U(2) invariant four dimensional Einstein metrics Peter PetersenShunhui Zhu Indiana University Mathematics Journal 1995 text pdf 10.1512/iumj.1995.44.1996 10.1512/iumj.1995.44.1996 en Indiana Univ. Math. J. 44 (1995) 451 - 466 state-of-the-art mathematics http://iumj.org/access/