# It is undecidable whether the growth rate of a given bilinear system is 1

We show that there exists no algorithm that decides for any bilinear system (B,v) if the growth rate of (B,v) is 1. This answers a question of Bui who showed that if the coefficients are positive the growth rate is computable (i.e., there is an algorithm that outputs the sequence of digits of the growth rate of (B,v)). Our proof is based on a reduction of the computation of the joint spectral radius of a set of matrices to the computation of the growth rate of a bilinear system. We also use our reduction to deduce that there exists no algorithm that approximates the growth rate of a bilinear system with relative accuracy ε in time polynomial in the size of the system and of ε. Our two results hold even if all the coefficients are nonnegative rationals.

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