(related to Proposition: Functional Equation of the Exponential Function of General Base (Revised))

- Assume that \(f:\mathbb R\to \mathbb R\) obeys the functional equation \(f(x+y)=f(x)\cdot f(y)\) for all \(x,y\in\mathbb R\).
- Because of the functional equation, we must have \[f(1)=f\left(\frac 12\right)^2.\]
- Therefore, in any case \(f(1)\ge 0\).
- Suppose \(f(1)=0\).
- Then, \(f\) is a constant function and equals \(0\), since:

\[\begin{array}{rcll} f(x)&=&f(x+0)&\text{existence of real zero}\\ f(x)&=&f(x+(-1)+1)&\text{existence of inverse real numbers with respect to addition}\\ &=&f((x-1)+1)&\text{associativity of addition}\\ &=&f(x-1)\cdot f(1)&\text{by assumed functional equation}\\ &=&f(x-1)\cdot 0&\text{by assumption }f(1)=0\\ &=&0&\text{existence of real zero}\\ \end{array}\]

Above, we have used the existence of real zero, the existence of inverse real numbers with respect to addition, and the associativity law of adding real numbers. Now, suppose \(f(1) > 0\). Set \(a:=f(1)\). Then, it follows from the uniqueness of 1 and from \(a=f(1 + 0)=f(1)\cdot f(0)=a\cdot f(0)\) that \(f(0)=1\).

Next, we will prove that

\((i)\) \(f(n)=a^n\) for all natural numbers \(n\in \mathbb N\)

\((ii)\) \(f(n)=a^n\) for all integers \(n\in \mathbb Z\)

\((iii)\) \(f\left(\frac pq\right)=\sqrt[q]a^p\) for all rational numbers \(\frac pq\in \mathbb Q\)

\((iv)\) \(f(x)=a^x\) for all real numbers \(x\in \mathbb R\), if \(f\) in addition is a continuous real function.

By induction, for the base case \(n=1\), \(f(1)=a\), and for the induction step \(f(n+1)=f(n)\cdot f(1)=a^n\cdot a=a^{n+1}.\)

From the functional equation, it follows \[1=0=f(n+(-n))=f(n)\cdot f(-n)\] and therefore \[f(-n)=\frac 1{f(n)}=\frac 1{a^n}=a^{-n}.\]

From \(a^p=f(p)=f\left(q\cdot \frac pq\right)=\left(f\left(\frac pq\right)\right)^q\), it follows \(\sqrt[q]a^p=f\left(\frac pq\right)\).

Let \(x\) be a real number, i.e. \(x\) is a class of all rational Cauchy sequences, which equal each other except a difference, which is a rational Cauchy sequence converging to zero, formally \[x\in\mathbb R\Longleftrightarrow x=(x_n)_{n\in\mathbb N}+I\] where \((x_n)_{n\in\mathbb N}\) is a rational Cauchy sequence representing the real number \(x\), and

\[I:=\{(i_n)_{n\in\mathbb N}~|~i_n\in\mathbb Q,\lim i_n=0\}\] is the set of all rational sequences, which converge to \(0\). Now, we have

\[\begin{array}{rcll} a^x&=&\lim_{n\to\infty}a^{x_n+i_n}&\text{due to the definition of real numbers}\\ &=&\lim_{n\to\infty}f(x_n+i_n)&\text{follows from }( i i i )\\ &=&\lim_{n\to\infty}f(x_n)\cdot f(i_n)&\text{by the assumed functional equation}\\ &=&f(x)\cdot f(0)&\text{continuity of }f\\ &=&f(x)\cdot 1&\text{according to }f(0)=1\\ &=&f(x)&\text{multiplication by }1. \end{array}\]

Above, we have used, in addition, the existence of the real number 1. Finally, it remains to be shown that \(f(x)=\exp_a(x)\) for all \(x\in\mathbb R\) and all positive real numbers \(a > 0\). This follows immediately from \((iv)\) and the proposition about general powers of positive numbers.

∎

**Forster Otto**: "Analysis 1, Differential- und Integralrechnung einer VerĂ¤nderlichen", Vieweg Studium, 1983