APA (7th ed.) Citation
Courant, R., & Robbins, H. (1941). What is mathematics?: An elementary approach to ideas and methods. Oxford university press.
Chicago Style (17th ed.) Citation
Courant, Richard, and Herbert Robbins. What Is Mathematics?: An Elementary Approach to Ideas and Methods. London: Oxford university press, 1941.
MLA (9th ed.) Citation
Courant, Richard, and Herbert Robbins. What Is Mathematics?: An Elementary Approach to Ideas and Methods. Oxford university press, 1941.
Warning: These citations may not always be 100% accurate.