Monday, 21 August 2006

nt.number theory - Can every finite graph be represented by an arithmetic sequence of natural numbers?

(This is a follow-up to my previous questions Natural models of graphs?.)



Erdös in The Representation of a Graph by Set Intersections (1966) states:




Theorem. Let G be an arbitrary
graph. Then there is a set S and a
family of subsets S1,S2,... of
S which can be put into one-to-one
correspondence with the vertices of
G in such a way that xi and xj are joined by an
edge of G iff ineqj
and SicapSjneqemptyset.




If we identify S with a set of prime numbers and each Si with the product of its members we get the following:




Corollary. Let G be an arbitrary finite
graph. Then there is a sequence of natural numbers (n1,n2,...,nk)
which can be put into one-to-one
correspondence with the vertices of
G in such a way that xi and xj are joined by an edge iff ineqj and GCD(ni,nj)>1.




We can choose the prime numbers (the elements of S, from which the ni are built) arbitrarily, and so the question arises, whether they can always be choosen in such a way, that the set (n1,n2,...,nk) is an arithmetic sequence.



Of course every complete graph on k nodes can be represented by an arithmetic sequence: just take some consecutive sequence of even numbers. Green-Tao's Theorem guarantees that also every empty graph on k nodes can be represented by an arithmetic sequence (p1,p2,...,pk) of primes.




Question: Can every graph on k nodes be represented by an arithmetic sequence
of natural numbers such that ni and nj are joined by an edge iff nineqnj and GCD(ni,nj)>1




This would be one kind of natural model of a graph, that I was looking for, originally.



Maybe some references?



Added: Due to Kevin's concise answer and Thomas' comment, I'd like to add the following question:




Question: If not every graph on k nodes can be represented by an arithmetic sequence
of natural numbers such that ni and nj are joined by an edge iff nineqnj and GCD(ni,nj)>1: Are there interesting classes of graphs with this property?


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