Partitioning the positive integers into finite sets with sums in geometric progression
问题内容
Here is a quite interesting problem I've come up with:
Let $\mathbb{N}^+ = \{1, 2, 3, \dots\}$.
Does there exist a sequence of sets $A_1, A_2, \dots$ such that:
$1$. $A_k \subset \mathbb{N}^+$ is nonempty and finite.
$2$. $A_i \cap A_j = \varnothing$ for $i \neq j$.
$3$. $\bigcup_{k=1}^{\infty} A_k = \mathbb{N}^+$.
$4$. The sums of the elements in each group form a geometric progression, i.e. $S_k = \sum_{x \in A_k} x = a r^{k-1}$ for some $a, r > 0$.
I'm curious whether such a partition exists. I would appreciate a proof or any ideas.
回答 (1)
Yes, there's a simple construction. Let $$A_k=\left\{n\in\mathbb N^+:\frac{3^{k-1}+1}{2}\le n\le \frac{3^k-1}{2}\right\}$$ Because $A_k$ has $3^{k-1}$ terms and the average term is $3^{k-1}$ the sum of each $A_k$ is simply $9^{k-1}$