The curve [math]x^3+y^3=6xy[/math] is called the "folium of Descartes" ("folium" is Latin for "leaf"). This equation first came out in a letter from Descartes to Fermat (Fermat was a lawyer by profession, but did mathematics as a hobby). Fermat claimed to have a method to find tangent lines to any curve, and Descartes invented this curve as a challenge to Fermat. Fermat was successful in finding tangent lines to the curve. Today, however, the problem of finding tangent lines is straightforward, due to the invention of calculus sometime later. This event was notable enough in the history of calculus that it is memorialized in the Albanian postage stamp above.
Source: http://goo.gl/JhLTgE
