Paul C. Bell, Mika Hirvensalo, Igor Potapov. 2(ℤ) is NP-complete. In Philip N. Klein, editor, Proceedings of the Twenty-Eighth Annual ACM-SIAM Symposium on Discrete Algorithms, SODA 2017, Barcelona, Spain, Hotel Porta Fira, January 16-19. pages 187-206, SIAM, 2017. [doi]
Abstract is missing.