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Published online by Cambridge University Press: 24 October 2008
Let H and K be groups with derived sub groups H´ and K´. Following (5) we say that H and K are orthogonal, and write H ⊥ K, if and only if the tensor product H/H´ ®K/K´ is trivial. If H and K are sub groups of a group we say that H and K permute if and only if HK = KH. We recall finally that the subgroup H of the group G is said to be subnormal in G, written H sn G, if and only if there exists a finite chain of subgroups
connecting H to G.