Spectroscopic observations and analysis of the unusual type Ia SN 1999ac - Garavini, G; Aldering, G; Amadon, A; Amanullah, R; Astier, P; Balland, C; Blanc, G; Conley, A; Dahlen, T; Deustua, SE; Ellis, R; Fabbro, S; et al. (ASTRONOMICAL JOURNAL, 2005)