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Published online by Cambridge University Press: 20 November 2018
If R 1 and R 2 are root systems and there is a linear map which maps R 1 ∪{0} onto R 2∪{0} we write R 1 —> R 2. We determine all pairs (R 1, R 2) of irreducible root systems such that R 1 —> R 2.