Steven Strogatz
The Calculus of Friendship
2009 BVT 510.922 STR
Don Joffray ( 1929 – 2020 ) taught asdf at the Loomis Chaffee private preparatory school in Windsor CT, retiring in 1999, followed by a mild stroke in 2004.
15 yo Strogatz (born August 1959) took Joffray's calculus class in 1974, and exchanged about one letter a year afterwards. Correspondence increased a decade later, while Strogatz earned a mathematics MS from Princeton, graduate work at Cambridge, and a PhD from Harvard in 1986. Strogatz joined the MIT faculty in 1989 and moved to Cornell in 1994.
The book is a collection of mathematics puzzles discussed those letters, mostly algebra with some simple calculus. The most memorable problem is this formula for the Nth term of the Fibonacci sequence:
F_n = { \Large { ( 1 + \sqrt{ 5 } )^n -( 1 - \sqrt{ 5 } )^n } \over { 2^n \sqrt{ 5 } } }