A fundamental problem in Distributed Computing is the Pattern Formation problem, where some independent mobile entities, called robots, have to rearrange themselves in such a way as to form a given figure from every possible (non-degenerate) initial configuration. In the present paper, we consider robots that operate in the Euclidean plane and are dimensionless, anonymous, oblivious, silent, asynchronous, disoriented, non-chiral, and non-rigid. For this very elementary type of robots, the

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... ility of the Pattern Formation problem has been settled, either in the positive or in the negative, for every possible pattern, except for one case: the Square Formation problem by a team of four robots. Here we solve this last case by giving a Square Formation algorithm and proving its correctness. Our contribution represents the concluding chapter in a long thread of research. Our results imply that in the context of the Pattern Formation problem for mobile robots, features such as synchronicity, chirality, and rigidity are computationally irrelevant.