Monday, June 8, 2009

Giovanni Domenico Cassini (1625–1712)

Giovanni Domenico Cassini
was born on June 8, 1625. He investigated the curves that are now known as the Cassini ovals defined by equations of the form

where q1 and q2 are fixed points, "dist" is the Euclidean distance in the plane, and b is a constant. The above picture displays a few Cassini ovals, with foci q1(-1, 0) and q2(1, 0), annotated with the value of b2.

In 1680, while director of the Paris Observatory he discovered the identity

satisfied by the Fibonacci numbers. This is now known as the Cassini identity. A quick proof by using determinants:

A bijective proof was provided by M. Werman and D. Zeilberger - "A bijective proof of Cassini's Fibonacci identity", Discrete Mathematics 58 (1), 109 (1986).

Also see the Cassini biography from MacTutor.