\Bibitem{Had45} \by J.~Hadamard \paper Probl\`emes \`a apparence difficile \jour Sb. Math. \yr 1945 \vol 59 \issue 1 \mathnet{http://mi.mathnet.ru//eng/sm6340} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=14571} \zmath{https://zbmath.org/?q=an:0063.01846}