Dania El-Khechen, Muriel Dulieu, John Iacono, Nikolaj van Omme. Packing 2×2 unit squares into grid polygons is NP-complete. In Proceedings of the 21st Annual Canadian Conference on Computational Geometry, Vancouver, British Columbia, Canada, August 17-19, 2009. pages 33-36, 2009. [doi]
Abstract is missing.