Schlichtkrull, Henrik. “One-Dimensional $K$-Types in Finite Dimensional Representations of Semisimple Lie Groups: A Generalization of Helgason’s Theorem”. MATHEMATICA SCANDINAVICA 54 (June 1, 1984): 279–294. Accessed April 5, 2025. https://www.mscand.dk/article/view/12059.