Sentence:  If * * f are the projections associated with the primary <WSD>decomposition</WSD> of T , then each * * f is a polynomial in T , and accordingly if a linear operator U commutes with T then U commutes with each of the * * f i.e. , each subspace * * f is invariant under U . 
Meaning of the word marked with <WSD> tags:  the analysis of a vector field