Strogatz, S. H., & Joffray, D. (2009). The calculus of friendship: What a teacher and a student learned about life while corresponding about math. Princeton University Press.

Strogatz, Steven H., and Don Joffray. The Calculus of Friendship: What a Teacher and a Student Learned About Life While Corresponding About Math. Princeton: Princeton University Press, 2009.

Strogatz, Steven H., and Don Joffray. The Calculus of Friendship: What a Teacher and a Student Learned About Life While Corresponding About Math. Princeton University Press, 2009.