@article{TAC_2008_20_a13,
     author = {F. William Lawvere},
     title = {Core varieties, extensivity, and rig geometry},
     journal = {Theory and applications of categories},
     pages = {497--503},
     publisher = {mathdoc},
     volume = {20},
     year = {2008},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/TAC_2008_20_a13/}
}