Simplification of linear equations uses the ‘elementary moves’ such as adding a multiple of one equation to another. These ‘moves’ have analogues in the matrix approach where an elementary move on the corresponding set of linear equations is mirrored by multiplication by elementary matrices. We did not specify over what ring we were working, and the group of elementary matrices will depend on that, but some of the relations between the ‘elementary moves’ are more universal and these are encoded in the presentation of the Steinberg group.
Let be an associative ring with 1. Recall that the ( unstable) Steinberg group, , has generators, , labelling the elementary matrices, , having
and in which all indices are positive integers less than or equal to .
Stable and unstable
The terminology unstable is to make the contrast with the group , the stable version. The unstable version, , models universal relations satisfied by the elementary matrices, whilst, in , the indices, , , etc. are not constrained to be less than or equal to .
There is an inclusion of into given by the obvious inclusion on the generating sets. The ‘union’ of the sequence of these groups is .
Elementary matrix groups
The subgroup of generated by the elementary matrices is denoted . In looking at the structure on elementary matrices we have a series of more minor results before a very neat result due to Henry Whitehead.
Lemma If are distinct positive integers, then
The proof is just calculation. This then makes the following obvious.
For , is a perfect group, i.e.,
Continuing with the properties of , let be any matrix over . (It is not assumed to be invertible.)
We note that in ,
so this is in . Similarly .
Next, let and note
(as is easily verified). We thus have
hence it is a product of commutators.
Lemma If , then
so is in .
Passing to the stable groups, we get the famous Whitehead lemma:
The first algebraic -group of the ring is , the abelianization of the stable general linear group of . By the above Whitehead lemma this is also .
The covering properties of .
The Steinberg relations are modelled on observed relations amongst the elementary matrices, so sending to defines an epimorphism .
This is a universal central extension and its kernel is Milnor's K2?.