For a sequence of $n$ distinct reals, determine the largest constant $c$ such that some monotonic subsequence always has sum exceeding $(c-o(1))\cdot(1/\sqrt{n})$ times the total sum. Resolved as $c = 1$.

Cambie makes the stronger conjecture that if $x_1,\ldots,x_{k^2}$ are distinct positive real numbers with $\sum x_i=1$ then there is always a monotonic subsequence with sum at least $1/k$. This is a weighted-form of the Erdős-Szekeres theorem, and is also mentioned (as an open question) in a survey on the latter by Steele [St95].

This stronger conjecture appears to have been first proved by Tidor, Wang, and Yang [TWY16], and is also implicit in work of Wagner [Wa17]. A proof was given and formalised by Aristotle (see the comments), with an alternative proof provided by Chan.