# Leaning dominoes

Is it possible to create a stable stack of frictionless homogeneous 3d blocks in which there are horizontal forces? We heard this question from Mikkel Thorup in a recent Combinatorics Study Group meeting – it is motivated by the Maximum Overhang Problem. For example, you might think that a circle of dominoes can lean against each other in a stable fashion, but apparently it will fall over (very slowly). Mikkel conjectures that no such stack exists. However, if this is true, the proof must use the “rectangular” nature of blocks, as such stacks do exist using triangles.

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