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For integers $x$, $y$ and $z$, if $x^2+y^2=z^2$ then the ordered triple $(x,y,z)$ is called a Pythagorean triple. It is well known that Pythagorean triples $(x,y,z)$ with $2\mid y$ have the form $(k(m^2-n^2),2kmn,k(m^2+n^2))$ with $k,m,n\in\mathbb Z$.

Ramanujan's tau function defined over $\mathbb Z^+=\{1,2,3,\ldots\}$ is given by $$q\prod_{n=1}^\infty (1-q^n)^{24}=\sum_{n=1}^\infty\tau(n)q^n\quad \ (|q|<1).$$ It plays an important role in the theory of modular forms.

Here I propose a conjecture connecting the tau function with Pythagorean triples.

Conjecture. There are no Pythagorean triples of the form $(\tau(k),\tau(m),\tau(n))$ with $k,m,n\in\mathbb Z^+$. In other words, the equation $x^2+y^2=z^2$ has no solution over the range of the tau function.

This conjecture implies Lehmer's conjecture which asserts that $\tau(n)\not=0$ for all $n\in\mathbb Z^+$.

I have checked the Conjecture via Mathematica and found no counterexamples.

QUESTION. Can one provide a concrete counter-example to the above conjecture?

Your comments are welcome!

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  • $\begingroup$ I have verified that there are no Pythagorean triples $(\tau(k),\tau(m),\tau(n))$ with $\max\{k,m,n\}\le 10000.$ $\endgroup$ Commented Dec 28, 2024 at 20:06
  • $\begingroup$ Based on some computations, I have made a further conjecture: $\tau(m)^2+\tau(n)^2$ with $m,n\in\mathbb Z^+$ can never be a perfect power. $\endgroup$ Commented Dec 29, 2024 at 11:18
  • $\begingroup$ It's a lot easier to conjecture that something doesn't happen, than to conjecture that something does happen. $\endgroup$ Commented Dec 30, 2024 at 4:09
  • $\begingroup$ Based on verification by Maple, the equation $\tau(x)^2+\tau(y)^2=\tau(z)^2$ has no solution for $1 \leq x \leq 10^5, 1 \leq y \leq 10^6 , 1 \leq z \leq 10^7.$ $\endgroup$ Commented Jan 16 at 13:01

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