“…In , he consider the case when is not a convex function (in this case, the system describes how the addition of a polymer affects the flow of water and oil in a reservoir) and prove the existence of a global weak solution to the Cauchy problem. Lu showed the existence of a global weak solution of the Cauchy problem for when f is a non negative convex function and P satisfies the following condition: Using delta‐shock waves, H. Cheng solved the Riemann problem to the non symmetric system of Keyfitz‐Kranzer type when is the function , , and is the indentity function, i.e. for all .…”