David_Hilbert_S_Cohn-Vossen_-_Geometry_and_the_Imagination.djvuPreprint_files/David_Hilbert_S_Cohn-Vossen_-_Geometry_and_the_Imagination.djvuhttp://www.dmi.unict.it/~frusso/DMI/Preprint_files/333_revised_version_november2010_7.pdfshapeimage_2_link_0