Editing Group extension (section)

====Warning on terminology====

In general in mathematics, an extension of a structure ''K'' is usually regarded as a structure ''L'' of which ''K'' is a substructure. See for example [[field extension]]. However, in group theory the opposite terminology has crept in, partly because of the notation <math>\operatorname{Ext}(Q,N)</math>, which reads easily as extensions of ''Q'' by ''N'', and the focus is on the group ''Q''.

A paper of [[Ronald Brown (mathematician)|Ronald Brown]] and Timothy Porter on [[Otto Schreier]]'s theory of nonabelian extensions uses the terminology that an extension of ''K'' gives a larger structure.<ref>{{cite journal|first1=Ronald|author-link1=Ronald Brown (mathematician)|last1= Brown |first2=Timothy|last2= Porter|title=On the Schreier theory of non-abelian extensions: generalisations and computations| journal=Proceedings of the Royal Irish Academy Sect A|volume=96 |year=1996|issue=2|pages=213–227|mr=1641218}}</ref>