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Abelian Groups - László Fuchs [Springer]

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570 14 Butler <strong>Groups</strong><br />

Superdecomposable Butler <strong>Groups</strong> We now turn our attention to the dual<br />

problem: the existence of superdecomposable Butler groups. Recall that a superdecomposable<br />

group was defined to be a group which had no indecomposable<br />

summands except for 0. (The superdecomposable groups constructed in Theorem<br />

1.5 in Chapter 13 are not Butler.)<br />

The method we adopt here is based on the construction of a special tree. Define<br />

atreeT of length ! whose nth level T n contains 2 n vertices:<br />

T n Df.n;0/;.n;1/;:::;.n;2 n<br />

1/g:<br />

The edges are directed, connecting vertex .n; m/ to vertices .n C 1; 2m/ and<br />

.n C 1; 2m C 1/ for all 0 m

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