(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 $S_1, S_2, ...$ of
$S$ which can be put into one-to-one
correspondence with the vertices of
$G$ in such a way that $x_i$ and $x_j$ are joined by an
edge of $G$ iff $i neq j$
and $S_i cap S_j neq emptyset$.
If we identify $S$ with a set of prime numbers and each $S_i$ 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 $(n_1, n_2, ..., n_k)$
which can be put into one-to-one
correspondence with the vertices of
$G$ in such a way that $x_i$ and $x_j$ are joined by an edge iff $i neq j$ and GCD$(n_i, n_j) > 1$.
We can choose the prime numbers (the elements of $S$, from which the $n_i$ are built) arbitrarily, and so the question arises, whether they can always be choosen in such a way, that the set $(n_1, n_2, ..., n_k)$ 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 $(p_1, p_2, ..., p_k)$ of primes.
Question: Can every graph on $k$ nodes be represented by an arithmetic sequence
of natural numbers such that $n_i$ and $n_j$ are joined by an edge iff $n_i neq n_j$ and GCD$(n_i, n_j) > 1$
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 $n_i$ and $n_j$ are joined by an edge iff $n_i neq n_j$ and GCD$(n_i, n_j) > 1$: Are there interesting classes of graphs with this property?
No comments:
Post a Comment