Stability of block-triangular stationary random matrices

The objective of this note is to prove, under certain technical conditions, that the top-Lyapunov exponent of a strictly stationary random sequence of block-triangular matrices is equal to the maximum of the top-Lyapunov exponents of its diagonal blocks. This study is partially motivated by a basic technical problem in the identification of GARCH processes. A recent extension of the above inheritance theorem in the context of LqLq-stability will also be briefly described.