Hodges, Wilfrid; Scanlon, Thomas (2018), "First-order Model Theory", in Zalta, Edward N. (ed.), The Stanford Encyclopedia of Philosophy (Winter 2018 ed.), Metaphysics Research Lab, Stanford University, retrieved 2019-10-31
wolfram.com
mathworld.wolfram.com
Weisstein, Eric W. "Theory". mathworld.wolfram.com. Retrieved 2019-10-31.