On the linear nature of the behavior of Iwasawa's $\mu$ invariant

In this paper the behavior of Iwasawa's $\mu$ invariant is studied on the projective space of $\mathbf Z_l$-extensions of an algebraic number field. The author proves a theorem to the effect that the set of $\mathbf Z_l$-extensions for which the values of $\mu$ are not less than an arbitrary constant forms a linear projective variety.
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