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Reference.org
Exponential integral
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<p>In mathematics, the exponential integral Ei is a <a href="/facts/Special_function/JETSeucA">special function</a> on the <a href="/facts/Complex_plane/vE0QrxJp">complex plane</a>. </p><p>It is defined as one particular <a href="/facts/Definite_integral/V7lyV997">definite integral</a> of the ratio between an <a href="/facts/Exponential_function/ewlYxvR2">exponential function</a> and its <a href="/facts/Argument_of_a_function/QkuPc28I">argument</a>. </p>