.. index:: Clausen functions
The Clausen function is defined by the following integral,
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.. math:: Cl_2(x) = - \int_0^x dt \log{\left( 2 \sin{(t/2)} \right)}
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Cl_2(x) = - \int_0^x dt log( 2 \sin(t/2) )
It is related to the :ref:`dilogarithm <dilog-function>` by
:math:`Cl_2(\theta) = \Im Li_2(\exp(i\theta))`.
The Clausen functions are declared in the header file
:file:`gsl_sf_clausen.h`.
.. function:: double gsl_sf_clausen (double x)
int gsl_sf_clausen_e (double x, gsl_sf_result * result)
These routines compute the Clausen integral :math:`Cl_2(x)`.