The Exponential Differential Equation
So we long ago defined the exponential function to be the inverse of the logarithm, and we showed that it satisfied the exponential property. Now we’ve got another definition, using a power series, which is its Taylor series at . And we’ve shown that this definition also satisfies the exponential property.
But what really makes the exponential function what it is? It’s the fact that the larger the function’s value gets, the faster it grows. That is, the exponential function satisfies the equation . We already knew this about , but there we ultimately had to use the fact that we defined the logarithm to have a specified derivative. Here we use this property itself as a definition.
This is our first “differential equation”, which relates a function to its derivative(s). And because differentiation works so nicely for power series, we can use them to solve differential equations.
So let’s take our equation as a case in point. First off, any function that satisfies this equation must by definition be differentiable. And then, since it’s equal to its own derivative, this derivative must itself be differentiable, and so on. So at the very least our function must be infinitely differentiable. Let’s go one step further and just assume that it’s analytic at . Since it’s analytic, we can expand it as a power series.
So we have some function defined by a power series around :
We can easily take the derivative
Setting these two power series equal, we find that , , , and so on. In general:
And we have no restriction on . Thus we come up with our series solution
which is just times the series definition of our exponential function ! If we set the initial value , then the unique solution to our equation is the function
which is our new definition of the exponential function. The differential equation motivates the series, and the series gives us everything else we need.