Although the⁻ me⁻re⁻ly formal conside⁻rations, which we⁻ will ne⁻e⁻d for the⁻ proof, are⁻ alre⁻ady mostly containe⁻d in a work by Troll H. Poincaré, for the⁻ sake⁻ of clarity I will not re⁻ly on that work.