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   http://functions.wolfram.com/01.31.07.0002.01
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    ProductLog[z] == Integrate[(-(1/Pi)) Im[D[ProductLog[x], x]] Log[1 - z/x], 
   {x, -Infinity, -(1/E)}] /; 
  !IntervalMemberQ[Interval[{-Infinity, -(1/E)}], z] 
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   Cell[BoxData[RowBox[List[RowBox[List[RowBox[List["ProductLog", "[", "z", "]"]], "\[Equal]", RowBox[List[SubsuperscriptBox["\[Integral]", RowBox[List["-", "\[Infinity]"]], RowBox[List["-", FractionBox["1", "\[ExponentialE]"]]]], RowBox[List[RowBox[List["-", FractionBox["1", "\[Pi]"]]], RowBox[List["Im", "[", RowBox[List[SubscriptBox["\[PartialD]", "x"], RowBox[List["ProductLog", "[", "x", "]"]]]], "]"]], " ", RowBox[List["Log", "[", RowBox[List["1", "-", FractionBox["z", "x"]]], "]"]], RowBox[List["\[DifferentialD]", "x"]]]]]]]], "/;", RowBox[List["Not", "[", RowBox[List["IntervalMemberQ", "[", RowBox[List[RowBox[List["Interval", "[", RowBox[List["{", RowBox[List[RowBox[List["-", "\[Infinity]"]], ",", RowBox[List["-", FractionBox["1", "\[ExponentialE]"]]]]], "}"]], "]"]], ",", "z"]], "]"]], "]"]]]]]] 
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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>  <mi> W </mi>  <mo> ⁡ </mo>  <mo> ( </mo>  <mi> z </mi>  <mo> ) </mo>  </mrow>  <mo> ⩵ </mo>  <mrow>  <msubsup>  <mo> ∫ </mo>  <mrow>  <mo> - </mo>  <mi> ∞ </mi>  </mrow>  <mrow>  <mo> - </mo>  <mfrac>  <mn> 1 </mn>  <mi> ⅇ </mi>  </mfrac>  </mrow>  </msubsup>  <mrow>  <mrow>  <mo> - </mo>  <mfrac>  <mn> 1 </mn>  <mi> π </mi>  </mfrac>  </mrow>  <mo> ⁢ </mo>  <mrow>  <mi> Im </mi>  <mo> ⁡ </mo>  <mo> ( </mo>  <mfrac>  <mrow>  <mo> ∂ </mo>  <mrow>  <mi> W </mi>  <mo> ⁡ </mo>  <mo> ( </mo>  <mi> x </mi>  <mo> ) </mo>  </mrow>  </mrow>  <mrow>  <mo> ∂ </mo>  <mi> x </mi>  </mrow>  </mfrac>  <mo> ) </mo>  </mrow>  <mo> ⁢ </mo>  <mrow>  <mi> log </mi>  <mo> ⁡ </mo>  <mo> ( </mo>  <mrow>  <mn> 1 </mn>  <mo> - </mo>  <mfrac>  <mi> z </mi>  <mi> x </mi>  </mfrac>  </mrow>  <mo> ) </mo>  </mrow>  <mo> ⁢ </mo>  <mrow>  <mo> ⅆ </mo>  <mi> x </mi>  </mrow>  </mrow>  </mrow>  </mrow>  <mo> /; </mo>  <mrow>  <mi> z </mi>  <mo> ∉ </mo>  <mrow>  <mo> ( </mo>  <mrow>  <mrow>  <mo> - </mo>  <mi> ∞ </mi>  </mrow>  <mo> , </mo>  <mrow>  <mo> - </mo>  <mfrac>  <mn> 1 </mn>  <mi> ⅇ </mi>  </mfrac>  </mrow>  </mrow>  <mo> ) </mo>  </mrow>  <mtext> ​ </mtext>  </mrow>  </mrow>  <annotation-xml encoding='MathML-Content'>  <mrow>  <mrow>  <mrow>  <mi> W </mi>  <mo> ⁡ </mo>  <mo> ( </mo>  <mi> z </mi>  <mo> ) </mo>  </mrow>  <mo> ⩵ </mo>  <mrow>  <msubsup>  <mo> ∫ </mo>  <mrow>  <mo> - </mo>  <mi> ∞ </mi>  </mrow>  <mrow>  <mo> - </mo>  <mfrac>  <mn> 1 </mn>  <mi> ⅇ </mi>  </mfrac>  </mrow>  </msubsup>  <mrow>  <mrow>  <mo> - </mo>  <mfrac>  <mn> 1 </mn>  <mi> π </mi>  </mfrac>  </mrow>  <mo> ⁢ </mo>  <mrow>  <mi> Im </mi>  <mo> ⁡ </mo>  <mo> ( </mo>  <mfrac>  <mrow>  <mo> ∂ </mo>  <mrow>  <mi> W </mi>  <mo> ⁡ </mo>  <mo> ( </mo>  <mi> x </mi>  <mo> ) </mo>  </mrow>  </mrow>  <mrow>  <mo> ∂ </mo>  <mi> x </mi>  </mrow>  </mfrac>  <mo> ) </mo>  </mrow>  <mo> ⁢ </mo>  <mrow>  <mi> log </mi>  <mo> ⁡ </mo>  <mo> ( </mo>  <mrow>  <mn> 1 </mn>  <mo> - </mo>  <mfrac>  <mi> z </mi>  <mi> x </mi>  </mfrac>  </mrow>  <mo> ) </mo>  </mrow>  <mo> ⁢ </mo>  <mrow>  <mo> ⅆ </mo>  <mi> x </mi>  </mrow>  </mrow>  </mrow>  </mrow>  <mo> /; </mo>  <mrow>  <mi> z </mi>  <mo> ∉ </mo>  <mrow>  <mo> ( </mo>  <mrow>  <mrow>  <mo> - </mo>  <mi> ∞ </mi>  </mrow>  <mo> , </mo>  <mrow>  <mo> - </mo>  <mfrac>  <mn> 1 </mn>  <mi> ⅇ </mi>  </mfrac>  </mrow>  </mrow>  <mo> ) </mo>  </mrow>  <mtext> ​ </mtext>  </mrow>  </mrow>  </annotation-xml>  </semantics>  </math> 
   
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  | Cell[BoxData[RowBox[List[RowBox[List["HoldPattern", "[", RowBox[List["ProductLog", "[", "z_", "]"]], "]"]], "\[RuleDelayed]", RowBox[List[RowBox[List[SubsuperscriptBox["\[Integral]", RowBox[List["-", "\[Infinity]"]], RowBox[List["-", FractionBox["1", "\[ExponentialE]"]]]], RowBox[List[RowBox[List["-", FractionBox[RowBox[List[RowBox[List["Im", "[", RowBox[List[SubscriptBox["\[PartialD]", RowBox[List["x"]]], RowBox[List["ProductLog", "[", "x", "]"]]]], "]"]], " ", RowBox[List["Log", "[", RowBox[List["1", "-", FractionBox["z", "x"]]], "]"]]]], "\[Pi]"]]], RowBox[List["\[DifferentialD]", "x"]]]]]], "/;", RowBox[List["!", RowBox[List["IntervalMemberQ", "[", RowBox[List[RowBox[List["Interval", "[", RowBox[List["{", RowBox[List[RowBox[List["-", "\[Infinity]"]], ",", RowBox[List["-", FractionBox["1", "\[ExponentialE]"]]]]], "}"]], "]"]], ",", "z"]], "]"]]]]]]]]]]  |  
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   Date Added to functions.wolfram.com (modification date)
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