1. K. Gustafson, A Perturbation Lemma, Bull. Amer. Math. Soc. 72 (1966), 334–338.
2. K. Gustafson, Positive Operator Products, Notices Amer. Math. Soc. 14 (1967), Abstract 67T-531, p. 717. See also Abstracts 67T-340, 67T-564, 67T-675.
3. K. Gustafson, A Note on Left Multiplication of Semi-group Generators, Pacific J. Math. 24 (1968a), 463–465.
4. K. Gustafson, The Angle of an Operator and Positive Operator Products, Bull. Amer. Math. Soc. 74 (1968b), 488–492.
5. K. Gustafson, Positive (noncommuting) Operator Products and Semigroups, Math. Zeitschrift 105 (1968c), 160–172.