gr.group theory - presentation for GL(n,K)

This is a sideways answer.

Let $E_{ij}(a)=I+a E_{ij}$, for $ineq j$ and $ain A$.These matrices generate the conmutator subgroup $$E(n, A)=[mathrm{GL}(n, A),mathrm{GL}(n, A)]subseteqmathrm{GL}(n, A).$$
One can easily check that the obvious relations satisfied by these elements are $$E_{ij}(a)E_{ij}(b)=E_{ij}(a+b),$$ $$[E_{ij}(a),E_{jk}(a)]=E_{ik}(a) mbox{ if $ineq k$,}$$ $$[E_{ij}(a),E_{kl}(b)]=1 mbox{ if $ineq l$ and $jneq k$.}$$
Yet the group presented by generators and this relations is not $E(n,A)$, but what we call the $*n$-th unstable Steinberg group* $mathrm{St}(n, A)$ of $A$. In general, this is larger than (precisely, an extension of) $E(n,A)$.

(NB: The following paragraph has been edited to make it match reality. Thanks to Allen for pointing the mistake in the comment bellow)

This is seen, for example, because the map $mathrm{St}(n, A)to E(n,A)$ has a non-trivial kernel. Indeed, after passing to the direct limit as $n$ goes to infinity, the kernel of that map is precisely the second algebraic $K$-theory group of $A$, $K_2(A)$. Milnor shows in his book that $K_2(mathbb{R})$ is uncountable, and describes $K_2(mathbb Q)$ (he also shows that $K_2(mathbb Z)$ is cyclic of order two, so this can be done for rings that are not fields too...)

A nice reference for all this is Jonathan Rosenberger's Algebraic $K$-theory and its applications, and there is John Milnor's Introduction to algebraic $K$-theory, which is also extremely nice.

A short intuitive description for $K_2(A)$ is: it measures how much more information is there in the elementary matrices of a ring which does not follow formally from the Steinberg relations.