This paper studies the behavior under iteration of the maps T_ab(x,y) = (F_ab(x)- y, x) of the plane R^2, in which F_ab(x)= ax if x>0 and bx if x<0. These maps are area-preserving homeomorphisms of the plane that map rays from the origin into rays from the origin. Orbits of the map correspond to solutions of the nonlinear difference equation x_n+2= 1/2(a-b)|x_n+1| + 1/2(a+b)x_n+1 - x_n. This difference equation can be written in an eigenvalue form for a nonlinear difference operator of Schrodinger type, in which μ= 1/2(a-b) is viewed as fixed and the energy E=2- 1/2(a+b). The paper studies the set of parameter values where T_ab has at least one nonzero bounded orbit, which corresponds to an l_∞ eigenfunction of the difference operator. It shows that the for transcendental μ the set of allowed energy values E for which there is a bounded orbit is a Cantor set. Numerical simulations suggest that this Cantor set may have positive one-dimensional measure, when μ is held fixed.