Curvature and Focal Points in Riemannian Foliations James Hebda Indiana University Mathematics Journal 1986 text pdf 10.1512/iumj.1986.35.35019 10.1512/iumj.1986.35.35019 en Indiana Univ. Math. J. 35 (1986) 321 - 331 state-of-the-art mathematics http://iumj.org/access/