AbstractLetL(X,Y) denote the class of linear transformationsT:D(T) ⊂X→YwhereXandYare normed spaces. A quantityfis called densely invariant if for every systemL(X, Y) and every operator T ϵL(X,Y) we havef(T/E)= f(T) wheneverEis a core ofT. The density invariance of certain well known quantities is established. In caseYis complete andTis closable, a stronger property is shown to hold for some of these quantitites, namely invariance under restriction to dense subsp
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