Board index Special Functions An imaginary part

## An imaginary part

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### An imaginary part

Sun Feb 07, 2016 9:13 am

Posts: 47

How would one prove that:

$$\Im({\rm Li}_2(iz))={\rm Ti}_2(z)$$

where ${\rm Ti}_2$ is the inverse tangent integral.

### Re: An imaginary part

Mon Feb 08, 2016 12:44 pm

Posts: 30
Location: India

Hi VJKey,

It follows pretty much from the definition for $z \in \mathbb{R}$: \begin{align*}\operatorname{Li}_2(iz) &= -\int_0^{z} \frac{\log (1-ix)}{x}\,dx \\&= -\int_0^{z} \frac{\log \left((1+x^2)^{1/2}e^{-i\arctan x}\right)}{x}\,dx \\&= -\frac{1}{2}\int_0^{z} \frac{\log \left(1+x^2\right)}{x}\,dx + i \int_0^{z} \frac{\arctan x}{x}\,dx \\&= -\frac{1}{4}\int_0^{-z^2} \frac{\log \left(1-x\right)}{x}\,dx + i \int_0^{z} \frac{\arctan x}{x}\,dx \\&= \frac{1}{4}\operatorname{Li}_2(-z^2) + i\operatorname{Ti}_2(z)\end{align*}

### Re: An imaginary part

Wed Feb 10, 2016 8:28 am

Posts: 47

Nicely done r9m. I had not thought that ${\rm Li}_2(iz)$ would have a real part. Thank you.