\Bibitem{Eru88} \by N.~P.~Erugin \paper The Riemann problem \jour Differ. Uravn. \yr 1988 \vol 24 \issue 12 \pages 2187--2190 \mathnet{http://mi.mathnet.ru/de9351} \zmath{https://zbmath.org/?q=an:0659.01009}