# Solenoid

Let $\mathbf n = \langle n _ {i} \rangle _ {i}$ be a sequence of positive integers. From $\mathbf n$ one constructs a topological space as follows.

Let $T _ {0}$ be a torus in $\mathbf R ^ {3}$; inside $T _ {0}$ one takes a torus $T _ {1}$ wrapped around longitudinally $n _ {1}$ times, in a smooth fashion without folding back; inside $T _ {1}$ one takes a torus $T _ {2}$ wrapped around $n _ {2}$ times in the same way. Continuing this procedure indefinitely, one obtains a decreasing sequence of tori. Its intersection is called the $\mathbf n$- adic solenoid $\Sigma _ {\mathbf n }$.

The basic properties of $\Sigma _ {\mathbf n }$ are that it is a one-dimensional continuum which, moreover, is indecomposable (cf. Indecomposable continuum).

$\Sigma _ {\mathbf n }$ is also a topological group; this can be seen if one considers an alternative construction of $\Sigma _ {\mathbf n }$ as the inverse limit of the following inverse sequence:

$${} \dots S _ {3} \rightarrow ^ { {f _ 3} } S _ {2} \mathop \rightarrow \limits ^ { {f _ {2} }} S _ {1} \rightarrow ^ { {f _ 1} } S _ {0} ,$$

where each $S _ {i}$ is the unit circle and $f _ {i} : S _ {i} \rightarrow S _ {i-} 1$ is defined by $f _ {i} ( z)= z ^ {n _ {i} }$. There are various other ways in which one can construct the solenoids, see, e.g., [a3].

Solenoids were first defined by L. Vietoris [a2] (for the sequence $\langle 2, 2 ,\dots \rangle$) and by D. van Dantzig [a1] (for all constant sequences).

Solenoids are also important in topological dynamics; on them one can define a flow (continuous-time dynamical system) structure [a4] which has a locally disconnected minimal set of almost-periodic motions.

There is a complete classification of the solenoids: first, without loss of generality one may assume that the numbers $n _ {i}$ are prime. Call two sequences of primes $\mathbf p$ and $\mathbf q$ equivalent if one can delete from each a finite number of terms such that in the reduced sequences $\mathbf p ^ \prime$ and $\mathbf q ^ \prime$ every prime is counted the same number of times. One can then show that $\Sigma _ {\mathbf p }$ and $\Sigma _ {\mathbf q }$ are homeomorphic if and only if $\mathbf p$ and $\mathbf q$ are equivalent. See [a5] and [a6].

Finally one can characterize the solenoids as those metric continua that are homogeneous and have the property that every proper subcontinuum is an arc. See [a7].

#### References

 [a1] D. van Dantzig, "Ueber topologisch homogene Kontinua" Fund. Math. , 15 (1930) pp. 102–125 [a2] L. Vietoris, "Über den höheren Zusammenhang kompakter Räume und eine Klasse von zusammenhangstreuen Abbildungen" Math. Ann. , 97 (1927) pp. 454–472 [a3] E. Hewitt, K.A. Ross, "Abstract harmonic analysis" , 1 , Springer (1979) [a4] V.V. Nemytskii, V.V. Stepanov, "Qualitative theory of differential equations" , Princeton Univ. Press (1960) (Translated from Russian) [a5] R.H. Bing, "A simple closed curve is the only homogeneous bounded plane continuum that contains an arc" Canad. Math. J. , 12 (1960) pp. 209–230 [a6] M.C. McCord, "Inverse limit sequences with covering maps" Trans. Amer. Math. Soc. , 114 (1965) pp. 197–209 [a7] C.L. Hagopian, "A characterization of solenoids" Pacific J. Math. , 68 (1977) pp. 425–435
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Solenoid. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Solenoid&oldid=48745