Smooth projective horospherical varieties with nef tangent bundles Qifeng Li Indiana University Mathematics Journal 2017 text pdf 10.1512/iumj.2017.66.6265 10.1512/iumj.2017.66.6265 en Indiana Univ. Math. J. 66 (2017) 1879 - 1893 state-of-the-art mathematics http://iumj.org/access/