between co-axially rotating black holes involves, as a first step, an analysis on the “boundary
regularity” of the so-called reduced singular harmonic maps. We carry out this analysis by
considering those harmonic maps as solutions to some homogeneous divergence systems
of partial differential equations with singular coefficients. Our results extend previous works
by Weinstein (Comm Pure Appl Math 43: 903–948, 1990; Comm Pure Appl Math 45: 1183 …