Two different functions of real argument x \in \mathbb{R} are called this way:
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which are related to each other as:
(3) | {\rm Ei}(x) = - E_1(-x) |
There is a trend to moving from \rm Ei definition which was dominating in the past towards \rm E_1.
For the small arguments |x| \ll 1 it can be approximated as:
-1 \ll x <0 | 0 < x \ll 1 | ||||
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where \gamma = 0.577 ... is Euler–Mascheroni constant.