He made big contributions in graph theory, calculus and more. I have attached a link with lots of information about this amazing man.

Leonhard Euler made many contributions to math but the one thing he is most famous for is changing the base of the natural logarithim by using the letter e which is aka Euler's number

Leonhard Euler (after whom it was named).

Leonhard Euler (after whom it was named).

Leonhard Euler (after whom it was named).

Leonhard Euler (after whom it was named).

Leonhard Euler was a Swiss mathematician and scientist in the 18th century.