books.google.co.uk - The main purpose of part I of these notes is to develop for a ring R a functional notion of R-completion of a space X. For R=Zp and X subject to usual finiteness condition, the R-completion coincides up to homotopy, with the p-profinite completion of Quillen and Sullivan; for R a subring of the rationals,...http://books.google.co.uk/books/about/Homotopy_Limits_Completions_and_Localiza.html?id=6Mw2plpWo2sC&q=homotopy+category&utm_source=gb-gplus-shareHomotopy Limits, Completions and Localizations
