Dear all,


According to the notation of Wilson's book "The finite simple groups" ( books.google.com/books?isbn=1848009879, page 9) $ A\_{.} B $ denotes an unspecified extension. Now, I want to know what does "unspecified extension of $A$ and $B$" mean? 



Notation of Wilson's book "The finite simple groups"
I don't have the book at hand, but I think the usual meaning of $G = A.B$ for groups $A$ and $B$ is that $G$ has a normal subgroup isomorphic to $A$ such that the quotient $G/A$ is isomorphic to $B$. Some authors only write non-split extensions in this way, but if the book states that $A.B$ denotes an "unspecified" extension, this means that it may be split or not. 


