@article{TAC_2011_25_a12,
     author = {Ulrich H\"ohle},
     title = {Covariant presheaves and subalgebras},
     journal = {Theory and applications of categories},
     pages = {342--367},
     publisher = {mathdoc},
     volume = {25},
     year = {2011},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/TAC_2011_25_a12/}
}