Published in Jornal de Sciencias Mathematicas, Physicas e Naturaes, Academia Real das Sciencias de Lisboa,, in 1898, in volume 5, pages 133-139

Published in J. Sci. math. phys. nat., in 1898, in volume 5, pages 157-162

Published in Jornal de Sciencias Mathematicas, Physicas e Naturaes, in 1898, in volume 5, pages 185-202