**Leonhard Euler** was a Swiss mathematician and physicist. He worked in almost all areas of mathematics as well as continuum physics, lunar theory and other areas of physics. He is a seminal figure in the history of mathematics. He made important discoveries in fields as diverse as infinitesimal calculus and graph theory. He also introduced much of the modern mathematical terminology and notation, particularly for mathematical analysis. He is renowned for his work in mechanics, fluid dynamics, optics, and astronomy. Leonhard Euler spent most of his adult life in Russia and in Berlin, Prussia. He is considered to be the preeminent mathematician of the 18th century, and one of the greatest mathematicians to have ever lived.

**Leonhard Euler**‘s eyesight worsened throughout his mathematical career. Three years after suffering a near-fatal fever in 1735 he became nearly blind in his right eye and later suffered a cataract in his good left eye, rendering him almost totally blind. Even so, his condition appeared to have little effect on his productivity, as he compensated for it with his mental calculation skills and photographic memory. For example, Euler could repeat the Aeneid of Virgil from beginning to end without hesitation, and for every page in the edition he could indicate which line was the first and which the last.

**Leonhard Euler** is the only mathematician to have two numbers named after him:

- the Euler’s Number in calculus, e, approximately equal to 2.71828
- and the Euler-Mascheroni Constant γ (gamma) sometimes referred to as just “Euler’s constant”, approximately equal to 0.57721.

Leonhard Euler was featured on the sixth series of the Swiss 10-franc banknote and on numerous Swiss, German, and Russian postage stamps. The asteroid 2002 Euler was also named in his honor.