Planck 2013 results. XXVI. Background geometry and topology of the Universe
P.A.R. Ade, N. Aghanim, C. Armitage-Caplan, M. Arnaud, M. Ashdown, F. Atrio-Barandela, Per Rex Christensen, Pavel Nasselski, Igor Dmitrievich Novikov, Hans Ulrik Nørgaard-Nielsen