We consider invariants of $m$-tuples of $n \times n$ matrices $X_1, \ldots, X_m$ under simultaneous conjugation. We show that any invariant can be expressed using the trace. We also consider concomitants and describe them as an algebra over the invariants generated by the projections on $X_i$. For the purpose of describing invariants and concomitants we introduce trace polynomials. We consider trace identities, i.e. trace polynomials describing the zero invariant or concomitant. We show that any identity is a consequence of the Cayley-Hamilton theorem.
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