DELL'OSSO, L., et al. “Some CONsideratiollS ON the Usual Derivation or the Pockels (or Helmholtz) Equation in Steady Two DimensionaI Fluid DYllamics”. Annals of Geophysics, vol. 28, no. 1, Nov. 1975, pp. 37-43, https://doi.org/10.4401/ag-4889.