Here is my list of open problems in wild topology. Some of these problems are very hard and fairly well-known. Some of them are less well known but still hard and some are probably answerable with some effort. This list does not speak for “the field” or really anyone but myself. However, if you have an interesting problem that you think should be on here, feel free to send it (with some context) to me. For example, I’m confident that I’m leaving some relevant homology and geometric group theory problems off of this list. I am happy to receive any corrections or updates as well.
Fundamental Groups and First Singular Homology
* Torsion Problem: Suppose and .
- Is torsion free? A stronger question would be: is residually locally free, i.e. a subgroup of a product of locally free groups?
- Is torsion free?
*Problem (Eda, 1992): Is torsion free?
Katsuya Eda asked the question in his 1992 paper Free sigma-products and noncommutatively slender groups and reminded us of it at the 2018 Arches Topology Conference. It was also asked later by other group theorists.
*Archipelago Group Problem (Conner-Hojka-Meilstrup): Consider the “archipelago group” of a non-trivial group constructed in
Conner, Hojka, Meilstrup, Archipelago groups, PAMS 143 no. 11 (2015) 4973-4988.
Are the isomorphism types groups independent of ?
*One-dimensional Surjectivity Problem: If is a Peano continuum, is there a one-dimensional metric space (ideally a Peano continuum) and a map which induces a surjection on fundamental groups?
I have asked several people about the above problem. Everyone thinks it’s true but no one seems to know how to prove it. I think having a positive answer would be very useful considering fundamental groups of 1-dimensional spaces have a usable calculus.
*One-dimensional Embedding Classification: Given a one-dimensional Peano continuum , characterize those one-dimensional Peano continua for which embeds as a subgroup of .
*One-point compactifications of infinite-type surfaces: suppose is an infinite type surface and is its one-point compactification. Does canonically inject into the first Cech homotopy group ? Is there a characterization of as a quotient of the earring group?
*Fundamental Groups of Free Topological Algebraic Objects: If is a path-connected, locally path-connected, metric space with basepoint . Let denote the infinitary abelianization of at as defined by Brazas-Gillespie. Consider the following spaces:
- the James reduced product ,
- the infinite symmetric product ,
- the path-component of the constant loop in the loop space .
Do all of the above spaces have fundamental group isomorphic to ?
The Infinite Earring
Let denote the smaller copies of the earring that consist of all circles with index , i.e. radius . Since this is a retract of , we may identify as a subgroup of .
Characterize n-slender Groups (Eda): A group is non-commutatively slender (or n-slender) if for every homomorphism , there exists a such that . Characterize n-slender groups.
*Pseudo-Earring Conjecture (Conner): If is the uncountable image of an endomorphism of the earring group, then is isomorphic to .
Let be the free subgroup generated by the homotopy classes of the loops , traversing the individual circles of .
*Transfinite Closure of Pseudo-Earring Groups (Brazas-Fischer): Suppose is a map and is the image of the induced homomorphism. If is a map such that , then must we have .
*Scattered-Dense Gap Problem (Brazas-Fischer): Let be the smallest normal subgroup of that contains the set and is closed under transfinite products. Must contain the homotopy class of the loop shown below?
The Scattered-Dense Gap Problem is equivalent to the problem in my paper Scattered products in fundamental groupoids asking “Must every space with well-defined scattered -products have well-defined transfinite -products? Although this may seem like a very specific statement, this is actually a very fundamental problem about how non-commutative infinitary operations work and it is definitely worth solving.
*Closures of Countable Subgroups Problem: Consider the -subgroup closure operators defined in the paper “Test-map characterizations of locally properties of fundamental groups.” Suppose is any space and is a countable index -closed subgroup. Must be closed according to other closure operators, e.g. must be -closed, -closed, -closed, or -closed?
Generalized covering maps and unique lifting
*Dydak’s Unique Path-Lifting Problem: Suppose is the closed unit disk and is a map such that is path connected and locally path connected and all paths lift uniquely, i.e. for every path and point , there is a unique path such that and . Must be a homeomorphism?
Note on Dydak’s Problem: I know of many related and partial results; however, Dydak’s Problem is a fundamental and general topological problem, which has broad consequences related to lifting properties. A positive answer essentially implies that unique lifting of paths alone implies all other unique lifting properties in covering space theory.
Overlays were introduced by R.H. Fox in the 1970’s as a refinement of the definition of covering map: an overlay is a covering map such that there is an open cover of consisting of neighborhoods evenly covered by such that if are intersecting neighborhoods, and and , then for every there is a unique such that .
Topological Group Overlay Problem (Eda): Let be a connected topological group and be an overlay. Does inherit the structure of a topological group so that is a group homomorphism?
The following problems are related to generalized covering maps. An excellent paper on the basics of generalized universal covering spaces is
- H. Fischer, A. Zastrow, Generalized universal covering spaces and the shape group, Fund. Math. 197 (2007) 167-196.
*Generalized Covering Space Problem for Peano Continua (Eda): Let be a Peano continuum. If is homotopically Hausdorff, does admit a generalized universal covering space?
*Fischer’s Problem: If is a planar set and is the generalized universal covering space, does admit a metric giving it the structure of a CAT(0) space?
Lifting Projection Problem: Give an example of a path-connected, locally path-connected metric space and a fibration with the unique path-lifting property (sometimes called a lifting projection), which is not an inverse limit of covering maps.
*Missing Ends: Is there a suitable theory of ends for generalized covering spaces?
Higher Homotopy and Homology
*Higher Homotopy Groups of the n-dimensional earring: Characterize , in terms of the higher homotopy groups of .
The reality is that very little is known about these groups. The next problem is a special case of interest – the smallest unknown case. Although we may have an idea of what it should be, the tools to complete the proof are still missing.
*Problem: Characterize .
*Problem (Eda-Karimov-Repovs): Does there exist a finite-dimensional non-contractible Peano continuum for which all homotopy groups are trivial?
Problem (Eda-Karimov-Repovs): Let be the shrinking wedge of tori (replace each in with a torus). Is trivial?
Note: it is known that .
*Problem (Eda-Kawamura): Let . If , are path-connected Hausdorff spaces and , must it be that ?
Note: Here the basepoint of a cone is the image of . The first question is problem is Question 1.3 in K. Eda, K. Kawamura, On the asphericity of one-point unions of cones, Topology Proceedings 36 (2010) 63-75. In this paper the authors affirmatively answer this question in dimension .
*Problem: If is a sequence of CW-comlexes, characterize the singular homology groups for .
Note: The singular homology groups of the infinite dimensional torus are uncountable in every dimension but even these groups have not been fully characterized for .
Topological homotopy groups
The n-th quasitopological homotopy group is the usual homotopy group endowed with the quotient topology with respect to the map sending a based map to its homotopy class.
*Problem: Characterize the locally path-connected spaces with a single wild point for which is a topological group.
*Problem: Find Peano continua with isomorphic fundamental groups whose quasitopological fundamental groups are both non-discrete and non-isomorphic.
Based on Eda’s homotopy classification of one-dimensional spaces, at least one of the spaces in an answer to the previous problem must have dimension at least 2.
*Problem (Brazas-Fabel): If is a compact metric space and satisfies the separation axiom, must also satisfy the separation axiom?
*Problem: Are there one-dimensional conditions that would ensure is a topological group for ? For example, if is 1-connected and locally 1-connected, must , be a topological group?
I’m going to hide this down here for those dedicated enough to read the whole list.
For any person or group who provides/publishes a verifiable solution to a problem with a * next to it, I will happily buy a pizza and round of beer (or other beverage of preference) as a friendly gesture of appreciation and admiration. Since I am neither omnipresent nor omniscient, it would be helpful if you contact me about your solution to redeem your refreshment prize. Pizzas redeemed in Philadelphia may be replaced with a cheesesteak and soft pretzel. If I cannot join you in person, I will find a way to get you your prize.
Updates and Solved Problems:
Some of the problems in this list have been solved after being posted here. When a solution to a listed problem is published, I’ll move the problem down here and link to the solution.
Update: Sam Corson has posted a preprint proving that these two groups are isomorphic. This was a starred problem so once the solution is refereed and published, I will offer Sam a delicious prize.
Wild Fundamental Group Actions Problem: Consider the one-point union for .
- Characterize .
- Describe the action of on . Does the image of under the action generate ?
Update: This problem has a proposed solution (by me) in 2021 in this paper. The group naturally embeds into . The characterization uses generalized universal covering maps and the Whitney covering lemma. The image of under the -action does NOT generate .