*(This is a follow-up to my previous questions [Natural models of graphs?][1].)* Erdös in [The Representation of a Graph by Set Intersections][2] (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][3]. 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][4] 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?** [1]: https://mathoverflow.net/questions/11647/natural-models-of-graphs [2]: https://www.renyi.hu/~p_erdos/1966-21.pdf [3]: https://en.wikipedia.org/wiki/Arithmetic_progression [4]: https://en.wikipedia.org/wiki/Green%E2%80%93Tao_theorem