Infinite Euclidean distance discriminants

We study infinite Euclidean distance discriminants of algebraic varieties, defined as the loci of data points whose fibers under the second projection from the Euclidean distance correspondence are positive-dimensional. In particular, these discriminants contain all data points with infinitely many critical points for the nearest-point problem. We present computer code that computes the infinite Euclidean distance discriminant, and use it to present numerous varieties with nonempty such discriminants. Moreover, we prove that for any data point, the fiber under the second projection is contained in a finite union of hyperspheres centered at that point. For curves, we include a complete characterization; their infinite Euclidean distance discriminants turn out to be affine linear spaces. Finally, we introduce and characterize skew-tube surfaces in three-dimensional space. By construction, these have a one-dimensional infinite Euclidean distance discriminant. We further demonstrate that many skew-tubes have significantly lower Euclidean distance degrees than generic surfaces of the same degree.