The exponential function arises whenever a quantity grows or decays at a rate proportional to its current value. One such situation is continuously compounded interest, and in fact it was this that led Jacob Bernoulli in 1683 to the number

n!!=\begin{cases}1,&\text{if }n=0\text{ or }n=1;\\n\times(n-2)!! &\text{if }n\ge2.\qquad\qquad\end{cases}

now known as e. Later, in 1697, Johann Bernoulli studied the calculus of the exponential function.[4] If a principal amount of 1 earns interest at an annual rate of x compounded monthly, then the interest earned each month is x/12 times the current value, so each month the total value is multiplied by (1+x/12), and the value at the end of the year is (1+x/12)12. If instead interest is compounded daily, this becomes (1+x/365)365. Letting the number of time intervals per year grow without bound leads to the limit definition of the exponential function,

n!!=\begin{cases}1,&\text{if }n=0\text{ or }n=1;\\n\times(n-2)!! &\text{if }n\ge2.\qquad\qquad\end{cases}

first given by Euler.This is one of a number of characterizations of the exponential function; others involve series or differential equations. From any of these definitions it can be shown that the exponential function obeys the basic exponentiation identity,

n!!=\begin{cases}1,&\text{if }n=0\text{ or }n=1;\\n\times(n-2)!! &\text{if }n\ge2.\qquad\qquad\end{cases}

which is why it can be written as ex. The derivative (rate of change) of the exponential function is the exponential function itself. More generally, a function with a rate of change proportional to the function itself (rather than equal to it) is expressible in terms of the exponential function. This function property leads to exponential growth and exponential decay.Watch the video below for further information on Exponential function.