In general topology, the **pseudo-arc** is the simplest nondegenerate hereditarily indecomposable continuum. The pseudo-arc is an arc-like homogeneous continuum, and played a central role in the classification of homogeneous planar continua. R.H. Bing proved that, in a certain well-defined sense, most continua in **R**^{n}, *n* ≥ 2, are homeomorphic to the pseudo-arc.

In 1920, Bronisław Knaster and Kazimierz Kuratowski asked whether a nondegenerate homogeneous continuum in the Euclidean plane **R**^{2} must be a Jordan curve. In 1921, Stefan Mazurkiewicz asked whether a nondegenerate continuum in **R**^{2} that is homeomorphic to each of its nondegenerate subcontinua must be an arc. In 1922, Knaster discovered the first example of a hereditarily indecomposable continuum *K*, later named the pseudo-arc, giving a negative answer to a Mazurkiewicz question. In 1948, R.H. Bing proved that Knaster's continuum is homogeneous, i.e. for any two of its points there is a homeomorphism taking one to the other. Yet also in 1948, Edwin Moise showed that Knaster's continuum is homeomorphic to each of its non-degenerate subcontinua. Due to its resemblance to the fundamental property of the arc, namely, being homeomorphic to all its nondegenerate subcontinua, Moise called his example *M* a **pseudo-arc**.^{ [lower-alpha 1] } Bing's construction is a modification of Moise's construction of *M*, which he had first heard described in a lecture. In 1951, Bing proved that all hereditarily indecomposable arc-like continua are homeomorphic — this implies that Knaster's *K*, Moise's *M*, and Bing's *B* are all homeomorphic. Bing also proved that the pseudo-arc is typical among the continua in a Euclidean space of dimension at least 2 or an infinite-dimensional separable Hilbert space.^{ [lower-alpha 2] } Bing and F. Burton Jones constructed a decomposable planar continuum that admits an open map onto the circle, with each point preimage homeomorphic to the pseudo-arc, called the circle of pseudo-arcs. Bing and Jones also showed that it is homogeneous. In 2016 Logan Hoehn and Lex Oversteegen classified all planar homogeneous continua, up to a homeomorphism, as the circle, pseudo-arc and circle of pseudo-arcs. In 2019 Hoehn and Oversteegen showed that the pseudo-arc is topologically the only, other than the arc, hereditarily equivalent planar continuum, thus providing a complete solution to the planar case of Mazurkiewicz's problem from 1921.

The following construction of the pseudo-arc follows ( Wayne Lewis 1999 ) .

At the heart of the definition of the pseudo-arc is the concept of a *chain*, which is defined as follows:

- A
**chain**is a finite collection of open sets in a metric space such that if and only if The elements of a chain are called its**links**, and a chain is called an**ε-chain**if each of its links has diameter less than ε.

While being the simplest of the type of spaces listed above, the pseudo-arc is actually very complex. The concept of a chain being *crooked* (defined below) is what endows the pseudo-arc with its complexity. Informally, it requires a chain to follow a certain recursive zig-zag pattern in another chain. To 'move' from the *m*th link of the larger chain to the *n*th, the smaller chain must first move in a crooked manner from the *m*th link to the (*n*-1)th link, then in a crooked manner to the (*m*+1)th link, and then finally to the *n*th link.

More formally:

- Let and be chains such that

- each link of is a subset of a link of , and
- for any indices
*i*,*j*,*m*, and*n*with , , and , there exist indices and with (or ) and and

- Then is
**crooked**in

For any collection *C* of sets, let denote the union of all of the elements of *C*. That is, let

The *pseudo-arc* is defined as follows:

- Let
*p*and*q*be distinct points in the plane and be a sequence of chains in the plane such that for each*i*,

- the first link of contains
*p*and the last link contains*q*, - the chain is a -chain,
- the closure of each link of is a subset of some link of , and
- the chain is crooked in .

- the first link of contains

- Let
- Then
*P*is a**pseudo-arc**.

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**Notes**

**Citations**

**Bibliography**

- R.H. Bing,
*A homogeneous indecomposable plane continuum*, Duke Math. J., 15:3 (1948), 729–742 - R.H. Bing,
*Concerning hereditarily indecomposable continua*, Pacific J. Math., 1 (1951), 43–51 - R.H. Bing and F. Burton Jones, "Another homogeneous plane continuum", Trans. Amer. Math. Soc. 90 (1959), 171–192
- Henderson, George W. "Proof that every compact decomposable continuum which is topologically equivalent to each of its nondegenerate subcontinua is an arc". Ann. of Math. (2) 72 (1960), 421–428
- L.C. Hoehn and Oversteegen, L., "A complete classification of homogeneous plane continua". Acta Math. 216 (2016), no. 2, 177-216.
- L.C. Hoehn and Oversteegen, L., "A complete classification of hereditarily equivalent plane continua". Adv. Math. 368 (2020), 107131, 8 pp; "arXiv:1812.08846".
- Trevor Irwin and Sławomir Solecki,
*Projective Fraïssé limits and the pseudo-arc*, Trans. AMS, 358:7 (2006), 3077-3096. - Kazuhiro Kawamura, "On a conjecture of Wood", Glasg. Math. J. 47 (2005) 1–5
- Bronisław Knaster,
*Un continu dont tout sous-continu est indécomposable*. Fundamenta Mathematicae 3 (1922): pp. 247–286 - Wayne Lewis,
*The Pseudo-Arc*, Bol. Soc. Mat. Mexicana, 5 (1999), 25–77. - Wayne Lewis and Piotr Minc,
*Drawing the pseudo-arc*, Houston J. Math. 36 (2010), 905-934. - Edwin Moise,
*An indecomposable plane continuum which is homeomorphic to each of its nondegenerate subcontinua*, Trans. Amer. Math. Soc., 63, no. 3 (1948), 581–594 - Nadler, Sam B., Jr. "Continuum theory. An introduction". Monographs and Textbooks in Pure and Applied Mathematics, 158. Marcel Dekker, Inc., New York, 1992. xiv+328 pp. ISBN 0-8247-8659-9
- Fernando Rambla, "A counterexample to Wood's conjecture", J. Math. Anal. Appl. 317 (2006) 659–667.
- Lasse Rempe-Gillen, "Arc-like continua, Julia sets of entire functions, and Eremenko's Conjecture", "arXiv:1610.06278v3"

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