show this:


I found wiki Continued fraction also not have this problem,maybe this problem can't have simple closed form? Thank you for you give someusefull

  • $\begingroup$ Your question edit makes my answer quite useless (and incomprehensible for future readers)! $\endgroup$
    – Martin R
    Dec 24, 2014 at 12:47

2 Answers 2


The entire function $g(z)=\frac{\sinh z}{z}$ satisfies the differential equation $(z g)''=z g$, or: $$ zg''+2g' = zg,\tag{1}$$ or: $$ \frac{g'}{g}=\frac{1}{\frac{2}{z}+\frac{g''}{g'}}.\tag{2}$$ By differentiating $(1)$, we get similar expressions for $\frac{g^{(n+1)}}{g^{n}}$, hence a continued fration representation for: $$ \frac{g'}{g}=\frac{d}{dz}\log g = \coth z - \frac{1}{z}, $$ as wanted.


According to The WOLFRAM Functions Site, that is the continued fraction of $ \alpha = \coth 1 = (e^2+1)/(e^2-1)$.

The continued fraction for $\coth z$ (with proof) can be found in the Handbook of Continued Fractions for Special Functions.


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