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+@cindex Clausen functions
+
+The Clausen function is defined by the following integral,
+@tex
+\beforedisplay
+$$
+Cl_2(x) = - \int_0^x dt \log(2 \sin(t/2))
+$$
+\afterdisplay
+@end tex
+@ifinfo
+
+@example
+Cl_2(x) = - \int_0^x dt \log(2 \sin(t/2))
+@end example
+
+@end ifinfo
+@noindent
+It is related to the dilogarithm by
+@c{$Cl_2(\theta) = \Im(Li_2(e^{i\theta}))$}
+@math{Cl_2(\theta) = \Im Li_2(\exp(i\theta))}.
+The Clausen functions are declared in the header file
+@file{gsl_sf_clausen.h}.
+
+@deftypefun double gsl_sf_clausen (double @var{x})
+@deftypefunx int gsl_sf_clausen_e (double @var{x}, gsl_sf_result * @var{result})
+These routines compute the Clausen integral @math{Cl_2(x)}.
+@end deftypefun