In the paper  V. MÜLLER,  A. PEPERKO,  Generalized spectral radius and its max algebra version, LAA, 2013, 1006-1016 we (also) gave quite a simple proof of the Berger- Wang formula on the equality of the joint and the generalized spectral radius of a bounded set of non-negative (entrywise) n times n matrices, by first proving in a direct way its max algebra version.

The problem that remains here is the following : the reduction of the Berger Wang formula from the general case of a bounded set of (real)n times n matrices to the non-negative case. This would give a new simple-simpler (or at least an alternative) proof of the Berger Wang formula (in the n times n matrix setting). V. Muller noticed that this reduction is possible in the case of the singelton set {A} by using e.g. upper triangular Schur form of a matrix. What about e.g the case {A,B}?

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