There are interesting theorems about groups as union of proper subgroups. The first result in this subject is the theorem of Scorza(1926): "*a groups if union of three proper subgroups if and only it has quotient $C_2\times C_2$.*" In 1959, Haber and Rosenfeld proved interesting theorems on the groups as union of subgroups. Then, in 1994, J. H. E. Cohn proved some interesting theorems about groups as union of *few* proper subgroups, and made conjectures.

While reading these three papers, which have large gaps in the publishing years, I couldn't find other initial references on "Groups as union of subgroups".

It will be a great pleasure, if one provides a list of references on the subject "Groups as union of proper subgroups", from 1926 to 1959 and from 1959 to 1994.

Especially, it is known that *a non-cyclic $p$-group can not be union of $p$-proper subgroups, and if it is union of $p+1$ proper subgroups, then all the subgroups are maximal, and theire intersection has index $p^2$ in $G$*. I would like to get original references for this theorem also.

Thanks in advance!!