We study the density of everywhere locally soluble diagonal quadric surfaces, parameterised by rational points that lie on a split surface.

We describe soluble groups in which the set of all subgroups is countable and show that locally (soluble-byfinite) groups with this property are soluble-by-finite. Further, we construct a nilpotent group with uncountably many subgroups in which the set of all abelian subgroups is countable.

is soluble for every pG.m and every gGG. Similarly we call a group G a Dor-group if the equation (1) (above) is not only soluble, but uniquely soluble. The class of Fur-groups and the class of Dor-groups will be denoted respectively by Em and Dm. Clearly Em~2)Dm. There is a less trivial relationship between these two classes of groups. If we denote by ©(C) the class of groups which occur as hom...