0.1 The partial differential equation (1) (Au)(x) = L aa(x)(Dau)(x) = f(x) m lal9 is called elliptic on a set G, provided that the principal symbol a2m(X,'¬) = L aa(x)'¬a lal=2m of the operator A is invertible on G X ( | Degenerate Elliptic Equations by Serge Levendorskii, Paperback | Indigo Chapters