Holmstrom Homotopy pullback square

See Hovey p. 184 for definition and properties. Or Dundas, page 39.

http://mathoverflow.net/questions/8684/homotopy-pullbacks-and-homotopy-pushouts

nLab page on Homotopy pullback square

Created on June 9, 2014 at 21:16:13 by Andreas Holmström