Abstract
This paper concerns the structure of the group of local unitary cocycles, also called the gauge group, of an E 0 -semigroup. The gauge group of a spatial E 0 -semigroup has a natural action on the set of units by operator multiplication. Arveson has characterized completely the gauge group of E 0 -semigroups of type I, and as a consequence it is known that in this case the gauge group action is transitive. In fact, if the semigroup has index k, then the gauge group action is transitive on the set of ( k + 1 ) -tuples of appropriately normalized independent units. An action of the gauge group having this property is called ( k + 1 ) -fold transitive. We construct examples of E 0 -semigroups of type II and index 1 which are not 2-fold transitive. These new examples also illustrate that an E 0 -semigroup of type II k need not be a tensor product of an E 0 -semigroup of type II 0 and another of type I k .
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