What is the branch of mathematics called that deals with differential equations over finite fields?

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What is the branch of mathematics called that deals with differential equations over finite fields? For example, Laplace's equation $$ \nabla^2 \phi = 0 $$ is usually defined for a scalar field $\phi$ that spans real or complex numbers. Is there an analogue equation for $\phi$ defined over a set of cyclic numbers, say, between $0$ and $2\pi$, or a Galois field, for example $GF(4)$? What is the branch of mathematics called that deals with how to solve these types of equations?