Number Theory – Infinitely Many Positive Integer Solutions to $(3+3k+l)^2=m\,(k\,l-k^3-1)$

diophantine equationsnt.number-theory

I usually work in the field of differential geometry, but I have encountered the following problem in my research: Are there infinitely many positive integers $k,l,m\in\mathbb N^{>0}$ such that $$(3+3k+l)^2=m\,(k\,l-k^3-1)\,?$$ Obviously, taking $l=k^2$
and $m=-(3+3k+l)^2$
gives infinitely many integer solutions, but $m<0$ is negative. As a non-expert, I imagine that there is either a simple answer to this question, or the problem is not so simple to solve. Of course, I've played around with the equations a bit, but other than finding numerous examples, I haven't made any progress.

I would appreciate an existence or non-existence statement for infinitely many positive integer solutions, but also some hints that the problem is most likely hard to solve would help me.

Background: I am looking for certain integer representations of a surface group, and I can show that integer solutions to this diophantine equation actually give rise to integer representations. The condition that $k,l,m$ are positive is equivalent to the condition that the corresponding representation is contained in a higher Teichmüller component (which is important for my differential geometric application).

Best Answer

It does have infinitely many positive solutions. Here is just one such series.

Consider the following recurrence sequence:

$$u_0=1,\ u_1=2,\ u_{n+1} = 23 u_n - u_{n-1} - 4\qquad (n\geq 1).$$

Let $t,k$ be any two consecutive terms of this sequence, then setting $l:=k^2+t$ produces the following equality: $$(3+3k+l)(t+1) = (k+26)(kl-k^3-1),$$ which gives solution $m:=\frac{(k+26)(3+3k+l)}{t+1}$ (which is an integer) to the original equation.


In fact, integrality of $m$ follows from the identity: $$(u_{n+2}+1)(u_n+1) = (u_{n+1}+26)(u_{n+1}+1),$$ which can be verified from the recurrence for $u_n$.

In summary, the values $(k,l,m)$ in this solution series are given by $$\begin{cases} k = u_{n+1}, \\ l = u_{n+1}^2 + u_n, \\ m = (u_{n+2}+2)(u_{n+1}+2) + 24. \end{cases}\qquad (n\in\mathbb{Z}_+) $$


ADDED. I've added $u_n$ to the OEIS as sequence A350917. Together with 9 other similar recurrences it gives all solutions $k$ to $(tk-1)\mid (k+1)^4$, which are now listed in sequence A350916.

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