Power Series Expansion for Logarithm of Tangent of x

Theorem
for all $x \in \R$ such that $0 < \size x < \dfrac \pi 2$.

Proof
From Power Series Expansion for Cosecant Function:

for $0 < \size {2 x} < \pi$.

From Power Series is Termwise Integrable within Radius of Convergence, $(1)$ can be integrated term by term:

for $0 < \size x < \dfrac \pi 2$.