Jan Kisyński. On cosine operator functions and one-parameter groups of operators. Studia Mathematica, 44:93-105, 1972. Collection of articles honoring the completion by Antoni Zygmund of 50 years of scientific activity, I.

• Mathematical Reviews review by J. A. Goldstein (subscription required)
• Zentralblatt MATH entry

• electronic copy of the paper from ICM (free access, PDF file, 4.8MB)