Chromatic Number of Graphs with Special Distance Sets-V ()

Venkataraman Yegnanarayanan, Angamuthu Parthiban

Department of Mathematics, Velammal Engineering College, Chennai, India.

**DOI: **10.4236/ojdm.2013.31001
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Department of Mathematics, Velammal Engineering College, Chennai, India.

An integer distance graph is a graph *G*(*Z*,*D*) with the set of integers as vertex set and an edge joining two vertices *u* and v if and only if ∣*u* - v∣*D *where *D* is a subset of the positive integers. It is known that *x*(*G*(*Z*,*D*) )=4 where *P* is a set of Prime numbers. So we can allocate the subsets *D* of *P* to four classes, accordingly as is 1 or 2 or 3 or 4. In this paper we have considered the open problem of characterizing class three and class four sets when the distance set *D *is not only a subset of primes *P* but also a special class of primes like Additive primes, Deletable primes, Wedderburn-Etherington Number primes, Euclid-Mullin sequence primes, Motzkin primes, Catalan primes, Schroder primes, Non-generous primes, Pell primes, Primeval primes, Primes of Binary Quadratic Form, Smarandache-Wellin primes, and Highly Cototient number primes. We also have indicated the membership of a number of special classes of prime numbers in class 2 category.

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Yegnanarayanan, V. and Parthiban, A. (2013) Chromatic Number of Graphs with Special Distance Sets-V. *Open Journal of Discrete Mathematics*, **3**, 1-6. doi: 10.4236/ojdm.2013.31001.

1. Introduction

The graphs considered in this paper are simple and undirected. A -coloring of a graph is an assignment of different colors to the vertices of such that adjacent vertices receive different colors. The minimum cardinality for which has a -coloring is called the chromatic number of and is denoted by Let and be graphs with, then it is easy to see that and so is a monotone function.

We begin with the plane coloring problem. What is the least number of colors needed to color all the points of the Euclidean plane such that no two points of distance one have the same color? The corresponding problem for the real line is easy. To see this, partition the vertex set of into two non empty disjoint sets such that their union is.

That is, where

and Now color all the vertices

Figure 1. A sample Distance graph with.

of with color 1 and the vertices of with color 2.

As are independent and is bipartite the result follows. Clearly is an infinite graph. The problem of finding the chromatic number of is enormously difficult. Paul Erdos has mentioned this problem as one of his favorite problems. Although he could not solve this problem he has made a significant progress towards the solution of this problem with a vital result given as follows: First let us state the famous Rado’s Lemma [1].

Lemma 1.1 Let and be arbitrary sets. Assume that to any there corresponds a finite subset of. Assume that to any finite subset, a choice function is given, which attaches an element of to each element of:. Then there exists a choice function defined for all (if) with the following property. If is any finite subset of, then there exists a finite subset, such that, as far as is concerned, the function coincides with.

Theorem 1.1 ([2]) Let be a positive integer, and let the graph have the property that any finite subgraph is -colorable. Then is -colorable itself.

Theorem 1.1 allows us to look for the maximum number of colors needed for the finite subsets. It is obvious that is not a trivial graph. Therefore

. The presence of at least one edgeviz., between (0,0) and (1,1) in conveys the information that. Similarly, the presence of a clique of size 3, viz., with vertices at

(0,0), , (1,1) shows that

.

Moreover, it is a fact that four points in the Euclidean two dimensional plane cannot have pairwise odd integer distances. Therefore, a clique of size 4, viz., cannot be an induced subgraph of. But it will be quite interesting to find the coordinates of a unit distance finite subgraph of such that The Moser Spindle graph in Figure 1 with a chromatic 4-coloring is a good example to deduce that

.

It is interesting to note that so far no unit distance graph that requires exactly five colors are known. Falconer [3] showed that if the color classes form meaurable sets of, then at least five colors are needed. Since the construction of non measurable sets requires the axiom of choice, we might have the answer turn out to depend on whether or not we accept the axiom of choice. We can tile the plane with hexagons to obtain a proper 7-coloring of the graph. The result is originally due to Hadwiger and Debrunner [4]. So.

See Figures 2 and 3 for the lower and upper bounds respectively. Despite the age of this problem, very little progress has been made since the initial bounds on were discovered shortly after the problem’s creation. This fact is a testament to the difficulty of the problem and in the absence of progress on the main problem, a number of restricted versions and related questions have been studied. We study here the open problem of characterizing the class 4 sets mentioned in the problem for the prime distance graph whose vertex set is and the distance set is a subset of not only primes but are also they are a special set of primes. The authors have already made some progress in [5-11].

Figure 2. The moser spindle.

Figure 3. The hexagonal 7 coloring.

2. Prime Distance Graph

A prime distance graph is a graph with the set of integers as vertex set and with an edge joining two vertices and if and only if where is the set of all prime numbers.

By a chromatic subgraph of a graph we mean a minimal subgraph of with the same chromatic number as.What class of graphs will include a chromatic subgraph of for?

A graph is color critical if its only chromatic subgraph is itself. For any positive integer, let be the graph comprising distinct vertices and distinct subgraphs each of which is a copy of (Where is the complete graph on n vertices) such that is adjacent to and each vertex of is adjacent to and for.

where is the cycle on vertices.

Certain Known Results

Lemma 2.1 ([12]) For any positive integers the graph is color critical with.

Theorem 2.1 ([13]) and hence.

Proof. Let each integer be assigned a color class precisely when, for Since integers assigned to the same color differ by a multiple of 4, they are not adjacent in, so

.

Since and is monotone, we have

.

But as we have

where is shown in the Figure 4 determined by the vertices and the subgraphs with vertex sets and respectively. The proof now follows from the Lemma 2.1.

In view of Theorem 2.1, we can allocate the subsets of to four classes, according as has chromatic number 1, 2, 3 or 4. Obviously empty set is the only member of class 1 and every singleton subset is in class 2.

Lemma 2.2 if consists only of odd integers.

Proof. Partition into two sets with

such that an integer if and only if . Now as the elements of are even and the elements of are odd we find that is an independent set for. Therefore is bipartite and hence. See Figure 5, where = indicates the presence of an edge between any two vertices.

Preposition 1 ([14]) If is finite and the greatest common divisor of the elements of is 1 then

(a) if all are odd,

Figure 4. In the figure and are isomorphic to.

Figure 5. = indicates the presence of an edge between any two vertices of V_{1}(Z) and V_{2}(Z).

(b) if no distance element is divisible by 3(c) if no distance element is divisible by 3 and at least one distance element is even.

Lemma 2.3 If contains no multiple of a fixed positive integer, then

Proof. Color each vertex in by mod. Since and have the same color if and only if is a multiple of, but contains no multiple of, this is a proper -coloring.

For simplicity, we assume that in the rest of this section. If, then it is not hard to see that. For the case of, where and are relatively prime, and so either they are both odd or they are of opposite parity.

Theorem 2.2 If and are relatively prime positive odd integers, then

Proof. The theorem follows immediately from Lemma 2.3 and the fact that for any nonempty.

Theorem 2.3 ([13]) Let with. Then may be classified as follows:

(a) is in class 2 if; otherwise is in class 3 or 4.

(b) If and, then is in class 3.

(c) If, then is in class 3.

(d) If, then is in class 3.

(e) If or, then is in class 4.

Theorem 2.4 ([13]) For any prime is in class 3.

Lemma 2.4 ([15]) There exists only a finite number of sets and with

Theorem 2.5 ([15]) Let be a set of primes with and. Then holds if and only if

The proofs of Lemma 2.4 and Theorem 2.5 are available in [15]. Moreover, the authors M. Voigt and H. Walther have shown that there are exactly eight pairs of primes with

Theorem 2.6 ([16]) Let with and Then

3. Some Special Set of Prime Numbers

Consider the following interesting set of prime numbers namely Additive Primes, Deletable Primes, WedderburnEtherington Number Primes, Euclid-Mullin Sequence Primes, Motzkin Primes, Schröder Primes, Catalan Primes, Non-generous Primes, Pell Primes, Primes of Binary Quadratic Form, Primeval Primes, Smarandache-Wellin Primes, Highly Cototient Number Primes, Annihilating Primes, Euclid primes, Fortunate Primes, SchröderHipparchus Number primes, Gaussian Primes, Mersenne Primes, Odd Primes, Primorial Primes, Proth Primes, Regular Primes, Self Primes, Solinas Primes, Super Primes, Delannoy Number Primes, Unique Primes, and Wagstaff Primes. For the definitions of these set of primes refer [17]. However, the existence of an interesting mathematical process that logically leads to the precise definition of an Annihilating prime we mention the same here for the sake of completeness and for the ease of the readers. Each of these set of primes have certain unique properties that make them special and are found to be useful from both theoretical and practical point of view.

Annihilating Primes

Definition 3.1 ([18]) For let denote the number of one-to-one sequences— these are sequences without repetitions—we can build with distinct objects.

(= the empty sequence), , , ,.

Hence,. Of course, it is easy to find a general expression for. Since there are

possible ways to choose objects from a set of (distinct) objects, and since (distinct) objects give rise to permutations, we get the following Lemma.

Lemma 3.1 Also the next representation for is elementary.

Lemma 3.2 For all positive we have

Remark: For the formula does not hold, since.

Proof. According to Lemma 3.1 we have

The following recursive relation for is an immediate consequence of the second formula in Lemma 3.1.

Lemma 3.3 For all positive we have Using this formula, we finally get the following integral representation of.

Lemma 3.4 For all we have

Notation: Throughout this text we adopt the standard notation to express that divides for . Moreover, if then

denotes the reminder of the division of by; and denotes the greatest common divisor of and.

We start our investigation on divisibility properties of with a simple fact which has first been proved in [19].

Lemma 3.5 For natural numbers, the following implication holds: If, then

and for any with

Definition 3.2 If is a sequence of natural numbers, we define its shadow to be the sequence given by where

are the shadow sets of the sequence

The shadow counts the sequence entries which are divisible by. So, the shadow measures (to a certain extent) how “divisible” the entries of the sequence are: For example, if only prime numbers occur in the sequence, then its shadow will reflect this fact by being small. If the entries of have many divisors, the shadow will typically be large.

If is an arithmetic sequence of first order, then its shadow is periodic, and for the shadow of Euler’s -function we have for all

Example: If for a function there exists an such that for all we have then for all we have. Vice versa, if for all, then equals the number of zeros in. Hence, it is easy to construct different functions which have the same shadow:

0 1 2 3 4 5 6 7···

0 1 2 3 4 5 6 7···

0 1 1 2 3 4 5 6···

0 1 1 1 2 3 4 5···

shadow 0 1 1 1 1 1 1 1···

Definition 3.3 A sequence is said to have the reduction property, if for all we have,

Lemma 3.6 The sequence has the reduction property.

Lemma 3.7 The shadow of a sequence which has the reduction property is multiplicative, i.e. if, then.

As an immediate consequence we get the following:

Corollary 1 If is the shadow of and if

is the prime decomposition of, then

Therefore, the shadow of is fully determined by its values on the powers of prime numbers. But what can we say about for prime?

By the reduction property, all elements

must be of the form for some

and some. Hence, we get inductively that if, then for all positive

Definition 3.4 A prime number with is called annihilating.

Preposition 2 If is divisible by an annihilating prime, then

Annihilating primes are primes such that, where is the shadow of a sequence of natural numbers. First few such primes are: 3, 7, 11, 17, 47, 53, 61, ···

4. Chromatic Number of Certain Prime Distance Graphs

Here we compute the chromatic number of the distance graph:, when is a set/subset of any of the above listed primes.

Theorem 4.1, when is a finite/ infinite set/subset as the case may be of WedderburnEtherington Number Primes.

Proof. Let denote the set of Wedderburn-Etherington Number Primes. Note that in, the set of primes appears as a proper subset. By Lemma 2.4 and Theorem 2.5, the set of primes is one of the eight sets of chromatic number 4 class. So

as.

Theorem 4.2, when is a finite/ infinite set/subset as the case may be of Additive Primes, Deletable Primes, and Euclid-Mullin sequence primes.

Proof. Let be the set of Additive primes/Deletable primes/ Euclid-Mullin sequence primes. As the set of primes is a proper subset of, by Theorem 1.1 and Theorem 2.1

as.

Theorem 4.3, when is a finite/ infinite set/subset as the case may be of 1) Motzkin primes, 2) Catalan primes, 3) Schröder primes, 4) Nongenerous primes, 5) Pell primes, 6) Primeval primes, 7) Primes of binary quadratic form, 8) Smarandache-Wellin primes, and 9) Highly Cototient number primes.

Proof. Let denote the set of any one of these special class of prime numbers listed in the theorem. All these set of primes have a unique property that an even prime integer 2 is in but the odd prime integer 3 is not appearing in. So by Theorem 2.3(b) and Preposition 1(c),.

Theorem 4.4, when is a finite/ infinite set/subset as the case may be of 1) Euclid primes, 2) Fortunate primes, 3) Schröder-Hipparchus Number primes, 4) Gaussian primes, 5) Mersenne primes, 6) Odd primes, 7) Primorial primes, 8) Proth primes, 9) Regular primes, 10) Self primes, 11) Solinas primes, 12) Super primes, 13) Delannoy Number primes, 14) Unique primes, 15) Wagstaff primes, and 16) Annihilating primes.

Proof. Let denote the set of any one of these special class of prime numbers listed in the theorem. All these special class of primes have a unique property that does not contain the even prime integer 2 but has an odd prime integer 3. So by Theorems 2.2 and 2.3(a),.

Conclusion

A complete characterization of class three sets and class four sets are available in the literature when the cardinality of the distance subset of are either 3 or 4. However when the cardinality of is more than 4 only a little is known. So in this aspect Theorem 4.1, Theorem 4.2 and Theorem 4.3 makes some progress towards the complete characterization of class three and class four sets. Moreover, Theorem 4.4 brings out certain special classes of prime numbers with membership in class 2 category.

5. Acknowledgements

This research is carried out with the financial grant and support of National Board for Higher Mathematics, Government of India under the grant sanction no. 2/ 48(10)/2005/R&D-II/11192/dated 26,Nov,2010.

Conflicts of Interest

The authors declare no conflicts of interest.

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