On critical normal sections for two-dimensional immersions in R4 and a Riemann-Hilbert problem

For orthonormal normal sections of two-dimensional immersions in R4 we define torsion coefficients and a functional for the total torsion. We discuss normal sections which are critical for this functional. In particular, a global estimate for the torsion coefficients of a critical normal section in terms of the curvature of the normal bundle is provided.