@TECHREPORT{AbsMahTru2013.01, author = "P.-A. Absil and Robert Mahony and Jochen Trumpf", title = "An extrinsic look at the {Riemannian Hessian}", institution = "U.C.Louvain", year = 2013, number = "UCL-INMA-2013.01-v2", month = "February", url = "http://sites.uclouvain.be/absil/2013.01", }