Euler's number
= Euler's number
{c}
{title2=$e$}
{wiki=E_(mathematical_constant)}
Euler's number is the value $\exp(1)=\sum_{n=0}^{\infty}1/n!$ of the <exponential function>, approximately $2.71828$. It is the base of the natural <logarithm>. Its factorial-series tail gives an elementary proof of the <irrationality of e>.