Lesser known unsolved mathematical problems I find unreasonably fascinating:
* Is the Leyland-mapping injective?
* We have Kadane's algorithm for the maximum subarray problem, which runs in O(n), is there an algorithm for the maximum submatrix problem which runs in O(n²)? What about higher-dimensional tenors?
* The polyhedral numbers conjecture: is every number the sum of m+1 n-hedral numbers, where m is the number of vertices of the polyhedron. Does it hold for any n-dimensional polytope?
* A closed-form solution for the problem describes in this video: https://m.youtube.com/watch?v=bRIL9kMJJSc (maybe not lesser-known? But it doesn't even have its own Wikipedia page!)
* the perfect cuboid problem
* What's the prob of a random normal Form game being a potential game
* generally all theorems of the form "we have mathematical object X, and Y as a special case of X. How many X are Y?"
* A closed-form equation that describes the number of preorders that can be defined for a set of size n