Given any $\beta \in R$, do there exist integers $k,l,m$, and a real number $x$ such that:
$l=4x^{3}m+6x^{2}m^{2}+4xm^{3}-2xm$,
as well as
$x^{4}-x^{2}=k+\beta$ ?
Asked
Active
Viewed 53 times
0
-
1Yes, sure. Take $k=-\beta$ and $x=0$, with $l=m=0$. – Dietrich Burde Jun 13 '19 at 15:14
-
1$\beta$ is not necessarily an integer, while $k$ is – Andrei Jun 13 '19 at 15:15
-
@DietrichBurde Doesn't work if $\beta\notin\mathbb Z$. – Adam Latosiński Jun 13 '19 at 15:16
1 Answers
2
From the first equation, $x$ is algebraic. This implies $\beta$ algebraic, which might not hold.
There are countably many values of $\beta$ that give solutions. We can enumerate them by considering all triples $(k,l,m)$, finding the roots in $x$ and computing $\beta=x^4-x^2-k$.
-
Yes, indeed. Thanks for pointing out that $\beta$ can only be countable. In general, sums and products of algebraic numbers are algebraic, and so we conclude the first statement. – Aritro Pathak Jun 13 '19 at 15:53