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Bibliography

[1]

C. A. Swanson, A comparison theorem for elliptic differential equations, Proc. Amer. Math. Soc. 17 (1966), 611-616. MR 0201781 (34:1663)

[2]

K. Kreith, A strong comparison theorem for selfadjoint elliptic equations, Proc. Amer. Math. Soc. 19 (1968), 989-990. MR 0227594 (37:3178)

[3]

E. Hopf, A remark on linear elliptic differential equations of the second order, Proc. Amer. Math. Soc. 3 (1952), 791-793. MR 0050126 (14:280b)

[4]

M. H. Protter and H. F. Weinberger, Maximum principles in differential equations, Prentice-Hall, Englewood Cliffs, N. J., 1967. MR 0219861 (36:2935)

[5]

C. A. Swanson, Comparison theorems for elliptic equations on unbounded domains, Trans. Amer. Math. Soc. 126 (1967), 278-285. MR 0203211 (34:3064)