@inproceedings{mogens1978jacobi,
title={Jacobi functions: The addition formula and the positivity of the dual convolution structure},
author={Mogens Flensted-Jensen, and Tom H. Koornwinder},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203505600923311},
booktitle={Arkiv for Matematik},
volume={17},
number={1},
pages={139-151},
year={1978},
}

Mogens Flensted-Jensen, and Tom H. Koornwinder. Jacobi functions: The addition formula and the positivity of the dual convolution structure. 1978. Vol. 17. In Arkiv for Matematik. pp.139-151. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203505600923311.