Power Series Expansion for Logarithm of 1 + x over 1 + x

Theorem
where $H_n$ denotes the $n$th harmonic number:
 * $H_n = \ds \sum_{r \mathop = 1}^n \dfrac 1 r = 1 + \dfrac 1 2 + \dfrac 1 3 \cdots + \dfrac 1 r$

valid for all $x \in \R$ such that $\size x < 1$.

Proof
Let $\map f x = \dfrac {\map \ln {1 + x} } {1 + x}$.

By definition of Maclaurin series:


 * $(1): \quad \map f x \sim \ds \sum_{n \mathop = 0}^\infty \frac {x^n} {n!} \map {f^{\paren n} } 0$

where $\map {f^{\paren n} } 0$ denotes the $n$th derivative of $f$ $x$ evaluated at $x = 0$.

From Nth Derivative of Natural Logarithm by Reciprocal:


 * $\dfrac {\d^n} {\d x^n} \dfrac {\map \ln {1 + x} } {1 + x} = \paren {-1}^{n + 1} n! \dfrac {H_n - \map \ln {1 + x} } {\paren {1 + x}^{n + 1} }$

The result follows by setting $x = 0$ and substituting for $\map {f^{\paren n} } 0$ in $(1)$.