Equivalence between Linear Tangle and Maximal Single Ideal

The concept of linear tangle was introduced as an obstruction to mixed searching number. The concept of (maximal) single ideal has been introduced as an obstruction to linear-width. Moreover, it was already known that mixed search number is equivalent to linear-width. Hence, by combining those results, we obtain a proof of the equivalence between linear tangle and maximal single ideal. This short report gives an alternative proof of the equivalence.


Introduction
A graph searching game is a game where searchers (or cops) want to capture a fugitive (or robber) and the fugitive want to escape from the searchers, and they move through a graph for their purpose.Graph searching games have been well-studied [1] [2] [3], since graph searching games have many practical and theoretical applications such as robot motion planning, network security, and artificial intelligence (see e.g.

[4]).
There are several variants of graph searching games such as edge search, node search, and mixed search (see e.g.[5]) and there are several graph width-parameters such as path-width, tree-width, and branch-width.In the study of graph searching games, it is known that there are some strong connections between graph searching games and graph width parameters, in which the minimum number of searchers (i.e., search number) usually corresponds to the value of width.For example, mixed search number is essentially equivalent to linear-width 1 (see [6] [7]).Open Journal of Discrete Mathematics The concept of linear tangle was introduced in [7] as an obstruction to the existence of mixed searching strategy: there is a linear tangle of order 1 k + iff there is no mixed searching strategy with k searchers, where k is a prefixed integer.From the equivalence, as mentioned above, between mixed search number and linear-width, a linear tangle of order 1 k + is an obstruction to being linear-width is at most k (see also [8]).
The concept of (maximal) single ideal has been introduced in [9] as an obstruction to linear-width: there is a maximal single ideal of order 1 k + iff the linear-width is more than k.Thus, a linear tangle of a large order and a maximal single ideal of a large order are both obstructions to small linear-width, which means that the concepts of linear tangle and maximal single ideal are the same.
In this short report, we give an alternative proof of the equivalence between linear tangle and maximal single ideal.

Definitions and Notations
In this paper, we consider a pair ( ) , E f rather than graphs, where E is an underlying set and f is a symmetric submodular function on E, and such a pair is called connectivity system (see cf. [10]).All sets considered in this paper are finite.For an underlying set E and a subset X of E, we denote \ E X by X .
It is known that a symmetric submodular function f satisfies the following properties [10]: for each ∈ , hence we have ( )

Definition 1 ([7]
).A linear tangle of order 1 k + 2 on a connectivity system ( ) , E f is a family L of k-efficient subsets of E, satisfying the following axioms:

holds. Definition 2 ([9]
).A maximal single ideal of order 1 k + on a connectivity system ( ) , E f is a family M of k-efficient subsets of E, satisfying the following axioms: ⊂ , B ∈ M , and ( )

Result
Lemma 1.A linear tangle L of order 1 k + is a maximal single ideal of order Proof.From the axioms (L1) and (L2), it is obvious that L satisfies the axioms (S1) and (S2).
We claim that L satisfies the axioms (S3).Suppose to, the contrary, that there exist k-efficient subsets A and B such that A B ⊆ , B ∈L , and A ∉L .Then, we have A ∈ L by (L2), and for any e E ∈ ,

{ }
A e B E =   holds, but this contradicts the axiom (L3).
Finally, we show that L satisfies the axioms (S4).Suppose to, the contrary, that there exists k-efficient subset A ∈L and an element e E ∈ such that { } ( )

{ } { }
A A e e E =    hold, however, this contradicts the axiom (L3).Lemma 2. A maximal single ideal M of order 1 k + is a linear tangle of order Proof.From the axioms (S1) and (S2), it is obvious that M satisfies the axioms (L1) and (L2).
We show that M satisfies the axiom (L3).Suppose to, the contrary, that there exists a triple  M holds by the axiom (S4).However, this contradicts the axiom (S2).
From lemmas 1 and 2, we have the following theorem.

≤
Throughout the paper, f means a symmetric submodular function, k a fixed positive integer, and we assume that for every e E

Theorem 1 .≤ 1 k
Under the assumption that for every e E ∈ , F is a linear tangle of order + iff F is a maximal single ideal of order1 k + .
however, this contradicts the choice of the triple.Thus, we have shown that X Y = ∅  .