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Fillmore Peter A.; Stampfli Joseph Gail; Williams J. P.: On the essential numerical range, the essential spectrum, and a problem of Halmos. In: Acta scientiarum mathematicarum, (33) 3-4. pp. 179-192. (1972)
Fillmore Peter A.: Sums of operators with square zero. In: Acta scientiarum mathematicarum, (28) 3-4. pp. 285-288. (1967)