Consider an $n \times n \times n$ cube. I would like to consider subsets of points in the cube with the two following constraints:
Each row in the cube (in any of the three directions) has exactly 2 or 0 points.
If two points are in the same row (any of three), connect them with an edge. The subset must be connected by these edges.
It is easy to build such a set which is $\Omega(n)$, for instance a staircase, or stacking squares. Is it the case that these structures are also $O(n)$? Or is there an $\Omega(n^2)$ bound?
Building such a set would require adding a linear number of points for each added dimension, which is hard due to the first restriction.
This problem may also be reformatted graph-theoretically. We may express points in the cube as 3-edges in a 3-uniform tripartite hypergraph. In this case our restraints become that each set of 2 points contained in a 3-edge is contained in exactly two 3-edges, and the 3-edges are connected.