On sets with more products than quotients

Abstract

Given a finite set $A\subset ℝ\setminus \left\{0\right\}$, define

$$\begin{array}{cccc}\hfill A\cdot A& =\left\{a_i\cdot a_j\mid a_i,a_j\in A\right\},\hfill & \hfill A∕A& =\left\{a_i∕a_j\mid a_i,a_j\in A\right\},\hfill \\ \hfill A+A& =\left\{a_i+a_j\mid a_i,a_j\in A\right\},\hfill & \hfill A-A& =\left\{a_i-a_j\mid a_i,a_j\in A\right\}.\hfill \end{array}$$

The set $A$ is said to be MPTQ (more product than quotient) if $|A\cdot A|>|A∕A|$ and MSTD (more sum than difference) if $|A+A|>|A-A|$. Since multiplication and addition are commutative while division and subtraction are not, it is natural to think that MPTQ and MSTD sets are very rare. However, they do exist. This paper first shows an efficient search for MPTQ subsets of $\left\{1,2,\dots ,n\right\}$ and proves that as $n\to \infty$, the proportion of MPTQ subsets approaches $0$. Next, we prove that MPTQ sets of positive numbers must have at least $8$ elements, while MPTQ sets of both negative and positive numbers must have at least $5$ elements. Finally, we investigate several sequences that do not have MPTQ subsets.