Weak subobjects and weak limits in categories and homotopy categories Marco Grandis (1997) Cahiers de Topologie et Géométrie Différentielle Catégoriques
What are sifted colimits? Adamek, J., Rosicky, J., Vitale, E.M. (2010) Theory and Applications of Categories [electronic only]
When the product-preserving functors preserve limits Věra Trnková (1970) Commentationes Mathematicae Universitatis Carolinae