Is REQ a maximal left ideal with invertible matrix Q?

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Discussion Overview

The discussion centers on whether the product of a maximal left ideal RE with an invertible matrix Q, denoted REQ, remains a maximal left ideal within the ring of n x n matrices over a field F. The scope includes theoretical considerations of ring theory and properties of ideals.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant asserts that RE is a maximal left ideal in the ring of n x n matrices over a field F.
  • Another participant argues that since RE equals R when E is the identity element, REQ would also equal R, which cannot be a maximal ideal as it is not a proper ideal.
  • A subsequent post clarifies that E is not the identity element due to its structure, which may affect the previous claims.
  • A different participant introduces a general result about maximal left ideals in associative rings, suggesting that if L is a maximal left ideal, then the transformed ideal q^{-1}Lq is also maximal when q is invertible.
  • This participant applies the general result to the specific case of RE and Q, proposing that REQ should also be a maximal left ideal based on the earlier assertion about RE.

Areas of Agreement / Disagreement

Participants express disagreement regarding the status of REQ as a maximal left ideal. Some assert that it cannot be maximal if it equals R, while others propose that it remains maximal based on the properties of maximal left ideals under ring automorphisms.

Contextual Notes

There are unresolved assumptions regarding the structure of E and its implications for the properties of RE and REQ. The discussion also highlights the dependence on definitions of maximal ideals and the conditions under which the transformations apply.

xixi
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Let R = Mn(F) be the ring consists of all n*n matrices over a field F and
E = E11 + E22 + ... + En-1,n-1, where Eii is the elementary matrix (Eij is matrix whose ij th element is 1 and the others are 0) .
I know that RE is a maximal left ideal . Let Q be an invertible matrix . Can we say that REQ is a maximal left ideal ?
 
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E is the identity element so RE=R. MQ^{-1}Q=M, so REQ is also R, which wouldn't normally be called a maximal ideal because the definition of maximal ideal requires a proper ideal.
 
Martin Rattigan said:
E is the identity element so RE=R. MQ^{-1}Q=M, so REQ is also R, which wouldn't normally be called a maximal ideal because the definition of maximal ideal requires a proper ideal.

Notice that E is not the identity element because its nth row and nth column are zero .
 
Sorry - totally missed that. I though there was something awry.
 
If R is any associative ring with 1 and q\in R has a multiplicative inverse, then \phi_q:R\rightarrow R defined by \phi_q:r\mapsto q^{-1}rq is a ring automorphism (with inverse \phi_{q^{-1}}).

If L\subset R is a maximal left ideal, then so is q^{-1}Lq. But then Lq=qq^{-1}Lq\subset q^{-1}Lq. Because L is a left ideal we have also q^{-1}Lq\subset Lq.

Hence Lq=q^{-1}Lq and therefore Lq is a maximal left ideal.

Using your assertion that RE in your question is a maximal left ideal and that Q is invertible, replacing L by RE and q by Q in the above gives the desired result.
 

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