The Lander–Parkin–Selfridge conjecture: if the sum of $n$ positive integer $k$-th powers equals the sum of $m$ positive integer $k$-th powers, with all values on the left distinct from all values on the right, then $n + m \geq k$.

Formally, for positive integers $k, n, m \in \mathbb{N}$ and sequences $x : {0, \ldots, n-1} \to \mathbb{N}$ and $y : {0, \ldots, m-1} \to \mathbb{N}$ with $x_i > 0$, $y_j > 0$, and $x_i \neq y_j$ for all $i, j$, if $$\sum_{i=0}^{n-1} x_i^k = \sum_{j=0}^{m-1} y_j^k,$$ then $k \leq n + m$.

Special case of the Lander–Parkin–Selfridge conjecture: there is no solution in positive integers to $$x_1^5 + x_2^5 + x_3^5 = y^5.$$ That is, for all $x_1, x_2, x_3, y \in \mathbb{N}$ with $x_1, x_2, x_3, y > 0$, $$x_1^5 + x_2^5 + x_3^5 \neq y^5.$$ This corresponds to the case $k = 5$, $n = 3$, $m = 1$ of the general conjecture, where $n + m = 4 < 5 = k$ would be required to yield a counterexample.