e(x) = (1+x+\frac {x^2}{2!} + ... + \frac {x^{10}} {10!})(1+x+\frac {x^2}{2!} + ... + \frac {x^{15}} {15!}) (1+x+\frac {x^2}{2!} + ... + \frac {x^{20}} {20!})