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2. On eigenvalues and eigenfunctions of strongly elliptic systems of differential equations of second order;Bochenek;Prace Mat.,1968
3. Comparison and Oscillation Theory of Linear Differential Equations;Swanson,1968
4. A Sturm theorem for strongly elliptic systems and applications;Kreith;Bull. Amer. Math. Soc.,1969
5. K. Kreith, A Picone identity for strongly elliptic systems, Duke Math. J., to appear.