If the system X_{k+1} = (A + u_{k}B)X_{k}, k = 0, 1, ..., is controllable with |u_{k}| for all \delta > 0 , then the eigenvalues of A lie on the unit circle. This is Goka's conjecture. Through algebraic transformation and discussion of invariant proper subsets, this note gives a proof of the conjecture, and shows that for more general discrete-time bilinear systems, the conjecture is still true.
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