Equivalence of Definitions of Real Exponential Function

Theorem
All the definitions of the exponential function are equivalent.


 * $(1): \quad y = \exp x \iff \ln y = x$:


 * $\quad \quad \ln y = \displaystyle \int_{t=1}^{t=y} \frac 1 t \ \mathrm dt$


 * $(2): \quad \exp x = e^x$


 * $(3): \quad \exp x = \displaystyle \lim_{n \to \infty} \left \langle {\left({1 + \frac x n}\right)^n} \right \rangle$


 * $(4): \quad \exp x = \displaystyle \sum_{n = 0}^\infty \frac {x^n} {n!}$


 * $(5): \quad y = f\left({x}\right)$: $y = \dfrac {\mathrm dy}{\mathrm dx}$, $f\left({0}\right) = 1$

1 implies 5
This proves that $y$ is a solution.

Now, let's check if it fulfills the initial condition.

... indeed it does.

5 implies 1
Now to solve for $C$, put $\left(x_0,y_0\right) = \left(0,1\right)$: