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.