What is known about maximum principles for strongly elliptic linear differential operators of even order (possibly higher than $2$)? By such an operator, I mean a linear differential operator with bounded coefficients whose principal symbol is positive (actually, maybe multiplied by some power of $-1$) and uniformly bounded away from $0$ in space.
I suspect that the maximum principle still holds in an analogous manner, but I've had trouble finding a reference that deals with the theory of elliptic operators in this general manner. Does anybody know where to find such a reference?