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   http://functions.wolfram.com/01.15.27.0004.01
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    ArcTan[x, y] == ArcTan[y/x] + (2 UnitStep[y] - 1) Pi /; 
 Element[y, Reals] && Re[x] < 0 
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   Cell[BoxData[RowBox[List[RowBox[List[RowBox[List["ArcTan", "[", RowBox[List["x", ",", "y"]], "]"]], "\[Equal]", RowBox[List[RowBox[List["ArcTan", "[", FractionBox["y", "x"], "]"]], "+", RowBox[List[RowBox[List["(", RowBox[List[RowBox[List["2", RowBox[List["UnitStep", "[", "y", "]"]]]], "-", "1"]], ")"]], "\[Pi]"]]]]]], "/;", RowBox[List[RowBox[List["Element", "[", RowBox[List["y", ",", "Reals"]], "]"]], "\[And]", RowBox[List[RowBox[List["Re", "[", "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>  <msup>  <mi> tan </mi>  <mrow>  <mo> - </mo>  <mn> 1 </mn>  </mrow>  </msup>  <mo> ( </mo>  <mrow>  <mi> x </mi>  <mo> , </mo>  <mi> y </mi>  </mrow>  <mo> ) </mo>  </mrow>  <mo> ⩵ </mo>  <mrow>  <mrow>  <msup>  <mi> tan </mi>  <mrow>  <mo> - </mo>  <mn> 1 </mn>  </mrow>  </msup>  <mo> ( </mo>  <mfrac>  <mi> y </mi>  <mi> x </mi>  </mfrac>  <mo> ) </mo>  </mrow>  <mo> + </mo>  <mrow>  <mrow>  <mo> ( </mo>  <mrow>  <mrow>  <mn> 2 </mn>  <mo> ⁢ </mo>  <mrow>  <mi> θ </mi>  <mo> ⁡ </mo>  <mo> ( </mo>  <mi> y </mi>  <mo> ) </mo>  </mrow>  </mrow>  <mo> - </mo>  <mn> 1 </mn>  </mrow>  <mo> ) </mo>  </mrow>  <mo> ⁢ </mo>  <mi> π </mi>  </mrow>  </mrow>  </mrow>  <mo> /; </mo>  <mrow>  <mrow>  <mi> y </mi>  <mo> ∈ </mo>  <semantics>  <mi> ℝ </mi>  <annotation encoding='Mathematica'> TagBox["\[DoubleStruckCapitalR]", Function[Reals]] </annotation>  </semantics>  </mrow>  <mo> ∧ </mo>  <mrow>  <mrow>  <mi> Re </mi>  <mo> ⁡ </mo>  <mo> ( </mo>  <mi> x </mi>  <mo> ) </mo>  </mrow>  <mo> < </mo>  <mn> 0 </mn>  </mrow>  </mrow>  </mrow>  <annotation-xml encoding='MathML-Content'>  <apply>  <ci> Condition </ci>  <apply>  <eq />  <apply>  <arctan />  <ci> x </ci>  <ci> y </ci>  </apply>  <apply>  <plus />  <apply>  <arctan />  <apply>  <times />  <ci> y </ci>  <apply>  <power />  <ci> x </ci>  <cn type='integer'> -1 </cn>  </apply>  </apply>  </apply>  <apply>  <times />  <apply>  <plus />  <apply>  <times />  <cn type='integer'> 2 </cn>  <apply>  <ci> θ </ci>  <ci> y </ci>  </apply>  </apply>  <cn type='integer'> -1 </cn>  </apply>  <pi />  </apply>  </apply>  </apply>  <apply>  <and />  <apply>  <in />  <ci> y </ci>  <reals />  </apply>  <apply>  <lt />  <apply>  <real />  <ci> x </ci>  </apply>  <cn type='integer'> 0 </cn>  </apply>  </apply>  </apply>  </annotation-xml>  </semantics>  </math> 
   
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  | Cell[BoxData[RowBox[List[RowBox[List["HoldPattern", "[", RowBox[List["ArcTan", "[", RowBox[List["x_", ",", "y_"]], "]"]], "]"]], "\[RuleDelayed]", RowBox[List[RowBox[List[RowBox[List["ArcTan", "[", FractionBox["y", "x"], "]"]], "+", RowBox[List[RowBox[List["(", RowBox[List[RowBox[List["2", " ", RowBox[List["UnitStep", "[", "y", "]"]]]], "-", "1"]], ")"]], " ", "\[Pi]"]]]], "/;", RowBox[List[RowBox[List["y", "\[Element]", "Reals"]], "&&", RowBox[List[RowBox[List["Re", "[", "x", "]"]], "<", "0"]]]]]]]]]]  |  
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   Date Added to functions.wolfram.com (modification date)
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