Travis, Stephen P. “A Representation for an Adjoint Operator”.
Anal. Numér. Théor. Approx.
4, no. 1 (February 1, 1975): 105–111. Accessed April 14, 2025.
https://ictp.acad.ro/jnaat/journal/article/view/1975-vol4-no1-art13
.