# Marshall Hall’s conjecture

Conjecture. (Marshall Hall, Jr.). With the exception of $n^{2}$ being a perfect sixth power, for any positive integer $n$, the inequality $|n^{2}-m^{3}|>C\sqrt{m}$, (with $m$ also being a positive integer and $C$ being a number less than 1 that nears 1 as $n$ tends to infinity) always holds.

The reason for the exception of perfect sixth powers (those cases of $n$ for which there is a solution to $n^{2}=h^{6}$ in integers) is a simple consequence of associativity: if $n^{2}=h^{6}$, then $h^{6}=h^{2}h^{2}h^{2}=h^{3}h^{3}$. Then $m=h$ and $n^{2}-m^{3}=0$. For example, $8^{2}-4^{3}=0$.

For small $n$, $C$ can’t be exactly 1. For example, $3^{2}-2^{3}=1$, and $\sqrt{2}>1$. But even among the smaller numbers, the conjecture generally holds even with $C=1$. After $n=3$, the next counterexample (that is not a perfect sixth power) to $C=1$ is $n=378661$, with the corresponding $m=5234$ producing a difference of just 17. A078933 in Sloane’s OEIS lists smaller values of $m$ with cubes being at a distance from the nearest square that is less than $\sqrt{m}$. Noam Elkies has found some fairly large counterexamples to setting $C=1$, such as $n=447884928428402042307918$ and $m=5853886516781223$, the difference between the square of the former and the cube of the latter being a relatively small 1641843.

## References

• 1 R. K. Guy, Unsolved Problems in Number Theory New York: Springer-Verlag 2004: D9
Title Marshall Hall’s conjecture MarshallHallsConjecture 2013-03-22 18:15:36 2013-03-22 18:15:36 PrimeFan (13766) PrimeFan (13766) 4 PrimeFan (13766) Conjecture msc 11D79 Marshall Hall conjecture PerfectPower