Rautenberg (2010, p. 57): "The formulas procured by (F1) and (F2) are said to be prime or atomic formulas, or simply called prime. As in propositional logic, prime formulas and their negations are called literals." Rautenberg, Wolfgang (2010). A Concise Introduction to Mathematical Logic. Universitext (3rd ed.). Springer. doi:10.1007/978-1-4419-1221-3. ISBN978-1-4419-1220-6.