#### 7.17.1 Exponential Integral

Function: double gsl_sf_expint_E1 (double x)
Function: int gsl_sf_expint_E1_e (double x, gsl_sf_result * result)

  These routines compute the exponential integral E_1(x),

      E_1(x) := \Re \int_1^\infty dt \exp(-xt)/t.

Function: double gsl_sf_expint_E2 (double x)
Function: int gsl_sf_expint_E2_e (double x, gsl_sf_result * result)

  These routines compute the second-order exponential integral E_2(x),

      E_2(x) := \Re \int_1^\infty dt \exp(-xt)/t^2.

Function: double gsl_sf_expint_En (int n, double x)
Function: int gsl_sf_expint_En_e (int n, double x, gsl_sf_result * result)

  These routines compute the exponential integral E_n(x) of order n,

      E_n(x) := \Re \int_1^\infty dt \exp(-xt)/t^n.