Corey Brennan, The Praetorship in the Roman Republic (Oxford University Press, 2000), p. 605 et passim; Clifford Ando, "From Republic to Empire," in The Oxford Handbook of Social Relations in the Roman World (Oxford University Press, pp.
Christopher Kelly, The Roman Empire: A Very Short Introduction (Oxford University Press, 2006), p. 4ff.; Claude Nicolet, Space, Geography, and Politics in the Early Roman Empire (University of Michigan Press, 1991, originally published in French 1988), pp. 1, 15; T. Corey Brennan, The Praetorship in the Roman Republic (Oxford University Press, 2000), p. 605 et passim; Clifford Ando, "From Republic to Empire," in The Oxford Handbook of Social Relations in the Roman World (Oxford University Press, pp. 39–40.