Abian, Alexander. “The Angle Between Two Sets and Their Inner Product in Any Banach Space”.
Anal. Numér. Théor. Approx.
19, no. 1 (February 1, 1990): 1–6. Accessed April 14, 2025.
https://ictp.acad.ro/jnaat/journal/article/view/1990-vol19-no1-art1
.