(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 GG be an arbitrary
graph. Then there is a set SS and a
family of subsets S1,S2,...S1,S2,... of
SS which can be put into one-to-one
correspondence with the vertices of
GG in such a way that xixi and xjxj are joined by an
edge of GG iff ineqjineqj
and SicapSjneqemptysetSicapSjneqemptyset.
If we identify SS with a set of prime numbers and each SiSi with the product of its members we get the following:
Corollary. Let GG be an arbitrary finite
graph. Then there is a sequence of natural numbers (n1,n2,...,nk)(n1,n2,...,nk)
which can be put into one-to-one
correspondence with the vertices of
GG in such a way that xixi and xjxj are joined by an edge iff ineqjineqj and GCD(ni,nj)>1(ni,nj)>1.
We can choose the prime numbers (the elements of SS, from which the nini are built) arbitrarily, and so the question arises, whether they can always be choosen in such a way, that the set (n1,n2,...,nk)(n1,n2,...,nk) is an arithmetic sequence.
Of course every complete graph on kk 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 kk nodes can be represented by an arithmetic sequence (p1,p2,...,pk)(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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