On unit solutions of the equation xyz = x+y+z in the ring of integers of a quadratic field R. Mollin, C. Small, K. Varadarajan, P. Walsh (1987) Acta Arithmetica
On unit solutions of the equation xyz=x+y+z in a number field with unit group of rank 1 Liang-Cheng Zhang, Jonathan Gordon (1991) Acta Arithmetica