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Abelian Groups - László Fuchs [Springer]

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354 11 p-<strong>Groups</strong> with Elements of Infinite Height<br />

Consequently, if N is a nice subgroup of A, then under the natural correspondence<br />

between subgroups of A containing N and subgroups of A=N, nice subgroups<br />

correspond to nice subgroups.<br />

F Notes. Niceness is a crucial concept in the theory. We shall see that nice subgroups play<br />

crucial role also in mixed groups; see Chapter 15.<br />

Exercises<br />

(1) If M; N are subgroups of a p-group A, andifa 2 A is proper with respect to<br />

both of them, then a is proper with respect to M \ N as well.<br />

(2) A subgroup N is nice in the p-group A if and only if the equality N C p A D<br />

\ < .N C p A/ holds for all limit ordinals .<br />

(3) If N is nice in A,thensoisN C p A for every .<br />

(4) (Hill) Let A be a p-group. For a subgroup N of A to be nice it is necessary and<br />

sufficient that N \ p A is nice in p A and .N C p A/=p A/ is nice in A=p A for<br />

all ordinals .<br />

(5) A p-group has the property that all of its subgroups are nice if and only if it is a<br />

direct sum of a bounded group and a divisible group.<br />

(6) The union of an ascending chain of nice subgroups need not be nice.<br />

(7) Let B denote an unbounded †-cyclic p-group, and B its torsion-completion.<br />

Show that (a) BŒp is nice in BŒp (it is a summand); (b) BŒp is nice in B;but(c)<br />

BŒp is not nice in B.<br />

(8) Give an example where N < M < A, M=N is nice in A=N, butM fails to be<br />

nice in A.<br />

(9) The extensions of a p-group A by a p-group C in which A is a nice subgroup<br />

form a subgroup in Ext.C; A/. [Hint: Baer sum.]<br />

3 Simply Presented p-<strong>Groups</strong><br />

We now embark on an ambitious generalization of Ulm’s theorem: this section<br />

and the three following ones are devoted to the theory of p-groups that can be<br />

characterized by their UK-invariants. This theory is undoubtedly one of the major<br />

achievements in the theory of p-groups of arbitrary cardinality; it provides the most<br />

penetrating results known today on p-groups. We develop the theory on parallel<br />

lines:<br />

1. simply presented p-groups;<br />

2. p-groups with nice systems (or nice composition series) of subgroups;<br />

3. balanced-projective p-groups; and<br />

4. totally projective p-groups.<br />

Simple Presentation In this section, we deal with p-groups that have a special<br />

kind of presentation. As a motivation, let us recall that a †-cyclic p-group can be

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