Another [irrational] number. Available in [expr] as ''exp(1)''.

base of natural logarithms; useful since d/dx(e^x) = e^x.

And Integral(1/x) = ln(x)  Integral(1/x) from 1 to e is ln(e) = 1.

2.718281828459... (see http://mathworld.wolfram.com/e.html) Note it does not recur, despite repeating 1828 twice in the first 10 digits.