MathSciNet bibliographic data MR872382 (88a:35155) 35L65 (35L67 58C27 76T05) Schaeffer, David G.; Shearer, Michael The classification of $2\times 2$$2\times 2$ systems of nonstrictly hyperbolic conservation laws, with application to oil recovery. Comm. Pure Appl. Math. 40 (1987), no. 2, 141–178. Article

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