Is z xy convex?

Excuse me, could you clarify what you mean by "z xy convex"? It seems like you're asking about a mathematical or geometrical property, but I'm not entirely sure what context you're referring to. Are you asking about the convexity of a function, surface, or some other mathematical object that involves the variables z, x, and y? If so, could you provide more details about the specific function or shape you're referring to? Without that information, it's difficult to give a definitive answer to your question.