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   http://functions.wolfram.com/06.34.04.0013.01
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    Limit[ExpIntegralE[\[Nu], x - I \[Epsilon]], \[Epsilon] -> Plus[0]] == 
  ExpIntegralE[\[Nu], x] - (2 Pi I x^(\[Nu] - 1))/
    (E^(Pi I \[Nu]) Gamma[\[Nu]]) /; x < 0 
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   Cell[BoxData[RowBox[List[RowBox[List[RowBox[List["Limit", "[", RowBox[List[RowBox[List["ExpIntegralE", "[", RowBox[List["\[Nu]", ",", RowBox[List["x", "-", RowBox[List["\[ImaginaryI]", " ", "\[Epsilon]"]]]]]], "]"]], ",", RowBox[List["\[Epsilon]", "\[Rule]", RowBox[List["+", "0"]]]]]], "]"]], "\[Equal]", " ", RowBox[List[RowBox[List["ExpIntegralE", "[", RowBox[List["\[Nu]", ",", "x"]], "]"]], "-", FractionBox[RowBox[List["2", " ", "\[Pi]", " ", "\[ImaginaryI]", " ", SuperscriptBox["\[ExponentialE]", RowBox[List[RowBox[List["-", "\[Pi]"]], " ", "\[ImaginaryI]", " ", "\[Nu]"]]], " ", SuperscriptBox["x", RowBox[List["\[Nu]", "-", "1"]]]]], RowBox[List["Gamma", "[", "\[Nu]", "]"]]]]]]], "/;", RowBox[List["x", "<", "0"]]]]]] 
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   <math xmlns='http://www.w3.org/1998/Math/MathML' mathematica:form='TraditionalForm' xmlns:mathematica='http://www.wolfram.com/XML/'>  <semantics>  <mrow>  <mrow>  <mrow>  <munder>  <mi> lim </mi>  <mrow>  <mi> ϵ </mi>  <semantics>  <mo> → </mo>  <annotation encoding='Mathematica'> "\[Rule]" </annotation>  </semantics>  <mrow>  <mo> + </mo>  <mn> 0 </mn>  </mrow>  </mrow>  </munder>  <mo> ⁢ </mo>  <mtext>   </mtext>  <mrow>  <msub>  <semantics>  <mi> E </mi>  <annotation encoding='Mathematica'> TagBox["E", ExpIntegralE] </annotation>  </semantics>  <mi> ν </mi>  </msub>  <mo> ( </mo>  <mrow>  <mi> x </mi>  <mo> - </mo>  <mrow>  <mi> ⅈ </mi>  <mo> ⁢ </mo>  <mi> ϵ </mi>  </mrow>  </mrow>  <mo> ) </mo>  </mrow>  </mrow>  <mo> ⩵ </mo>  <mrow>  <mrow>  <msub>  <semantics>  <mi> E </mi>  <annotation encoding='Mathematica'> TagBox["E", ExpIntegralE] </annotation>  </semantics>  <mi> ν </mi>  </msub>  <mo> ( </mo>  <mi> x </mi>  <mo> ) </mo>  </mrow>  <mo> - </mo>  <mfrac>  <mrow>  <mn> 2 </mn>  <mo> ⁢ </mo>  <mi> π </mi>  <mo> ⁢ </mo>  <mi> ⅈ </mi>  <mo> ⁢ </mo>  <msup>  <mi> ⅇ </mi>  <mrow>  <mrow>  <mo> - </mo>  <mi> π </mi>  </mrow>  <mo> ⁢ </mo>  <mi> ⅈ </mi>  <mo> ⁢ </mo>  <mi> ν </mi>  </mrow>  </msup>  <mo> ⁢ </mo>  <msup>  <mi> x </mi>  <mrow>  <mi> ν </mi>  <mo> - </mo>  <mn> 1 </mn>  </mrow>  </msup>  </mrow>  <mrow>  <mi> Γ </mi>  <mo> ⁡ </mo>  <mo> ( </mo>  <mi> ν </mi>  <mo> ) </mo>  </mrow>  </mfrac>  </mrow>  </mrow>  <mo> /; </mo>  <mrow>  <mi> x </mi>  <mo> < </mo>  <mn> 0 </mn>  </mrow>  </mrow>  <annotation-xml encoding='MathML-Content'>  <apply>  <ci> Condition </ci>  <apply>  <eq />  <apply>  <limit />  <bvar>  <ci> ϵ </ci>  </bvar>  <condition>  <apply>  <tendsto />  <ci> ϵ </ci>  <apply>  <plus />  <cn type='integer'> 0 </cn>  </apply>  </apply>  </condition>  <apply>  <ci> ExpIntegralE </ci>  <ci> ν </ci>  <apply>  <plus />  <ci> x </ci>  <apply>  <times />  <cn type='integer'> -1 </cn>  <apply>  <times />  <imaginaryi />  <ci> ϵ </ci>  </apply>  </apply>  </apply>  </apply>  </apply>  <apply>  <plus />  <apply>  <ci> ExpIntegralE </ci>  <ci> ν </ci>  <ci> x </ci>  </apply>  <apply>  <times />  <cn type='integer'> -1 </cn>  <apply>  <times />  <cn type='integer'> 2 </cn>  <pi />  <imaginaryi />  <apply>  <power />  <exponentiale />  <apply>  <times />  <apply>  <times />  <cn type='integer'> -1 </cn>  <pi />  </apply>  <imaginaryi />  <ci> ν </ci>  </apply>  </apply>  <apply>  <power />  <ci> x </ci>  <apply>  <plus />  <ci> ν </ci>  <cn type='integer'> -1 </cn>  </apply>  </apply>  <apply>  <power />  <apply>  <ci> Gamma </ci>  <ci> ν </ci>  </apply>  <cn type='integer'> -1 </cn>  </apply>  </apply>  </apply>  </apply>  </apply>  <apply>  <lt />  <ci> x </ci>  <cn type='integer'> 0 </cn>  </apply>  </apply>  </annotation-xml>  </semantics>  </math> 
   
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  | Cell[BoxData[RowBox[List[RowBox[List["HoldPattern", "[", RowBox[List["Limit", "[", RowBox[List[RowBox[List["ExpIntegralE", "[", RowBox[List["\[Nu]_", ",", RowBox[List["x_", "-", RowBox[List["\[ImaginaryI]", " ", "\[Epsilon]_"]]]]]], "]"]], ",", RowBox[List["\[Epsilon]_", "\[Rule]", RowBox[List["+", "0"]]]]]], "]"]], "]"]], "\[RuleDelayed]", RowBox[List[RowBox[List[RowBox[List["ExpIntegralE", "[", RowBox[List["\[Nu]", ",", "x"]], "]"]], "-", FractionBox[RowBox[List["2", " ", "\[Pi]", " ", "\[ImaginaryI]", " ", SuperscriptBox["\[ExponentialE]", RowBox[List[RowBox[List["-", "\[Pi]"]], " ", "\[ImaginaryI]", " ", "\[Nu]"]]], " ", SuperscriptBox["x", RowBox[List["\[Nu]", "-", "1"]]]]], RowBox[List["Gamma", "[", "\[Nu]", "]"]]]]], "/;", RowBox[List["x", "<", "0"]]]]]]]]  |  
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   Date Added to functions.wolfram.com (modification date)
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