If you have a second order elliptic equation but the coefficients of the second order terms only form a nonnegative (instead of positive definite) matrix, then, do you know if there is any literature about this? (For example, the matrix of the second order terms can be diag{1,1,0,0} )
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closed as offtopic by Ricardo Andrade, Theo JohnsonFreyd, Andrey Rekalo, Willie Wong, Stefan Kohl Nov 27 '13 at 11:01
This question appears to be offtopic. The users who voted to close gave this specific reason:
 "This question does not appear to be about research level mathematics within the scope defined in the help center." – Ricardo Andrade, Andrey Rekalo, Willie Wong, Stefan Kohl
They are called degenerate elliptic equations. 

