We introduce a higher dimensional quasiregular map analogous to the
trigonometric functions and we use the dynamics of this map to define, for d>1,
a partition of d-dimensional Euclidean space into curves tending to infinity
such that two curves may intersect only in their endpoints and such that the
union of the curves without their endpoints has Hausdorff dimension one.
The equation in the title describes the number of bright images of a point
source under lensing by an elliptic object with isothermal density. We prove
that this equation has at most 6 solutions. Any number of solutions from 1 to 6
can actually occur.