit and Foote write the following:
I must confess I do not understand or follow this argument ... I hope someone can clarify (slowly and clearly
Hope someone can help ... ...
Peter================================================================================To give readers the background and context to the above text from Dummit and Foote, I am providing the introductory page of Section 7.4 as follows ... ...