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They are called http://www.google.it/search?client=safari&rls=en&q=degenerate+elliptic+equation&ie=UTF-8&oe=UTF-8&redir_esc=&ei=rqn9T4LwDdTc4QTmhLmJBwdegenerate elliptic equations.
http://www.google.it/search?client=safari&rls=en&q=degenerate+elliptic+equation&ie=UTF-8&oe=UTF-8&redir_esc=&ei=rqn9T4LwDdTc4QTmhLmJBw
They are called degenerate elliptic equations.
