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Some Cardinal and Geometric Properties of the Space of Permutation Degree
Abstract
Abstract contributionclaim_typeThis paper is devoted to the investigation of cardinal invariants such as the hereditary density, hereditary weak density, and hereditary Lindelöf number. The relation between the spread and the extent of the space SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) of permutation degree of the Hattori space is discussed. In particular, it is shown that the space SP 2 ( 𝑅 , 𝜏 𝑆 ) contains a closed discrete subset of cardinality 𝑐 . Moreover, it is shown that the functor SP 𝐺 𝑛 preserves the homotopy and the retraction of topological spaces. In addition, we prove that if the spaces X and Y are homotopically equivalent, then the spaces SP 𝐺 𝑛 𝑋 and SP 𝐺 𝑛 𝑌 are also homotopically equivalent. As a result, it has been proved that the functor SP 𝐺 𝑛 is a covariant homotopy functor. Keywords: extent; Lindelöf number; hereditary cardinal invariant; homotopically equivalent; covariant homotopy functor; retract MSC: 18F60; 18A05; 54A25; 55P99
1. Introduction
mechanisms↗concepts↗concepts↗unhoused_relationssensescontent_tags↗content_tags↗The cardinal invariants are considered as topological invariants with values in the class of all cardinal numbers, and are used to describe various topological properties of spaces. For example, the weight, 𝜋 -weight, network weight, density, character, Lindelöf number, tightness, and cellularity of a topological space X are some classical cardinal invariants. Many researches have been devoted to the investigation of cardinal invariants and hereditary cardinal invariants (see, for example, [1,2,3,4,5,6]) and their important role in topology. Recall that a function 𝜑 : Top → Card from the class Top of topological spaces to the set Card of infinite cardinals such that 𝜑 ( 𝑋 ) = 𝜑 ( 𝑌 ) whenever X and Y are homeomorphic, is called a cardinal function (or cardinal invariant). The hereditary version of a cardinal function 𝜑 , denoted ℎ 𝜑 , is defined as ℎ 𝜑 ( 𝑋 ) = sup { 𝜑 ( 𝑌 ) : 𝑌 ⊂ 𝑋 } [1,4,5,6].
In recent research, the interest in the theory of cardinal invariants and their behavior under the influence of various covariant functors is increasing (see, for example, [7,8,9]). In [10], the authors investigated several cardinal invariants under the influence of some seminormal and normal functors. In the investigations in [11,12], the concept of symmetric product of a topological space is introduced. In particular, in [13] the functor SP 𝐺 𝑛 is studied, and some cardinal and topological properties of this functor were investigated. In [14], some propositions about homotopy properties of the topological spaces were proved. For instance, it was proved that, contractibility, connectedness, and pathwise connectedness are homotopy properties of the spaces. In our work, we prove that if the mappings 𝑓 , 𝑔 : 𝑋 → 𝑌 are homotopic, then the mappings SP 𝐺 𝑛 𝑓 , SP 𝐺 𝑛 𝑔 : SP 𝐺 𝑛 𝑋 → SP 𝐺 𝑛 𝑌 are also homotopic.
object_of_studyresearch_questionconcepts↗claim_relationsThe current paper is devoted to the investigation of hereditary cardinal invariants (such as the hereditary density, the hereditary weak density, and the hereditary Lindelöf number) in the space of permutation degree. Additionally, the relation between the spread and the extent of the space SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) of the permutation degree of Hattori space ( 𝑅 , 𝜏 ( 𝐴 ) ) is studied. Moreover, it is shown that the functor SP 𝐺 𝑛 preserves the homotopy and the retraction of topological spaces. As a consequence, it has been proved that the functor SP 𝐺 𝑛 is a covariant homotopy functor. Our research complements and extends existing results in the fields of cardinal invariants and the theory of covariant functors.
The paper is organized as follows. In Section 2, we recall basic notions and notation that will be used in the rest of the study. In Section 3, we study hereditary cardinal invariants and obtain some results for the space SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) of permutation degree of the Hattori space ( 𝑅 , 𝜏 ( 𝐴 ) ) . Finally, in Section 4, we study some geometric properties of the space SP 𝐺 𝑛 𝑋 of permutation degree of a space X.
limitationsconcepts↗concepts↗concept_edgesThroughout the paper, all spaces are assumed to be completely regular; 𝜏 denotes an infinite cardinal number; and by 𝜔 and 𝑐 we denote the countable cardinal number and the cardinality of continuum, respectively. The real line with the Sorgenfrey topology [4] is denoted by ( 𝑅 , 𝜏 𝑆 ) ; and for 𝐴 ⊂ 𝑅 by ( 𝑅 , 𝜏 ( 𝐴 ) ) we denote the Hattori space over A. Recall that in [15], the following generalization of the Sorgengfey line was defined: if 𝐴 ⊂ 𝑅 , then 𝜏 ( 𝐴 ) denotes the topology on 𝑅 , in which each point 𝑎 ∈ 𝐴 has the usual Euclidean neighborhoods, and basic neighborhoods of a point 𝑥 ∈ 𝑅 \ 𝐴 are of the form [ 𝑥 , 𝜀 ) , 𝜀 > 0 . Notice that for 𝐴 ⊂ 𝑅 , the topology 𝜏 ( 𝐴 ) is finer than the usual Euclidean topology on 𝑅 and weaker than the Sorgenfrey topology 𝜏 𝑆 [16].
2. Preliminaries
For convenience of the reader, we give some notation, concepts, and statements that are widely used in this article. For a space X, the group of all permutations of X is denoted by 𝑆 ( 𝑋 ) and called the permutation group of X. If 𝑋 = { 1 , 2 … , 𝑛 } , then we write 𝑆 𝑛 instead of 𝑆 ( 𝑋 ) .
concepts↗concepts↗content_tags↗Let X be a space. The permutation group 𝑆 𝑛 acts on the n-th power 𝑋 𝑛 of X as the permutation of coordinates: the points ( 𝑥 1 , 𝑥 2 , … , 𝑥 𝑛 ) , ( 𝑦 1 , 𝑦 2 , … , 𝑦 𝑛 ) ∈ 𝑋 𝑛 are equivalent if there exists a permutation 𝜎 ∈ 𝑆 𝑛 for which 𝑦 𝑖 = 𝑥 𝜎 ( 𝑖 ) . This equivalence relation is called the symmetric equivalence relation [17], and the set of all orbits of the action of 𝑆 𝑛 on 𝑋 𝑛 with the quotient topology is denoted by SP 𝑛 𝑋 and called the space of n-permutation degree of X [17].
concepts↗concepts↗claim_relationsconcept_edgescontent_tags↗The following generalization of the permutation degree will be also used in what follows. If G be a subgroup of the group 𝑆 𝑛 , then G also acts on 𝑋 𝑛 as the group of permutations of coordinates, and generates an equivalence relation called the G-symmetric equivalence relation [17]. The quotient space of 𝑋 𝑛 under this relation is called G-permutation degree of X and is denoted by SP 𝐺 𝑛 𝑋 , and the quotient mapping from 𝑋 𝑛 to SP 𝐺 𝑛 𝑋 is denoted by 𝛽 𝑛 , 𝐺 𝑠 . Observe that SP 𝐺 𝑛 is a covariant functor in the category of compact spaces and is called the functor of G-permutation degree [17]. Clearly, if 𝐺 = 𝑆 𝑛 , then SP 𝐺 𝑛 = SP 𝑛 , and if G contains only the identity element, SP 𝐺 𝑛 𝑋 = 𝑋 𝑛 [17].
In [12], it is proved that the quotient mapping 𝛽 𝑛 , 𝐺 𝑠 : 𝑋 𝑛 → SP 𝐺 𝑛 𝑋 is continuous, open, and closed surjection.
mechanisms↗For every mapping 𝑓 : 𝑋 → 𝑌 , the mapping SP 𝐺 𝑛 𝑓 : SP 𝐺 𝑛 𝑋 → SP 𝐺 𝑛 𝑌 is defined [17] by the formula SP 𝐺 𝑛 𝑓 [ ( 𝑥 1 , 𝑥 2 , … , 𝑥 𝑛 ) ] 𝐺 = [ ( 𝑓 ( 𝑥 1 ) , 𝑓 ( 𝑥 2 ) , … , 𝑓 ( 𝑥 𝑛 ) ) ] 𝐺 .
concepts↗concepts↗content_tags↗content_tags↗A set 𝐴 ⊂ 𝑋 is dense in X if 𝐴 ¯ = 𝑋 . The density of X, denoted by 𝑑 ( 𝑋 ) , is defined as 𝑑 ( 𝑋 ) = min { | 𝐴 | : 𝐴 𝑖 𝑠 𝑑 𝑒 𝑛 𝑠 𝑒 𝑖 𝑛 𝑋 } [4]. A collection 𝐵 of nonempty open sets in X is said to be a π-base of X if for every nonempty open set 𝐺 ⊂ 𝑋 there is a 𝐵 ∈ 𝐵 with 𝐵 ⊂ 𝐺 . The π-weight of a space X is defined as 𝜋 𝑤 ( 𝑋 ) = min { | 𝐵 | : 𝐵 is a 𝜋 -base of 𝑋 } [1,4,6]. The weak density of a space X, denoted by 𝑤 𝑑 ( 𝑋 ) , is the smallest cardinal number 𝜏 ≥ 𝜔 such that there is a 𝜋 -base 𝐵 = ∪ { 𝐵 𝛼 : 𝛼 < 𝜏 } in X, and for each 𝛼 < 𝜏 , 𝐵 𝛼 is a centered system of open sets in X [7,9,10].
For definitions of the following cardinal functions, see [1,4,5,6].
concepts↗concepts↗concepts↗sensescontent_tags↗content_tags↗The extent of a space X, denoted by 𝑒 ( 𝑋 ) , is defined as 𝑒 ( 𝑋 ) = sup { | 𝑌 | : 𝑌 is a closed discrete subspace in 𝑋 } . The Souslin number or cellularity of the space X, denoted by 𝑐 ( 𝑋 ) , is the smallest cardinal number 𝜏 ≥ 𝜔 such that every family or pairwise, disjoint, non-empty open subset of X has cardinality ≤ 𝜏 . The Lindelöf number 𝑙 ( 𝑋 ) of X is the smallest cardinal number 𝜏 such that each open cover of X has a subcover of cardinality ≤ 𝜏 .
A continuous mapping 𝑓 : [ 0 , 1 ] → 𝑋 is called a path in X. 𝑓 ( 0 ) is called the initial point, and 𝑓 ( 1 ) the final point of this path. If 𝑥 ∈ 𝑋 , then the constant path 𝑒 𝑥 : 𝐼 → 𝑋 is defined by 𝑒 𝑥 ( 𝑡 ) = 𝑥 for all 𝑡 ∈ 𝐼 . A space X is path connected if for any two points 𝑥 0 , 𝑥 1 ∈ 𝑋 there is a path from 𝑥 0 to 𝑥 1 [18].
concepts↗Continuous mappings 𝑓 , 𝑔 : 𝑋 → 𝑌 are homotopic, denoted by 𝑓 ≃ 𝑔 , if there is a continuous mapping 𝐹 : 𝑋 × 𝐼 → 𝑌 such that 𝐹 ( 𝑥 , 0 ) = 𝑓 ( 𝑥 ) and 𝐹 ( 𝑥 , 1 ) = 𝑔 ( 𝑥 ) . F is called a homotopy between f and g [18].
Consider the mappings 𝑓 ( 𝑥 ) = ( cos ( 𝜋 𝑥 ) , sin ( 𝜋 𝑥 ) ) and 𝑔 ( 𝑥 ) = ( cos ( 𝜋 𝑥 ) , − sin ( 𝜋 𝑥 ) ) . These mappings are homotopic. We can define the homotopy 𝐹 : 𝐼 × 𝐼 → 𝑅 2 between f and g as follows: 𝐹 ( 𝑥 , 𝑡 ) = ( cos ( 𝜋 𝑥 ) , ( 1 − 2 𝑡 ) sin ( 𝜋 𝑥 ) ) . Indeed, F is continuous and 𝐹 ( 𝑥 , 0 ) = ( cos ( 𝜋 𝑥 ) , sin ( 𝜋 𝑥 ) ) = 𝑓 ( 𝑥 ) , 𝐹 ( 𝑥 , 1 ) = ( cos ( 𝜋 𝑥 ) , − sin ( 𝜋 𝑥 ) ) = 𝑔 ( 𝑥 ) (see [18]).
concepts↗concepts↗sensescontent_tags↗A continuous mapping 𝑓 : 𝑋 → 𝑌 is said to be a homotopy equivalence [18] if there exists a continuous mapping 𝑔 : 𝑌 → 𝑋 such that the compositions 𝑔 ∘ 𝑓 and 𝑓 ∘ 𝑔 are homotopic to the identity mappings on X and Y, respectively. Two topological spaces X and Y are said to be homotopically equivalent (notation 𝑋 ≃ 𝑌 ) if there exists a homotopy equivalence 𝑓 : 𝑋 → 𝑌 [18].
concepts↗sensescontent_tags↗By a covariant homotopy functor [17], we mean an operator 𝜙 which assigns to each topological space X a space 𝜙 ( 𝑋 ) , and to each continuous mapping 𝑓 : 𝑋 → 𝑌 , a mapping 𝜙 ( 𝑓 ) : 𝜙 ( 𝑋 ) → 𝜙 ( 𝑌 ) satisfying the following three conditions:
(i) 𝜙 preserves the identity mapping; that is, if f is the identity mapping of X, then 𝜙 ( 𝑓 ) is the identity mapping of 𝜙 ( 𝑋 ) .
(ii) 𝜙 preserves compositions; that is, if 𝑓 : 𝑋 → 𝑌 and 𝑔 : 𝑌 → 𝑍 are continuous mappings, then 𝜙 ( 𝑔 ∘ 𝑓 ) = 𝜙 ( 𝑔 ) ∘ 𝜙 ( 𝑓 ) .
(iii) 𝜙 preserves homotopy; that is, if a mapping 𝐹 ( 𝑥 , 𝑡 ) is a homotopy between the continuous mappings 𝑓 , 𝑔 : 𝑋 → 𝑌 , then 𝜙 ( 𝐹 ( 𝑥 , 𝑡 ) ) is a homotopy between the mappings 𝜙 ( 𝑓 ) , 𝜙 ( 𝑔 ) : 𝜙 ( 𝑋 ) → 𝜙 ( 𝑌 ) .
concepts↗concepts↗concepts↗content_tags↗A space X which is homotopy equivalent to a point is called contractible. A subset A of a space X is a retract of X if there exists a continuous mapping 𝑟 : 𝑋 → 𝐴 , called a retraction, such that 𝑟 | 𝐴 = 1 𝐴 [18].
A property P of topological spaces is called a homotopy property if it is preserved by all homotopy equivalences. More precisely, P is a homotopy property if and only if for an arbitrary homotopy equivalence 𝑓 : 𝑋 → 𝑌 , if X has P, then Y also has P [18].
3. Some Cardinal Properties of the Space of Permutation Degree
limitationsconcepts↗In this section, we study some (hereditary) cardinal invariants (the spread, extent, density, weak density, 𝜋 -weight) of the space SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) of permutation degree of the Hattori space ( 𝑅 , 𝜏 ( 𝐴 ) ) . Let us observe that the space SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) has a Sorgenfrey-type topology.
We begin with the following two lemmas.
phenomenonconcepts↗content_tagsThe space SP 2 ( 𝑅 , 𝜏 𝑆 ) contains a closed discrete subset of cardinality 𝑐 .
Note that the subset 𝑌 = { ( 𝑥 , 𝑦 ) ∈ ( 𝑅 , 𝜏 𝑆 ) 2 : 𝑥 ≥ 𝑦 } of ( 𝑅 , 𝜏 𝑆 ) 2 is homeomorphic to the space SP 2 ( 𝑅 , 𝜏 𝑆 ) , and the set 𝑍 = { ( 𝑥 , 𝑦 ) ∈ 𝑌 : 𝑦 = − 𝑥 , 𝑥 > 0 } is closed and discrete in Y and has cardinality 𝑐 . □
Let Y be a subset of a Hausdorff topological space X and 𝑍 = { 𝐹 ∈ SP 2 𝑋 : 𝐹 ⊂ 𝑌 } ⊂ SP 2 𝑋 . Then:
(i) The space SP 2 𝑌 is homeomorphic to the subspace Z of the space SP 2 𝑋 ;
(ii) The set Z is open in SP 2 𝑋 whenever Y is open in X;
(iii) The set Z is closed in SP 2 𝑋 whenever Y is closed in X;
(iv) The set Z is clopen in SP 2 𝑋 whenever Y is clopen in X.
(i) It is known [17] and easy to check that the space exp 2 𝑌 is homeomorphic to the subspace Z of the space exp 2 𝑋 . In [17], it is shown that the space SP 2 𝑌 is homeomorphic to the space exp 2 𝑌 . Hence, we have that the space SP 2 𝑌 is homeomorphic to 𝑍 ⊂ SP 2 𝑋 .
(ii), (iii), and (iv) follow from the fact that the set Z is open (closed, clopen) in exp 2 𝑋 whenever Y is open (closed, clopen) in X [17] and the mentioned result that SP 2 𝑋 is homeomorphic to exp 2 𝑋 . □
From Lemmas 1 and 2 we have:
Let A be a subset of 𝑅 and 𝐵 ⊆ 𝑅 \ 𝐴 . If B is a (closed) subset of ( 𝑅 , 𝜏 ( 𝐴 ) ) which is homeomorphic to the space ( 𝑅 , 𝜏 𝑆 ) , then the space SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) contains a (closed) discrete subset of cardinality 𝑐 .
In [16], it was shown that if 𝐴 ⊆ 𝑅 and 𝐵 ⊆ 𝑅 \ 𝐴 , then 𝜏 ( 𝐴 ) | 𝐵 = 𝜏 𝑆 | 𝐵 . Recall that the set ( 𝑅 , 𝜏 ( 𝐴 ) ) is a closed subset of SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) . Hence, each (closed) discrete subset M of ( 𝑅 , 𝜏 ( 𝐴 ) ) with cardinality 𝑐 (which exists by Lemma 1) is a (closed) discrete subset of SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) with cardinality 𝑐 . □
Let A be a subset of 𝑅 , and Y be a subspace of ( 𝑅 , 𝜏 ( 𝐴 ) ) . Then, | 𝜏 ( 𝐴 ) 𝑌 | ≤ 𝑐 . In addition, | SP 2 𝑌 | ≤ 𝑐 .
It is enough to show that | 𝜏 ( 𝐴 ) | ≤ 𝑐 . Let 𝐵 be a base for ( 𝑅 , 𝜏 ( 𝐴 ) ) of cardinality ≤ 𝑐 . Since the space ( 𝑅 , 𝜏 ( 𝐴 ) ) is hereditary Lindelöf, each open subset of ( 𝑅 , 𝜏 ( 𝐴 ) ) is an union of countably many elements of 𝐵 . Hence, | 𝜏 ( 𝐴 ) | ≤ 𝑐 𝜔 = ( 2 𝜔 ) 𝜔 = 2 𝜔 = 𝑐 . □
content_tagsLet A be a subset of 𝑅 and 𝐵 ⊆ 𝑅 \ 𝐴 . If B is a closed subset of ( 𝑅 , 𝜏 ( 𝐴 ) ) which is homeomorphic to the space ( 𝑅 , 𝜏 𝑆 ) , then 𝑠 ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) = 𝑒 ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) = 𝑐 .
By Proposition 1 we have 𝑒 ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) ≥ 𝑐 . Note that 𝑒 ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) ≤ 𝑠 ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) ≤ | ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) | .
It follows from Proposition 2 that | ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) | ≤ | 𝑒 𝑥 𝑝 ( 𝑅 , 𝜏 ( 𝐴 ) ) | ≤ 𝑐 .
Thus, we get 𝑠 ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) = 𝑒 ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) = 𝑐 .
□
Let A be a subset of 𝑅 and 𝐵 ⊆ 𝑅 \ 𝐴 . If B is a subset of ( 𝑅 , 𝜏 ( 𝐴 ) ) which is homeomorphic to the space ( 𝑅 , 𝜏 𝑆 ) , then 𝑠 ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) = 𝑐 .
By Proposition 1 we have 𝑠 ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) ≥ 𝑐 . Note that 𝑠 ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) ≤ | ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) | . It follows from Proposition 2 that | ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) | ≤ 𝑐 . Thus, we get 𝑠 ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) = 𝑐 . □
Let A be a subset of 𝑅 and 𝜑 ∈ { 𝑑 , 𝑒 , 𝑐 } (resp. 𝜑 ∈ { 𝑤 𝑑 , 𝑙 , 𝜋 𝑤 } ). If B is a subset of ( 𝑅 , 𝜏 ( 𝐴 ) ) which is homeomorphic to the space ( 𝑅 , 𝜏 𝑆 ) , then ℎ 𝜑 ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) = 𝑐 (viz., ℎ 𝜑 ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) ≥ 𝑐 ).
In fact, note that ℎ 𝑒 = 𝑠 and ℎ 𝑐 = 𝑠 . Since ℎ 𝑐 ( SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) ≥ 𝑐 (by Proposition 1), we have the equalities. The inequalities also trivially follow from Proposition 1. □
Let 𝐴 ⊆ 𝑅 . Note that the family 𝐵 = { ( 𝑟 1 , 𝑟 2 ) : 𝑟 1 , 𝑟 2 ∈ 𝑄 , 𝑟 1 < 𝑟 2 } is a π-base for the space ( 𝑅 , 𝜏 ( 𝐴 ) ) and 𝐵 𝑛 = { ∏ 𝑖 = 1 𝑛 𝐵 𝑖 : 𝐵 𝑖 ∈ 𝐵 } is also a π-base for the space ( 𝑅 , 𝜏 ( 𝐴 ) ) 𝑛 . Hence, 𝜋 𝑤 ( 𝑅 , 𝜏 ( 𝐴 ) ) 𝑛 = 𝑐 ( 𝑅 , 𝜏 ( 𝐴 ) ) 𝑛 = 𝜔 . Similarly, the family SP 𝑛 𝐵 𝑛 = { SP 𝑛 𝐵 𝑖 = 𝛽 𝑛 𝑠 ( 𝐵 𝑖 ) : 𝐵 𝑖 ∈ 𝐵 𝑛 } is a π-base for the space SP 𝑛 ( 𝑅 , 𝜏 ( 𝐴 ) ) . This shows that 𝜋 𝑤 ( SP 𝑛 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) = 𝑐 ( SP 𝑛 ( 𝑅 , 𝜏 ( 𝐴 ) ) ) = 𝜔 .
4. Some Geometric Properties of the Space of Permutation Degree
method↗content_tagsNow we study some geometric properties of the space SP 𝐺 𝑛 𝑋 of permutation degree. In particular, we show the functor SP 𝐺 𝑛 preserves the homotopy and the retraction of topological spaces. In fact, we prove that if spaces X and Y are homotopically equivalent, then the spaces SP 𝐺 𝑛 𝑋 and SP 𝐺 𝑛 𝑌 are also homotopically equivalent, and conclude that the functor SP 𝐺 𝑛 is a covariant homotopy functor.
If the mappings 𝑓 , 𝑔 : 𝑋 → 𝑌 are homotopic, then the mappings SP 𝐺 𝑛 𝑓 , SP 𝐺 𝑛 𝑔 : SP 𝐺 𝑛 𝑋 → SP 𝐺 𝑛 𝑌 are also homotopic.
Assume that the mappings 𝑓 , 𝑔 : 𝑋 → 𝑌 are homotopic. Then there exists a continuous mapping 𝐹 : 𝑋 × 𝐼 → 𝑌 such that 𝐹 ( 𝑥 , 0 ) = 𝑓 ( 𝑥 ) and 𝐹 ( 𝑥 , 1 ) = 𝑔 ( 𝑥 ) . On the other hand, we have SP 𝐺 𝑛 𝑓 [ ( 𝑥 1 , 𝑥 2 , … , 𝑥 𝑛 ) ] 𝐺 = [ ( 𝑓 ( 𝑥 1 ) , 𝑓 ( 𝑥 2 ) , … , 𝑓 ( 𝑥 𝑛 ) ) ] 𝐺 , SP 𝐺 𝑛 𝑔 [ ( 𝑥 1 , 𝑥 2 , … , 𝑥 𝑛 ) ] 𝐺 = [ ( 𝑔 ( 𝑥 1 ) , 𝑔 ( 𝑥 2 ) , … , 𝑔 ( 𝑥 𝑛 ) ) ] 𝐺 .
Now we define the mapping SP 𝐺 𝑛 𝐹 ( [ ( 𝑥 1 , 𝑥 2 , … , 𝑥 𝑛 ) ] 𝐺 , 𝑡 ) = [ ( 𝐹 ( 𝑥 1 , 𝑡 ) , 𝐹 ( 𝑥 2 , 𝑡 ) , … , 𝐹 ( 𝑥 𝑛 , 𝑡 ) ) ] 𝐺 .
mechanisms↗It is clear that since the mapping F is continuous, the mapping SP 𝐺 𝑛 𝐹 is also continuous. Now we will show that the mapping SP 𝐺 𝑛 𝐹 is a homotopy between the mappings SP 𝐺 𝑛 𝑓 and SP 𝐺 𝑛 𝑔 . Indeed, SP 𝐺 𝑛 𝐹 ( [ ( 𝑥 1 , 𝑥 2 , … , 𝑥 𝑛 ) ] 𝐺 , 0 ) = [ ( 𝐹 ( 𝑥 1 , 0 ) , 𝐹 ( 𝑥 2 , 0 ) , … , 𝐹 ( 𝑥 𝑛 , 0 ) ) ] 𝐺 = [ ( 𝑓 ( 𝑥 1 ) , 𝑓 ( 𝑥 2 ) , … , 𝑓 ( 𝑥 𝑛 ) ) ] 𝐺 = SP 𝐺 𝑛 𝑓 [ ( 𝑥 1 , 𝑥 2 , … , 𝑥 𝑛 ) ] 𝐺 ; and SP 𝐺 𝑛 𝐹 ( [ ( 𝑥 1 , 𝑥 2 , … , 𝑥 𝑛 ) ] 𝐺 , 1 ) = [ ( 𝐹 ( 𝑥 1 , 1 ) , 𝐹 ( 𝑥 2 , 1 ) , … , 𝐹 ( 𝑥 𝑛 , 1 ) ) ] 𝐺 = [ ( 𝑔 ( 𝑥 1 ) , 𝑔 ( 𝑥 2 ) , … , 𝑔 ( 𝑥 𝑛 ) ) ] 𝐺 = SP 𝐺 𝑛 𝑔 [ ( 𝑥 1 , 𝑥 2 , … , 𝑥 𝑛 ) ] 𝐺 .
This means that SP 𝐺 𝑛 𝑓 ≃ SP 𝐺 𝑛 𝑔 . □
content_tagsIf the spaces X and Y are homotopically equivalent, then the spaces SP 𝐺 𝑛 𝑋 and SP 𝐺 𝑛 𝑌 are also homotopically equivalent.
Suppose that the spaces X and Y are homotopically equivalent. Then there exist two continuous mappings 𝑓 : 𝑋 → 𝑌 and 𝑔 : 𝑌 → 𝑋 such that 𝑓 ∘ 𝑔 ≃ 𝑖 𝑑 𝑌 and 𝑔 ∘ 𝑓 ≃ 𝑖 𝑑 𝑋 . This means that there are two homotopy 𝐹 ( 𝑦 , 𝑡 ) and 𝐻 ( 𝑥 , 𝑡 ) such that 𝐹 ( 𝑦 , 0 ) = ( 𝑓 ∘ 𝑔 ) ( 𝑦 ) , 𝐹 ( 𝑦 , 1 ) = 𝑦 𝑎 𝑛 𝑑 ( 𝑥 , 0 ) = ( 𝑔 ∘ 𝑓 ) ( 𝑥 ) , 𝐻 ( 𝑥 , 1 ) = 𝑥 .
Consider the compositions SP 𝐺 𝑛 𝑓 ∘ SP 𝐺 𝑛 𝑔 : SP 𝐺 𝑛 𝑌 → SP 𝐺 𝑛 𝑌 and SP 𝐺 𝑛 𝑔 ∘ SP 𝐺 𝑛 𝑓 : SP 𝐺 𝑛 𝑋 → SP 𝐺 𝑛 𝑋 of the mappings SP 𝐺 𝑛 𝑓 : SP 𝐺 𝑛 𝑋 → SP 𝐺 𝑛 𝑌 and SP 𝐺 𝑛 𝑔 : SP 𝐺 𝑛 𝑌 → SP 𝐺 𝑛 𝑋 defined by 𝑆 𝑃 𝐺 𝑛 𝑓 ∘ 𝑆 𝑃 𝐺 𝑛 𝑔 ( 𝑦 1 , 𝑦 2 , … , 𝑦 𝑛 ) 𝐺 = ( 𝑓 ∘ 𝑔 ) ( 𝑦 1 ) , ( 𝑓 ∘ 𝑔 ) ( 𝑦 2 ) , … , ( 𝑓 ∘ 𝑔 ) ( 𝑦 𝑛 ) 𝐺 and 𝑆 𝑃 𝐺 𝑛 𝑔 ∘ 𝑆 𝑃 𝐺 𝑛 𝑓 ( 𝑥 1 , 𝑥 2 , … , 𝑥 𝑛 ) 𝐺 = ( 𝑓 ∘ 𝑔 ) ( 𝑥 1 ) , ( 𝑓 ∘ 𝑔 ) ( 𝑥 2 ) , … , ( 𝑓 ∘ 𝑔 ) ( 𝑥 𝑛 ) 𝐺 .
One can easily check that the mapping 𝑆 𝑃 𝐺 𝑛 𝐹 [ ( 𝑦 1 , 𝑦 2 , … , 𝑦 𝑛 ) ] 𝐺 , 𝑡 = ( 𝐹 ( 𝑦 1 , 𝑡 ) , 𝐹 ( 𝑦 2 , 𝑡 ) , … , 𝐹 ( 𝑦 𝑛 , 𝑡 ) ) 𝐺 is a homotopy between 𝑆 𝑃 𝐺 𝑛 𝑓 ∘ 𝑆 𝑃 𝐺 𝑛 𝑔 and 𝑖 𝑑 𝑆 𝑃 𝐺 𝑛 𝑌 .
Similarly, 𝑆 𝑃 𝐺 𝑛 𝐻 [ ( 𝑥 1 , 𝑥 2 , … , 𝑥 𝑛 ) ] 𝐺 , 𝑡 = ( 𝐻 ( 𝑥 1 , 𝑡 ) , 𝐻 ( 𝑥 2 , 𝑡 ) , … , 𝐻 ( 𝑥 𝑛 , 𝑡 ) ) 𝐺 is a homotopy between 𝑆 𝑃 𝐺 𝑛 𝑔 ∘ 𝑆 𝑃 𝐺 𝑛 𝑓 and 𝑖 𝑑 𝑆 𝑃 𝐺 𝑛 𝑋 .
By Theorem 1, 𝑆 𝑃 𝐺 𝑛 𝑋 and 𝑆 𝑃 𝐺 𝑛 𝑌 are homotopically equivalent. □
If a set A is a retract of the topological space X, then the set SP 𝐺 𝑛 𝐴 is a retract of the topological space SP 𝐺 𝑛 𝑋 .
Suppose that a set A is a retract of X. Then there exists a continuous mapping 𝑟 : 𝑋 → 𝐴 such that 𝑟 ( 𝑎 ) = 𝑎 for all 𝑎 ∈ 𝐴 . Now we consider the mapping SP 𝐺 𝑛 𝑟 : SP 𝐺 𝑛 𝑋 → SP 𝐺 𝑛 𝐴 . For every [ ( 𝑎 1 , 𝑎 2 , … , 𝑎 𝑛 ) ] 𝐺 ∈ SP 𝐺 𝑛 𝐴 we have SP 𝐺 𝑛 𝑟 ( [ ( 𝑎 1 , 𝑎 2 , … , 𝑎 𝑛 ) ] 𝐺 ) = [ ( 𝑟 ( 𝑎 1 ) , 𝑟 ( 𝑎 2 ) , … , 𝑟 ( 𝑎 𝑛 ) ) ] 𝐺 = [ ( 𝑎 1 , 𝑎 2 , … , 𝑎 𝑛 ) ] 𝐺 .
This means that the mapping SP 𝐺 𝑛 𝑟 : SP 𝐺 𝑛 𝑋 → SP 𝐺 𝑛 𝐴 is a retraction. Hence, the set SP 𝐺 𝑛 𝐴 is a retract of the space SP 𝐺 𝑛 𝑋 . □
content_tagsThe functor 𝑆 𝑃 𝐺 𝑛 is a covariant homotopy functor.
Now we will show that the functor SP 𝐺 𝑛 satisfies the above three conditions.
(i) Let 𝑖 𝑑 𝑋 be identity mapping in the topological space X. Then we have that SP 𝐺 𝑛 𝑖 𝑑 𝑋 [ ( 𝑥 1 , 𝑥 2 , … , 𝑥 𝑛 ) ] 𝐺 = [ ( 𝑖 𝑑 𝑋 ( 𝑥 1 ) , 𝑖 𝑑 𝑋 ( 𝑥 2 ) , … , 𝑖 𝑑 𝑋 ( 𝑥 𝑛 ) ) ] 𝐺 = [ ( 𝑥 1 , 𝑥 2 , … , 𝑥 𝑛 ) ] . This means that the mapping SP 𝐺 𝑛 𝑖 𝑑 𝑋 is the identity mapping in the topological space SP 𝐺 𝑛 𝑋 .
(ii) Let 𝑓 : 𝑋 → 𝑌 , 𝑔 : 𝑌 → 𝑍 be continuous mappings. Then it follows that SP 𝐺 𝑛 ( 𝑔 ∘ 𝑓 ) [ ( 𝑥 1 , 𝑥 2 , … , 𝑥 𝑛 ) ] 𝐺 = [ ( ( 𝑔 ∘ 𝑓 ) ( 𝑥 1 ) , ( 𝑔 ∘ 𝑓 ) ( 𝑥 2 ) , … , ( 𝑔 ∘ 𝑓 ) ( 𝑥 𝑛 ) ) ] 𝐺 = SP 𝐺 𝑛 𝑔 [ ( 𝑓 ( 𝑥 1 ) , 𝑓 ( 𝑥 2 ) , … , 𝑓 ( 𝑥 𝑛 ) ) ] 𝐺 = SP 𝐺 𝑛 𝑔 ∘ SP 𝐺 𝑛 𝑓 .
(iii) It follows easily from Theorem 1. □
claim_relationsIn [14], some propositions about homotopy properties of the topological spaces were given. For instance, it was proved that contractibility, connectedness, and pathwise connectedness are homotopy property of the spaces.
If a topological space X is contractible, then the space SP 𝐺 𝑛 𝑋 is also contractible.
If a topological space X is connected (viz., pathwise connected), then the space SP 𝐺 𝑛 𝑋 is also connected (viz., pathwise connected).
If f and g are two paths in X with 𝑓 ( 1 ) = 𝑔 ( 0 ) , then by the product of f and g we mean the path 𝑓 ∗ 𝑔 , which is defined by ( 𝑓 ∗ 𝑔 ) ( 𝑡 ) = 𝑓 ( 2 𝑡 ) , if 0 ≤ 𝑡 ≤ 1 / 2 , 𝑔 ( 2 𝑡 − 1 ) , if 1 / 2 ≤ 𝑡 ≤ 1 .
Let 𝑓 ≃ 𝑔 and 𝑔 ≃ ℎ , where F is a homotopy from f to g and G is a homotopy from g to h. Then, 𝑓 ≃ ℎ . Define a homotopy 𝐻 : 𝑋 × 𝐼 → 𝑌 between f and h as follows: 𝐻 ( 𝑥 , 𝑡 ) = 𝐹 ( 𝑥 , 2 𝑡 ) , if 0 ≤ 𝑡 ≤ 1 / 2 , 𝐺 ( 𝑥 , 2 𝑡 − 1 ) , if 1 / 2 ≤ 𝑡 ≤ 1 .
If the mappings 𝑓 𝑖 : 𝐼 → 𝑋 are paths in X from the points 𝑥 0 𝑖 to the points 𝑥 1 𝑖 , 𝑖 = 1 , 2 , … , 𝑛 , respectively, then the mapping SP 𝐺 𝑛 𝑓 𝑛 : 𝐼 → SP 𝐺 𝑛 𝑋 defined by SP 𝐺 𝑛 𝑓 𝑛 ( 𝑡 ) = [ ( 𝑓 1 ( 𝑡 ) , 𝑓 2 ( 𝑡 ) , … , 𝑓 𝑛 ( 𝑡 ) ) ] 𝐺 is also a path from the point [ ( 𝑥 0 1 , 𝑥 0 2 , … , 𝑥 0 𝑛 ) ] 𝐺 to the point [ ( 𝑥 1 1 , 𝑥 1 2 , … , 𝑥 1 𝑛 ) ] 𝐺 in SP 𝐺 𝑛 𝑋 .
Let SP 𝐺 𝑛 𝑓 𝑛 and SP 𝐺 𝑛 𝑔 𝑛 be paths from [ ( 𝑥 0 1 , 𝑥 0 2 , … , 𝑥 0 𝑛 ) ] 𝐺 to [ ( 𝑥 1 1 , 𝑥 1 2 , … , 𝑥 1 𝑛 ) ] 𝐺 and from [ ( 𝑥 2 1 , 𝑥 2 2 , … , 𝑥 2 𝑛 ) ] 𝐺 to [ ( 𝑥 3 1 , 𝑥 3 2 , … , 𝑥 3 𝑛 ) ] 𝐺 , respectively, with SP 𝐺 𝑛 𝑓 𝑛 ( 1 ) = SP 𝐺 𝑛 𝑔 𝑛 ( 0 ) . Then we define the multiplication of the paths in SP 𝐺 𝑛 𝑋 as follows: ( SP 𝐺 𝑛 𝑓 𝑛 ∗ SP 𝐺 𝑛 𝑔 𝑛 ) ( 𝑡 ) = [ ( ( 𝑓 1 ∗ 𝑔 1 ) ( 𝑡 ) , ( 𝑓 2 ∗ 𝑔 2 ) ( 𝑡 ) , … , ( 𝑓 𝑛 ∗ 𝑔 𝑛 ) ( 𝑡 ) ) ] 𝐺 .
This path sharing the points [ ( 𝑥 0 1 , 𝑥 0 2 , … , 𝑥 0 𝑛 ) ] 𝐺 and [ ( 𝑥 3 1 , 𝑥 3 2 , … , 𝑥 3 𝑛 ) ] 𝐺 .
In [18], it is shown that the multiplication of equivalence classes of paths is associative; in other words, ( [ 𝑓 ] [ 𝑔 ] ) [ ℎ ] = [ 𝑓 ] ( [ 𝑔 ] [ ℎ ] ) .
Let f and g be paths from the initial point 𝑥 0 to the final point 𝑥 1 . If there is a homotopy F from f to g such that for each 𝑡 ∈ 𝐼 , 𝐹 ( 0 , 𝑡 ) = 𝑥 0 and 𝐹 ( 1 , 𝑡 ) = 𝑥 1 , then f and g are said to be path-homotopic [18]. For a path f, [ 𝑓 ] denotes the equivalence class of all paths path-homotopic to f.
The operation ∗ defined above can be applied to homotopy classes as well. Let 𝑓 : 𝐼 → 𝑋 be a path from 𝑥 0 to 𝑥 1 and 𝑔 : 𝐼 → 𝑋 a path from 𝑥 1 to 𝑥 2 . Then, one defines [ 𝑓 ] ∗ [ 𝑔 ] = [ 𝑓 ∗ 𝑔 ] .
concepts↗Let X be a space and 𝑥 0 𝑖 𝑛 𝑋 . A path in X beginning and ending at 𝑥 0 is called a loop [18] based at 𝑥 0 . Denote by 𝜋 1 ( 𝑋 , 𝑥 0 ) the set of all equivalence classes [ 𝑓 ] of loops in X based at 𝑥 0 . 𝜋 1 ( 𝑋 , 𝑥 0 ) with the operation ∗ is a group, where the identity element of the group is [ 𝑒 𝑥 ] and the inverse element of [ 𝑓 ] is [ 𝑓 ¯ ( 𝑡 ) ] = [ 𝑓 ( 1 − 𝑡 ) ] . We call 𝜋 1 ( 𝑋 , 𝑥 0 ) the fundamental group [18].
Suppose ( 𝐺 , ∗ ) and ( 𝐺 1 , ∗ 1 ) are groups. A homomorphism is a mapping such that 𝑓 ( 𝑥 ∗ 𝑦 ) = 𝑓 ( 𝑥 ) ∗ 1 𝑓 ( 𝑦 ) for all 𝑥 , 𝑦 ∈ 𝐺 . A homomorphism f is called an isomorphism if it is bijective.
The fundamental groups of a space and its quotient space are not always isomorphic.
Let 𝑋 = [ 0 , 1 ] and let 𝑆 1 be the unit circle. Clearly, 𝑆 1 is the quotient space of the space X, where the quotient mapping is defined as 𝑞 ( 𝑥 ) = ( 𝑐 𝑜 𝑠 ( 2 𝜋 𝑥 ) ; 𝑠 𝑖 𝑛 ( 2 𝜋 𝑥 ) ) . We know that 𝑋 = [ 0 , 1 ] has the trivial fundamental group (the group consisting of the identity), and the fundamental group of 𝑆 1 is isomorphic to the group ( 𝑍 , + ) .
content_tagsThe fundamental groups 𝜋 1 ( 𝑋 , 𝑥 0 ) and 𝜋 1 ( 𝑆 𝑃 𝐺 𝑛 𝑋 , [ 𝑥 0 ] 𝐺 ) of the topological spaces X and 𝑆 𝑃 𝐺 𝑛 𝑋 are not always isomorphic for every 𝑥 0 ∈ 𝑋 , where [ 𝑥 0 ] 𝐺 = [ ( 𝑥 0 , 𝑥 0 , … , 𝑥 0 ) ] 𝐺 = ( 𝑥 0 , 𝑥 0 , … , 𝑥 0 ) .
Let ℎ : 𝑋 → 𝑌 be a continuous mapping sending the point 𝑥 0 ∈ 𝑋 to the point 𝑦 0 ∈ 𝑌 ; We denote this fact by ℎ : ( 𝑋 , 𝑥 0 ) → ( 𝑌 , 𝑦 0 ) . If f is a loop in X based at 𝑥 0 , then the composition ℎ ∘ 𝑓 : 𝐼 → 𝑌 is a loop in Y based at 𝑦 0 . In this way the correspondence 𝑓 → ℎ ∘ 𝑓 gives rise to a mapping from 𝜋 1 ( 𝑋 , 𝑥 0 ) to 𝜋 1 ( 𝑌 , 𝑦 0 ) . Define ℎ ∗ : 𝜋 1 ( 𝑋 , 𝑥 0 ) → 𝜋 1 ( 𝑌 , 𝑦 0 ) by ℎ ∗ ( [ 𝑓 ] ) = [ ℎ ∘ 𝑓 ] . In [19] it is proved that ℎ ∗ is a homomorphism.
The fundamental groups 𝜋 1 ( 𝑋 , 𝑥 0 ) and 𝜋 1 ( SP 𝐺 𝑛 𝑋 , [ 𝑥 0 ] ) of the topological spaces X and SP 𝐺 𝑛 𝑋 are homomorphic for every 𝑥 0 ∈ 𝑋 , and the homomorphism is defined by ( 𝜋 𝑛 , 𝐺 𝑠 ) ∗ [ 𝑓 ] = [ 𝜋 𝑛 , 𝐺 𝑠 ∘ 𝑓 ] .
We know that 𝜋 𝑛 , 𝐺 𝑠 : 𝑋 𝑛 → 𝑆 𝑃 𝐺 𝑛 𝑋 is a continuous mapping that carries the point ( 𝑥 0 , 𝑥 0 , … , 𝑥 0 ) of 𝑋 𝑛 to the point [ ( 𝑥 0 , 𝑥 0 , … , 𝑥 0 ) ] of 𝑆 𝑃 𝐺 𝑛 𝑋 . If f is a loop in X based at 𝑥 0 , then 𝑓 𝑛 is a loop in 𝑋 𝑛 based at ( 𝑥 0 , 𝑥 0 , … , 𝑥 0 ) and the composition 𝜋 𝑛 , 𝐺 𝑠 ∘ 𝑓 𝑛 : 𝐼 → 𝑆 𝑃 𝐺 𝑛 𝑋 is a loop in 𝑆 𝑃 𝐺 𝑛 𝑋 based at [ ( 𝑥 0 , 𝑥 0 , … , 𝑥 0 ) ] . The correspondence 𝑓 → 𝜋 𝑛 , 𝐺 𝑠 ∘ 𝑓 𝑛 thus gives rise to a mapping carrying 𝜋 1 ( 𝑋 , 𝑥 0 ) into 𝜋 1 ( 𝑆 𝑃 𝐺 𝑛 𝑋 , [ ( 𝑥 0 , 𝑥 0 , … , 𝑥 0 ) ] ) . Define 𝜋 𝑛 , 𝐺 𝑠 ∗ : 𝜋 1 ( 𝑋 , 𝑥 0 ) → 𝜋 1 ( 𝑆 𝑃 𝐺 𝑛 𝑋 , [ ( 𝑥 0 , 𝑥 0 , … , 𝑥 0 ) ] ) by the equation 𝜋 𝑛 , 𝐺 𝑠 ∗ ( [ 𝑓 ] ) = [ 𝜋 𝑛 , 𝐺 𝑠 ∘ 𝑓 𝑛 ] . The mapping 𝜋 𝑛 , 𝐺 𝑠 ∗ is a homomorphism (as we said before). □
5. Conclusions
In this article we continue the study of the functor of permutation degree—one of important functors in topology. Our results extend and complement the existing results in this field. We obtained several relations among cardinal invariants in the space SP 2 ( 𝑅 , 𝜏 ( 𝐴 ) ) of the permutation degree of the Hattori space ( 𝑅 , 𝜏 ( 𝐴 ) ) . These cardinal invariants include the hereditary density, hereditary weak density, spread, extent, 𝜋 -weight, and (hereditary) Lindelöf number. Additionally, we proved that if the spaces X and Y are homotopically equivalent, then the spaces SP 𝐺 𝑛 𝑋 and SP 𝐺 𝑛 𝑌 are homotopically equivalent too. As a consequence, one obtains that the functor SP 𝐺 𝑛 is a covariant homotopy functor. It preserves a few topological properties, including retracts.
We believe that our results can be applied to similar investigation of other topological properties and other functors.
Relations on this page carry a quotation for the claim. Where a relation was separately evidenced — and so tested against the vocabulary for its type — the row says so; 2 of 2 relations here carry that mark. The rest are the tagger’s reading of the sentence quoted beside them.
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| facet | tag | field | conf. | whose words |
|---|---|---|---|---|
| object_of_study | hereditary cardinal invariants and geometric properties of the space of permutation degree of a topological space, in particular of the Hattori space | object_of_study | the paper's words | |
| research_question | how cardinal invariants such as spread, extent, density and Lindelof number behave in the permutation-degree space, and whether the permutation-degree functor preserves homotopy and retraction | research_question | the paper's words | |
| contribution | relations among cardinal invariants in the permutation-degree space of the Hattori space, and a proof that the functor of G-permutation degree preserves homotopy and retraction and is a covariant homotopy functor | contribution | the paper's words | |
| claim_type | existence | claim_type | the paper's words | |
| phenomenon | the space of permutation degree of the Hattori space carries a Sorgenfrey-type topology and contains a closed discrete subset of the cardinality of the continuum | phenomenon | the paper's words | |
| method | mathematical-proof also in: mathematical-proof | method | the paper's words | |
| mechanisms | The functor of G-permutation degree sends a map f to the map carrying an orbit of an n-tuple to the orbit of the tuple of images, lifting maps to the permutation-degree space. also in: functor lifts maps | mechanisms[0] | 0.96 | the tagger's words |
| mechanisms | A homotopy between two maps lifts to a homotopy between their images under the functor, applying the homotopy coordinatewise on representatives. also in: homotopy lifts through the functor | mechanisms[1] | 0.95 | the tagger's words |
| mechanisms | The hereditary version of a cardinal function is the supremum of that function over all subspaces. also in: hereditary invariant by subspaces | mechanisms[2] | 0.96 | the tagger's words |
| limitations | All spaces are assumed to be completely regular, so the results are stated under that standing separation assumption. | limitations[0] | 0.95 | the tagger's words |
| limitations | The cardinal-invariant equalities are established for the specific second permutation-degree space of the Hattori space, not for arbitrary spaces. | limitations[1] | 0.94 | the tagger's words |
| concepts | cardinal invariant also in: cardinal invariant | concepts[0] | 0.97 | the paper's words |
| concepts | cardinal function also in: cardinal function | concepts[1] | 0.96 | the paper's words |
| concepts | hereditary cardinal invariant also in: hereditary cardinal invariant | concepts[2] | 0.96 | the paper's words |
| concepts | density also in: density | concepts[3] | 0.96 | the paper's words |
| concepts | weak density also in: weak density | concepts[4] | 0.95 | the paper's words |
| concepts | extent also in: extent | concepts[5] | 0.96 | the paper's words |
| concepts | spread also in: spread | concepts[6] | 0.95 | the paper's words |
| concepts | cellularity also in: cellularity | concepts[7] | 0.95 | the paper's words |
| concepts | Lindelöf number also in: Lindelöf number | concepts[8] | 0.95 | the paper's words |
| concepts | permutation degree also in: permutation degree | concepts[9] | 0.96 | the paper's words |
| concepts | symmetric equivalence relation also in: symmetric equivalence relation | concepts[10] | 0.96 | the paper's words |
| concepts | G-permutation degree also in: G-permutation degree | concepts[11] | 0.96 | the paper's words |
| concepts | covariant functor also in: covariant functor | concepts[12] | 0.96 | the paper's words |
| concepts | homotopy also in: homotopy | concepts[13] | 0.96 | the paper's words |
| concepts | homotopy equivalence also in: homotopy equivalence | concepts[14] | 0.96 | the paper's words |
| concepts | homotopically equivalent also in: homotopically equivalent | concepts[15] | 0.95 | the paper's words |
| concepts | covariant homotopy functor also in: covariant homotopy functor | concepts[16] | 0.96 | the paper's words |
| concepts | retract also in: retract | concepts[17] | 0.95 | the paper's words |
| concepts | retraction also in: retraction | concepts[18] | 0.95 | the paper's words |
| concepts | contractible also in: contractible | concepts[19] | 0.95 | the paper's words |
| concepts | fundamental group also in: fundamental group | concepts[20] | 0.95 | the paper's words |
| concepts | Hattori space also in: Hattori space | concepts[21] | 0.95 | the paper's words |
| concepts | Sorgenfrey topology also in: Sorgenfrey topology | concepts[22] | 0.95 | the paper's words |
| concepts | closed discrete subset also in: closed discrete subset | concepts[23] | 0.95 | the paper's words |
| unhoused_relations | homotopy equivalence → cardinal invariant A cardinal invariant is a homeomorphism invariant, not in general a homotopy invariant, so homotopy-equivalent spaces need not share it. — the relation stated: Cardinal invariants agree on homeomorphic spaces, a stricter relation than homotopy equivalence. | unhoused_relations[0] | 0.93 | the tagger's words |
| senses | cardinal invariant - topology sense: a cardinal-valued function invariant under homeomorphism, describing a space | senses[0] | 0.95 | the tagger's words |
| senses | extent versus spread - cardinal-function senses: closed-discrete versus discrete subspace cardinalities | senses[1] | 0.94 | the tagger's words |
| senses | functor - covariant homotopy sense: preserves identities, compositions and homotopies | senses[2] | 0.95 | the tagger's words |
| senses | equivalent - homeomorphic versus homotopically equivalent sense: two strengths of topological sameness | senses[3] | 0.94 | the tagger's words |
| claim_relations | extends → existing results on cardinal invariants under covariant functors note: The paper states it complements and extends existing results in cardinal invariants and the theory of covariant functors. | claim_relations[0] | 0.95 | the tagger's words |
| claim_relations | builds-on → the theory of the symmetric-product / permutation-degree functor note: The functor of G-permutation degree and its basic properties are taken from prior work. | claim_relations[1] | 0.96 | the tagger's words |
| claim_relations | builds-on → prior results on homotopy properties of topological spaces note: Uses the earlier result that contractibility, connectedness and pathwise connectedness are homotopy properties. | claim_relations[2] | 0.95 | the tagger's words |
| concept_edges | G-permutation degree —is-a→ covariant functor the relation itself is separately evidenced · strength of the relation: 1 · note: Flat categorical membership; kind cue 'is a covariant functor'. · the relation stated: The functor of G-permutation degree is a covariant functor in the category of compact spaces.
Audit 2026-09-28: DOUBTFUL - read against its full span and kept, but the span calls the FUNCTOR SP_G^n covariant; 'G-permutation degree of X' names the quotient space, so is-a is loose. also in: G-permutation degree · covariant functor | concept_edges[0] | 0.96 | the tagger's words |
| concept_edges | Hattori space —distinct-from→ Sorgenfrey topology the relation itself is separately evidenced · strength of the relation: 0.85 · note: Differentiator stated by comparison; cue 'weaker than'. · the relation stated: The Hattori-space topology is finer than the Euclidean topology and weaker than the Sorgenfrey topology.
Audit 2026-09-28: DOUBTFUL - read against its full span and kept, but the span orders the topologies (finer than Euclidean, weaker than Sorgenfrey); distinct-from is loose. also in: Hattori space · Sorgenfrey topology | concept_edges[2] | 0.94 | the tagger's words |
| content_tags | The permutation-degree space of the Sorgenfrey line contains a closed discrete subset of the cardinality of the continuum. | content_tags.findings[0] | 0.96 | the tagger's words |
| content_tags | For the Hattori space, the spread and the extent of its second permutation-degree space both equal the cardinality of the continuum. | content_tags.findings[1] | 0.96 | the tagger's words |
| content_tags | The functor of G-permutation degree preserves the homotopy and the retraction of topological spaces. | content_tags.findings[2] | 0.96 | the tagger's words |
| content_tags | If two spaces are homotopically equivalent, then their images under the functor of G-permutation degree are homotopically equivalent. | content_tags.findings[3] | 0.96 | the tagger's words |
| content_tags | The functor of G-permutation degree is a covariant homotopy functor. | content_tags.findings[4] | 0.96 | the tagger's words |
| content_tags | The fundamental groups of a space and its permutation-degree space are not always isomorphic. | content_tags.findings[5] | 0.95 | the tagger's words |
| content_tags | a function from topological spaces to infinite cardinals that agrees on homeomorphic spaces also in: cardinal function | content_tags.definitions[0] | 0.96 | the tagger's words |
| content_tags | the supremum of a cardinal function over all subspaces of the space also in: hereditary cardinal invariant | content_tags.definitions[1] | 0.96 | the tagger's words |
| content_tags | the least cardinality of a dense subset of the space also in: density | content_tags.definitions[2] | 0.96 | the tagger's words |
| content_tags | the smallest cardinal for which there is a pi-base that is a union of that many centered systems of open sets also in: weak density | content_tags.definitions[3] | 0.95 | the tagger's words |
| content_tags | the supremum of the cardinalities of closed discrete subspaces of the space also in: extent | content_tags.definitions[4] | 0.96 | the tagger's words |
| content_tags | the smallest cardinal such that every open cover has a subcover of that cardinality also in: Lindelöf number | content_tags.definitions[5] | 0.95 | the tagger's words |
| content_tags | two n-tuples are equivalent when a permutation of coordinates carries one to the other also in: symmetric equivalence relation | content_tags.definitions[6] | 0.96 | the tagger's words |
| content_tags | the quotient of the n-th power of a space by the action of a subgroup of the symmetric group on coordinates also in: G-permutation degree | content_tags.definitions[7] | 0.96 | the tagger's words |
| content_tags | a map with a homotopy inverse, whose two composites are homotopic to the identities also in: homotopy equivalence | content_tags.definitions[8] | 0.96 | the tagger's words |
| content_tags | an operator on spaces and maps that preserves identities, compositions, and homotopies also in: covariant homotopy functor | content_tags.definitions[9] | 0.96 | the tagger's words |
| content_tags | a subset admitting a continuous map from the space that fixes the subset pointwise also in: retract | content_tags.definitions[10] | 0.95 | the tagger's words |
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