@Misc{ Oro22a, author = { Oronzio, Francesca }, title = { An example in dimension four of a Riemannian manifold such that its sectional curvature is positive and its curvature operator is not nonnegative }, journal = { }, year = { 2022 }, pages = { }, URL = { http://cvgmt.sns.it/paper/5789/ }, note = { cvgmt preprint} }