APA (7th ed.) Citation

Courant, R., & Robbins, H. (1969). What is mathematics ? Oxford University Press.

Chicago Style (17th ed.) Citation

Courant, Richard, and Herbert Robbins. What Is Mathematics ? Oxford University Press, 1969.

MLA (8th ed.) Citation

Courant, Richard, and Herbert Robbins. What Is Mathematics ? Oxford University Press, 1969.

Warning: These citations may not always be 100% accurate.