Ebbinghaus, H.-D.; Flum, J.; Thomas, W. (1994). Mathematical logic (2. ed., 4. pr. ed.). New York [u.a.]: Springer. pp. 50. ISBN978-0-387-94258-2. http://www.springer.com/mathematics/book/978-0-387-94258-218 July 2012閲覧. "However, in first-order logic we may not form infinitely long disjunctions. Indeed, we shall later show that there is no set of first-order formulas whose models are precisely the periodic groups."