Ei(x) - GNU Scientific Library -- Reference Manual
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#### 7.17.2 Ei(x)

— Function: double gsl_sf_expint_Ei (double x)
— Function: int gsl_sf_expint_Ei_e (double x, gsl_sf_result * result)

> These routines compute the exponential integral Ei(x),

>               Ei(x) := - PV(\int_{-x}^\infty dt \exp(-t)/t)

> where PV denotes the principal value of the integral.

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