Mathematics - 1974
Campbell, S. L. (1974). Commutation properties of the coefficient matrix in the differential equation of an inner function. Proceedings of the American Mathematical Society, 42(2), 507–512.
Campbell, S. (1974). Linear operators for which T∗T and TT∗ commute. II. Pacific Journal of Mathematics, 53(2), 355–361. https://doi.org/10.2140/pjm.1974.53.355