Characteristic polynomials and eigenvalues of tensors

We lay the geometric foundations for the study of the characteristic polynomial of tensors. For symmetric tensors of order $d\ge 3$ and dimension $2$ and symmetric tensors of order $3$ and dimension $3$, we prove that only finitely many tensors share any given characteristic polynomial, unlike the case of symmetric matrices and the case of non-symmetric tensors. We propose precise conjectures for the dimension of the variety of tensors sharing the same characteristic polynomial, in the symmetric and in the non-symmetric setting.