The **Euler Number**, symbolized *e*, is an irrational transcendental number approximately equal to 2.718281828459045 . It can be used in logarithms as the base, called a natural logarithm. It is named for Swiss mathematician Leonhard Euler, though he did not discover the constant.

It has some remarkable properties. For example:

(i.e. the exponential function is an eigenfunction of the derivative operator, with eigenvalue 1).