↑Varian, Hal (1992). "Choice". Microeconomic Analysis (Third ed.). New York: W.W. Norton. p. 117. ISBN978-0-393-95735-8. สืบค้นเมื่อ 4 October 2019. [...] as the consumer gets more income, he consumes more of both goods but proportionally more of one good (the luxury good) than of the other (the necessary good).