Sentence:  Such an operator is obviously the zero operator ; for since it is nilpotent , the minimal polynomial for this operator is of the form * * f for some * * f ; but then since the operator is diagonalizable , the minimal polynomial cannot have a repeated <WSD>root</WSD> ; hence * * f and the minimal polynomial is simply x , which says the operator is 0 . 
Meaning of the word marked with <WSD> tags:  a number that, when multiplied by itself some number of times, equals a given number