Jean-Robert Argand (UK: /ˈɑːrɡænd/, US: /ˌɑːrˈɡɑːn(d)/, French: [ʒɑ̃ ʁɔbɛʁ aʁɡɑ̃]; July 18, 1768 – August 13, 1822) was an amateur mathematician. In 1806, while managing a bookstore in Paris, he published the idea of geometrical interpretation of complex numbers known as the Argand diagram and is known for the first rigorous proof of the Fundamental Theorem of Algebra.