I know that it is undecidable to determine if a set of tiles can tile the plane, a result of Berger using Wang tiles. My question is whether it is also known to be undecidable to determine if a single given tile can tile the plane, a monohedral tiling.
If this remains unsettled, I would be interested to know what is the minimum cardinality of a set of tiles for which there is an undecidability proof. (I have not yet accessed Berger's proof.)