1. Bonnet A., A positivity related property for the biharmonic operator. Preprint.
2. On the characteristic roots of power-positive matrices;Brauer;Duke math. J.,1961
3. Conditional gauge and potential theory for the Schrödinger operator;Craston;Trans. Am. math. Soc.,1988
4. Maximum principles for a class of noncooperative elliptic systems;Caristi;Delft Progress Report,1990
5. Sign-definite solutions in elliptic systems with constant coefficients;Cosner,1988