Can non-linear objects be defined using linear ones?

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Tensors are said to be multi-linear maps. My question is if tensors can be used to describe non-linear systems? Or if non-linear mappings can be made using tensors?

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They certainly can, some famous nonlinear systems that are nonlinear and are described using tensors are General Relativity, where the equations involve the Riemann curvature tensor, and also non-abelian Yang-Mills theory, where you have the field strength tensor. In these theories, the field acts as its own source, so it's in general not true that a linear combination of solutions will be a solution itself.

Think of the case of the one-dimensional derivative. It is a linear map between functions, and can be thought as an operator on a vector space. But that does not mean everything you write with derivatives will be a linear differential equation.